Boiler, Piping and Controls Design

EPRI Project ManagerJ. Kesselring

EPRI • 3412 Hillview Avenue, Palo Alto, California 94304 • PO Box 10412, Palo Alto, California 94303 • USA800.313.3774 • 650.855.2121 • [email protected] • www.epri.com

Hydronic Heating: A PracticalOverview

TR-114793

Final Report, March 2000

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ORGANIZATION(S) THAT PREPARED THIS DOCUMENT

Saturn Resource Management

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iii

CITATIONS

This report was prepared by

Saturn Research Management324 Fuller Avenue N-13Helena, MT 59601

Principal InvestigatorJ. Krigger

This report describes research sponsored by EPRI.

The report is a corporate document that should be cited in the literature in the following manner:

Hydronic Heating: A Practical Overview, EPRI, Palo Alto, CA: 2000. TR-114793.

v

REPORT SUMMARY

This booklet is a hydronic-heating primer for utility representatives, contractors, andhomeowners. Its purpose is to foster a general knowledge and interest in modem hydronicheating.

BackgroundHydronic heating uses water or steam as the source for heat transfer. Driven by the comfortbenefits of hydronic radiant heat, this technique has seen a resurgence in the United States. Well-insulated homes and radiant-floor heating permit use of low-temperature water. Combined withnew energy efficient models of hydronic heat pumps and combustion boilers, radiant heat offersa high level of comfort and energy efficiency. New piping materials, improved pumps andvalves, and a variety of versatile control systems have made hydronics the heating choice ofmany home buyers.

Objectives• To introduce consumers, designers, and builders to the advantages and disadvantages of

hydronic heating systems.

• To explain the process of designing and installing hydronic systems.

ApproachThe project team used existing reference material coupled with extensive site visits anddiscussions with hydronic system installers to develop a firm background in hydronic systemtechnology. The draft material was then reviewed by hydronic practitioners for accuracy andcompleteness.

ResultsThis booklet discusses how hydronic systems work and briefly compares them with forced-airheating. It describes hydronic heat pumps, electric boilers, electric thermal storage, and fossil-fuel boilers. Selecting heat emitters is detailed, with descriptions of baseboard convectors, fancoils, panel radiators, and radiant floors. The booklet covers key elements in designingdistribution and control systems. Also covered are domestic hot water, cooling, and installationissues.

EPRI PerspectiveHydronic heating has many virtues and advantages—including comfort, energy efficiency, anddurability—but is generally more expensive and complex to design and install than forced-airheating. If design and planning are adequate, however, an experienced hydronic-heatingcontractor can install a system that will give customers a heating system generally superior to

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more conventional forced-air systems. This booklet introduces designers and builders to theprocess of designing and installing hydronic systems.

TR-114793

KeywordsResidential energy useHydronic heatingHeating

vii

ABSTRACT

Hydronic heating has enjoyed a resurgence in the United States, driven by the comfort benefitsof hydronic radiant heat. Well-insulated homes and radiant-floor heating allow the use of low-temperature water. Combined with new energy-efficient models of hydronic heat pumps andcombustion boilers, radiant heat offers a high level of comfort and energy efficiency. New pipingmaterials, improved pumps and valves, and a variety of versatile control systems make hydronicsthe heating choice of many discriminating home buyers. Domestic water heating is easy tointegrate with the modern hydronic system. Cooling is still a challenge to integrate with hydronicheating and commonly requires the addition of an air handler with ducts. However, a radiantfloor combined with room-sized fan-coils will provide effective cooling in well-designed homes,where indoor relative humidity can be closely controlled.

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ACKNOWLEDGMENTS

Most of the information contained in this booklet originates from just a few sources. JohnSiegenthaler has chronicled this field better than anyone else in his text, Modern HydronicHeating and in the 1999 Standard Guidelines for the Radiant Panel Association (RPA) and in theRPA’s book, Radiant Basics. John is an experienced and inventive engineer, whose talent fortechnical communication is a great benefit to the hydronics industry.

Dale Pickard answered a host of practical questions and loaned his expert technical review to thisdocument. His 15 years of experience installing hydronic systems was a great help to the author.Thanks also to Dale’s partner Bob Knebel for answering questions and arranging site visits.Many of the photographs in this booklet come from the files of Radiant Engineering.

Illustrations were provided by B. Starkey, J. Krigger, and M. Lord. Photography was done by J.Krigger, D. Pickard, and B. Knebel. The contributions of all are gratefully acknowledged.

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FOREWORD

This booklet is a hydronic-heating primer for utility representatives, contractors andhomeowners. Its purpose is to encourage a general knowledge and interest in modern hydronicheating.

Hydronic heating has been used extensively for over 100 years. The word hydronic refers tocentral heating that employs a hot-water or steam distribution system. Steam is consideredobsolete for heating buildings because of its excessive cycle losses and its hot radiators thatresulted in room air stratification.

Both hydronic hot-water and steam heating were popular choices for central heating in the earlypart of the 20th century. In the 1950s forced-air heating gained the dominant market share whenthe modern furnace was introduced. Furnaces and ducts can be installed less expensively than aboiler and pipes.

In the past 15 years, hydronic hot-water heating has been gaining back some market share fromforced-air heating. New system components, pioneered in Europe, are leading the way. Modernhydronic systems feature: easy-to-install plastic pipe, innovative valve designs, and electroniccontrols that can provide superior comfort and energy efficiency, when installed correctly.

xiii

CONTENTS

1.0 CHOOSING HYDRONICS ....................................................................................................1

1.1 How a hydronic system works ..........................................................................................3

1.2 Comparing hydronic heating with forced-air heating .........................................................3

1.3 Energy efficiency ..............................................................................................................4

1.4 The building shell..............................................................................................................5

1.5 Planning and design .........................................................................................................7

1.6 Quality installation ............................................................................................................8

1.7 Choosing a fuel ................................................................................................................9

2.0 SELECTING THE HEATER................................................................................................10

2.1 Hydronic heat pumps......................................................................................................10

2.1.1 Open-loop systems .................................................................................................10

2.1.2 Closed-loop systems ..............................................................................................11

2.2 Electric boilers and electric thermal storage....................................................................12

2.3 Fossil-fuel boilers............................................................................................................12

2.3.1 Energy-efficient gas combustion .............................................................................12

2.3.2 Energy-efficient oil combustion ...............................................................................14

2.3.3 Cast-iron and steel boilers ......................................................................................15

2.3.4 Low-mass and condensing boilers ..........................................................................17

3.0 SELECTING HEAT EMITTERS..........................................................................................19

3.1 Baseboard convectors....................................................................................................19

3.2 Fan coils.........................................................................................................................20

3.3 Panel radiators ...............................................................................................................22

3.4 Radiant floors .................................................................................................................22

4.0 DESIGNING THE DISTRIBUTION SYSTEM AND CONTROL SYSTEM............................28

4.1 Hydronic distribution systems .........................................................................................29

xiv

4.1.1 Piping choices.........................................................................................................30

4.1.2 Circulators...............................................................................................................32

4.1.3 Valves.....................................................................................................................34

4.1.4 Expansion tanks......................................................................................................36

4.1.5 Air eliminators .........................................................................................................37

4.1.6 Manifolds.....................................................................................................................38

4.2 Control systems..............................................................................................................38

4.2.1 Aquastats and reset controls...................................................................................39

4.2.2 Electronic controls...................................................................................................40

4.2.3 Modular boiler systems ...........................................................................................41

5.0 DOMESTIC HOT WATER, COOLING, AND INSTALLATION ISSUES..............................43

5.1 Domestic hot water.........................................................................................................43

5.2 Hydronic cooling .............................................................................................................45

5.3 Putting it all together.......................................................................................................45

GLOSSARY....................................................................................................................... .......48

REFERENCES .........................................................................................................................52

Choosing Hydronics

1

1.0 ChoosinghydronicsThe primary purpose of thisbooklet is to introduceconsumers, designers, andbuilders to the advantages anddisadvantages of hydronic heatingsystems, and also, to explain theprocess of designing andinstalling hydronic systems.Hydronic heating uses water orsteam as the source of heattransfer. This booklet discusseshydronic heating with hot wateronly. (Hydronic cooling systemsare also discussed, but to a lesserextent.)

Hydronic heating has manyvirtues and advantages, includingcomfort, energy efficiency, anddurability, but it is generally moreexpensive and complex to designand install than forced-airheating. If the design andplanning are adequate, however,an experienced hydronic-heatingcontractor can install a systemthat will give the customer aheating system generally superiorto a more conventional forced-airsystem.

Cost is a key issue sincehydronic heating is substantiallymore expensive than forced-airheating, and can vary widely

depending on the choices madeby the customer and designer.High-efficiency heat sources likecondensing boilers and hydronicheat pumps are more expensivethan conventional boilers.Radiant-floor heating is morelabor-intensive and expensivethan heating with baseboardconvectors and wall-hungradiators. However, a growingnumber of consumers appear tobelieve that the benefits of aradiant floor outweigh the costs.

The best way to providecomfort is to create an indoorenvironment where the bodyeffortlessly remains at thermalequilibrium with its surroundings.Thermal equilibrium means thatthe body is losing the requiredamount of heat—not too much ortoo little. Forced-air and radiantheating accomplish this goal intwo different ways. Forced-airheating moves warm airthroughout the living space,raising the air temperature andheating the objects in the room byconvection. Radiant heat warms abuilding’s contents andoccupants, in turn, warming theair.

As we metabolize our food andoperate our bodies in a relaxedway, we must release about 400

2

Choosing Hydronics

British thermal units per hour(Btuh) (117 watts) to remain at aconstant 98.6° F (37.0° C).Almost half of this, or 190 Btuh(56 watts), is typically lostthrough radiation, withconvection contributing about120 Btuh and evaporationcontributing 80 Btuh (23 watts).Radiant-heating experts professthat controlling the mean radianttemperature (MRT) is the mosteffective and efficient way toprovide comfort because thehuman body’s preferred heat-control mechanism is radiation.Every 1-degree Fahrenheit (0.6-degree Celsius) increase in MRTis equivalent to a 1.4°-F (0.8°-C)increase in air temperature.

The more a hydronic heatingsystem relies on radiation, themore of these radiation-relatedbenefits it provides. Largeradiation surfaces like radiantfloors theoretically provide themost comfort and efficiencyadvantages by reducing indoortemperature differences andtemperature-induced airstratification common withforced-air heating and high-temperature hydronic convectors.The opportunity to use low-temperature water in radiantfloors allows them to be coupled

Convection versus radiation

Forced-air heating dependsexclusively on convection. Theroom’s objects are warmed bymoving air. Air stratificationand cool floors can result.Windows are well dried bymoving air, and windowdowndraft is overpowered byairflow from the register.

Hydronic radiant convectorsheat primarily by convectionwith some radiation. Windowdowndrafts and condensationare prevented. Stratificationand pressurizationdisadvantages can be avoidedby using low watertemperatures.

Panel radiators heatprimarily by radiation withsome convection to preventmost window downdraftsand condensation.Convection plays a lesserrole, improving comfort.

Radiant floors heat exclusivelyby radiation. Radianttemperature controls bodilyheat loss most comfortably.Radiant floors may need helpfrom hydronic baseboardconvectors to counteractsudden outdoor temperaturechanges, and to avoidcondensation and downdrafts atlarge windows.

Choosing Hydronics

3

with condensing boilers andhydronic heat pumps to providethe most efficient heating systemsavailable.

1.1 How a hydronicsystem worksHydronic heating systems adjusttemperature in a home with acombination of piped water andheat emitters. Hydronic systemsoften include domestic hot-waterheating as part of the system. Thisdocument is almost entirely abouthydronic heating because it is farmore common than hydroniccooling. (See Section 5.2 for ashort discussion of hydroniccooling.)

The main component of ahydronic system is its heatsource; the most common arecombustion boilers and hydronicheat pumps. A combustion boilerburns gas or oil to produce theneeded heat. A hydronic heatpump collects heat from theground or from a water source,such as a well or lake.

Heat from the boiler or heatpump is circulated through theheat emitters by a pump orcirculator. A circulator adds themechanical energy needed tomove the heated water throughthe heat emitters.

The most common heat emitteris the baseboard convector,although European-style wall-hung radiators are becomingmore common. The popularity ofradiant-floor heat is driving arevival of interest in hydronicsystems, as evidenced by salesincreases of these systems andtheir component parts.

Hydronic piping systemscontain an expansion tank toallow the water to expand andcontract as it’s heated and cooled.Hydronic piping also containsvents to rid the system of air—acause of corrosion and otherproblems.

1.2 Comparinghydronic heating withforced-air heatingThe two main types ofdistribution systems for centralheating are hydronic heating andforced-air heating. Forced-airheating uses ducts to distributeheated air from a furnace or air-source heat pump. Hydronic heatuses pipes and heat emitters todistribute heated water from aboiler or hydronic heat pump.Forced-air heating is the mostpopular residential applicationbecause of its lower initial costand easier integration with central

4

Choosing Hydronics

cooling compared to hydronicsystems.

When properly designed andinstalled, hydronic heating offersthe best comfort and the highestenergy efficiency of any commonheating system, although athigher cost and greatercomplexity. This cost-and-complexity disadvantage fades asa home gets larger, or with amultifamily building.

Advantages of hydronic heatinginclude:

• Ability to zone rooms,• More efficient use of

space for pipescompared to ducts,

• Ability to control fluidtemperature accordingto outdoor temperature,

• Ability to use low fluidtemperatures in radiantfloors, and

• Distributing heat moreefficiently with pumpsthan fans.

Advantages of forced-airheating include:

• Less expense andcomplexity thanhydronics,

• Easy and economicalinclusion of coolingsystems,

• Ready availability ofhigh-efficiencyfurnaces that require no

special installationprocedures, and

• Reasonably pricedelectric heating andcooling from air-sourceheat pumps.

1.3 Energy efficiencyForced-air and hydronic systemefficiency varies widely,depending on design, equipmentselection, and installation. Someforced-air systems are moreefficient than their hydroniccounterparts. However, hydronicsystems usually have an energy-efficiency advantage overforced-air systems for thefollowing reasons:

• Pipes waste less heatbecause of theirsmaller surface areaand relative absence ofleaks compared toducts;

• Water moves heat manytimes more efficientlythan air because waterstores four times asmuch heat by weightand 3450 times asmuch heat by volumeas air;

• Hydronic systems canbe zoned far moreeasily andeconomically thanforced-air systems;

• It’s far easier to adjustwater temperature up

Choosing Hydronics

5

or down, according toboth indoor andoutdoor temperature,compared to airtemperature; and

• A correctly designedhydronic system canavoid temperature-induced airstratification and roompressurization—energyproblems commonlyassociated with forced-air heating.

A variety of design elementsdetermine how close the hydronicsystem will come to its theoreticallimit of energy efficiency:

• Matching the heatsource to the loadthrough multi-boilersystems or two-stagecompressors inhydronic heat pumps;

• Designing pipingsystems to minimizehydraulic resistancewithout sacrificingoperating effectivenessor control; and

• Using the lowestacceptable supply-water temperature byadjusting boiler-watertemperature, valvesetting, or pump speed,according to outdoortemperature.

These energy-savingadvantages can be maximized,and can provide superior energy

efficiency, if combined with anenergy-efficient building shell.

1.4 The building shellThe performance of any heatingsystem depends on theconstruction of the building shell.A well-insulated and airtightbuilding will make almost anyheating system shine. Andconversely, a poorly insulated,drafty building may tarnish theperformance of a well-designedand well-installed heating system.Flaws in the building shell havebeen the cause of manycomplaints to heating contractorsabout inadequate comfort. Forthis reason, many experts in theheating, ventilation, and airconditioning (HVAC) field areadvising HVAC contractors to getinvolved in home heating andcooling projects early and toadvise the building contractor andthe customer to construct anenergy-efficient building shell.

Air leakage through the shell isparticularly difficult for a heatingengineer or contractor to predict,and could range from 0.10 to 1.0air changes per hour. Flaws ininsulation can reduce the R-valueof insulation by 15 to 30 percent,depending on their severity.Given this unpredictable

6

Choosing Hydronics

variability, HVAC contractorsoften install a significantlyoversized heating or coolingsystem. Installing oversizedheating and cooling equipmentcosts the customer more initially,reduces system efficiency, andmakes providing optimal comfortmore difficult than installing acorrectly sized system.

Home heat loss is dividedbetween heat transmission and airleakage. The following are somesimple thermal resistancestandards to consider forminimizing heat transmission:

• U-0.40 or less windowU factor;

• R-38 attic insulationwith energy trusses thatallow at least R-24over top of outsidewalls;

• R-19 above-grade wallinsulation;

• R-11 below-grade wallinsulation;

• R-19 floor insulationbetween living spacesand uninsulatedbasement or crawlspace; and

• Insulation should beinstalled touching theinterior sheetingmaterial. Battinsulation should becut and sized very

accurately, allowing nogaps or voids.

A home’s air leakage must bemeasured, because experienceshows it can’t be estimated byvisual inspection. If the airleakage is unknown and variestenfold from one house to thenext, how can a contractor orutility representative calculate thecorrect equipment size?Instruments called blower doorsare used to measure air leakage,and the natural airflow rate iscalculated from blower-doorreadings. The American Societyof Heating, Refrigerating, andAir-Conditioning Engineers(ASHRAE) has established alower limit for air leakage inhomes without mechanicalventilation systems. ASHRAEStandard 62-1989 states that ahouse should have 0.35 airchanges of the complete housevolume each hour under typicaloutdoor conditions. Homes withmechanical ventilation systemsare allowed to be completelyairtight, although it’s rare for ahome’s leakage rate to be muchless than 0.10 air changes perhour.

Planning an airtight home andmechanical ventilation system issafer and more realistic than

Choosing Hydronics

7

expecting the building contractorto build the home “tight but nottoo tight.” Most buildingscientists agree that airtightbuilding-shell components protecta home from moisture-bearing airintrusion and resultingcondensation damage. There is noaccepted way of leaving gaps infloors, walls, or ceilings, toachieve the “tight but not tootight” specification. Recentscientific studies indicate thatmechanical ventilation systemsperform better in airtight homesthan in looser homes. The samestudies suggest that mechanicalventilation systems should haveboth fan-powered intake andexhaust to be effective atremoving pollutants and keepingindoor air fresh.

1.5 Planning anddesignThe vast array of hydronicsystems and components offersthe contractor and customerabundant flexibility. However, thecomplexity and uniqueness ofhydronic systems requires carefuldesign and planning.

It’s ideal to match a home’sheating system to its design, andto include drawings andspecifications for the heating

system in the home’s blueprints.Heating systems occupy spaceand require dedicated room orarea. The heat source along withits circulators, valves, andcontrols need a dedicated spacelarge enough to contain theequipment, while allowing extraworking space for maintenanceand repair. Baseboard convectors,wall-hung radiators, and fan coilsalso need some empty spacearound them to function properly,and should be located tominimize conflicts with furniture.

The home’s heating load isdetermined by its climate, size,insulation levels, airtightness, andthe energy efficiency of itswindows and doors. The heatingload is expressed in Btuh orkilowatts (kW), and is calculatedusing the design temperature of aparticular climatic region. Designtemperature is the temperaturethat a particular climate exceeds97.5 percent of the time.

Better designers and contractorscalculate both the total heatingload for a home and the room-by-room heat loads needed to selectthe heat emitters and providethem with the proper flow rateand water temperature. Thesecalculations are usuallyperformed using a computer

8

Choosing Hydronics

program designed for thispurpose. Heat loads commonlyrange from 10 to 50 Btuh persquare foot of floor space (32 to158 watts per square meter),depending on climate and thecharacteristics of the buildingshell.

The design process continueswith the designer and customerschoosing the heat emitters anddetermining their location. Nextthe designer determines whichheat emitters should be groupedtogether in zones. A pipingdiagram is next, combined withdetermination of hydraulicresistance for the heat emitters,piping, and fittings from themanufacturer’s specifications.Each heat emitter, pipe, andfitting removes head ormechanical energy from themoving water, depending on itshydraulic resistance. Each of thesystem’s piping circuits can bedescribed by a graph of flow rateversus head loss. Head loss is theamount of mechanical energyremoved from the water by ahydronic component or a wholepiping circuit.

Planning, designing, andinstalling the system require thefollowing tasks:

• Selecting the heater,

• Selecting the heatemitters,

• Designing thedistribution and controlsystem, and

• Installing and testing theheating system.

1.6 Quality installationPlanning and installing

hydronic systems will toleratefew errors and omissions. A fewminor service calls or failure of asingle component could defeatthe economic advantagesanticipated by the customer.Simpler systems pose lesspotential problems and may be abetter choice for less-experiencedcontractors.

Hydronic systems are oftenone-of-a-kind engineeredsystems. All components shouldbe used only as recommended bythe manufacturer. Pitfallsinvolving unsuitability orincompatibility of systemcomponents are more numerousthan those found in forced-airsystems.

Most hydronic systems haveiron and steel components thatwill corrode if oxygen migratesinto the system. There are twocommon ways oxygen gets in.The first is oxygen dissolved inmakeup water. A design or

Choosing Hydronics

9

installation flaw that allows waterto escape through the pressure-relief valve or a leak will bring asteady supply of oxygen into thesystem through the makeup watervalve. The second is too lowpressure. If the circulator createstoo great a suction within thevalves and piping, oxygen fromthe air can be drawn in throughvents or other openings.

A particular threat to boilers iscondensation of water out ofcombustion gases that corrodesboilers and chimneys. The mostcommon cause of thiscondensation is return watertemperature to the boiler that istoo cool. Control systems must bedesigned, installed, and adjustedto avoid this condition.

Selecting the proper pipe,fittings, and valves is critical tothe system’s durability. Mistakesin the application of thesecomponents have rendered newsystems ineffective and in need ofexpensive modifications.

1.7 Choosing a fuelThe three fuels discussed in this

book are electricity, gas, and oil.Price is a major concern toconsumers, especially to thosewho have a choice of fuels.Natural gas and electricity have

remained fairly stable in cost forabout the past 15 years, withelectricity about three times moreexpensive than natural gas perunit of heat. However, a hydronicheat pump with a Coefficient ofPerformance (COP) of between 3and 4 is competitive with acondensing boiler with an AnnualFuel Utilization Efficiency(AFUE) of around 95 percent.(Efficiency comparisons areexplained in Section 2.)

The efficiency of oil-firedboilers is about the same as gas-fired boilers, except thathigh-efficiency oil-firedcondensing boilers aren’tcommonly available. Oil hasexperienced more cost increasesand reductions than electricityand natural gas. However, it’sanyone’s guess how or if energyprices will change. Oil may riseand fall more than natural gas andelectricity because oil is theleading transportation fuel andbecause the cost of oil is affectedby politics of the oil-producingcountries.

10

Selecting the Heater

2.0 Selecting theheaterThe heaters described here haveone of three design types. Theyare fossil-fuel boilers, hydronicheat pumps, and electric boilers.Hydronic heat pumps and electricboilers have simple designscompared to fossil-fuel boilers,which have many differentdesigns featuring variouscombustion systems, heatexchangers, and controls. Fossil-fuel boilers include those fired byoil, natural gas, and propane.Propane boilers are nearlyidentical to natural-gas boilers,except they have slightly differentgas valves and burner orifices.

2.1 Hydronic heatpumpsHydronic heat pumps use therefrigeration cycle to move heatfrom the earth or a body of waterinto the home during the heatingseason. Heat-pump systems thatget their heat from a body ofwater, such as a well or lake, arecalled open-loop systems becausethe system is open to theatmosphere. Closed-loop systemsextract heat from the earth usingloops of buried pipe. These loopsrecirculate the same cold-water/

antifreeze solution to move heatfrom the earth to the home.

The energy efficiency of ahydronic heat pump is expressedas a Coefficient of Performance(COP). Most hydronic heatpumps list a COP of between 3.0and 3.5, meaning that they supply3.0 to 3.5 kilowatt-hours (kWh)of heat for each kWh ofelectricity they consume.

Hydronic heat pumps can’tproduce the high temperaturesrequired by most radiators andconvectors. Radiant floors are alow-temperature heat emitter andare more compatible withhydronic heat pumps. Hydronicheat pumps offer the opportunityfor both heating and cooling ifmatched to radiant floors, radiantwalls or ceilings, fan coils, or airhandlers with ducts.

Hydronic heat pumps, like air-source heat pumps, work best inwell-insulated and airtight homesbecause they provide heat at arelatively low temperature.

2.1.1 Open-loop systemsAn open-loop system pumpswater from a lake, pond, orunderground aquifer through awater-to-refrigerant heatexchanger. Hydronic heat pumpsrequire water flow of at least one

11


gallon per minute (gpm) (0.13liters per second [lps]) for each6000 Btuh of heating capacity.Medium-to-large-sized systemsmay demand too much water forsome wells. The well should havea verified flow rate of at least 10gpm (0.63 lps) to handle the heatpump and domestic water. If youlive in an area where wellcapacity varies from season toseason or year to year, be sure toallow for this variation in yourdesign. Also consider theelectricity drawn by the waterpump when comparing the heatpump’s efficiency with other heatsources.

Water from lakes and wellsmust be returned to the groundunpolluted by water treatments.Lake and well water should betested to analyze its potential tocorrode the heat exchanger ordeposit silt or minerals.

2.1.2 Closed-loop systemsThe key to a successful closed-loop system is a contractor orengineer who understands thedesign of the ground loop. Theground loop’s design andinstallation, including materials,soil type, and the loop’s depth andconfiguration must be appropriateto the site, climate, and size of the

Components and functioning of a hydronic heat pump

Vertical Ground Loop

Horizontal Ground Loop

Open-Loop Water-Source Heat Pump

condenser

compressor

evaporator

thermostatic expansion valve

Hydronic heat pumps extract heat from the ground loop or water source andtransfer that heat to the indoors by way of a hydronic distribution system.

drain field

well

12


heating system. Piping materialsand connections in particular willtolerate no compromise. A well-designed and well-installedground loop will last decades, anda poorly designed and installedground loop may never workproperly, or fail in a few years.

2.2 Electric boilers andelectric thermal storageAlthough electricity as a fuel istypically two to four times asexpensive as fossil fuels, electricboilers have some advantagesover combustion boilers. Electricboilers have two or more heatingelements that can be staged tomatch the load. And, electricboilers are tolerant of low return-water temperature, which cancause thermal shock or corrosionin fossil-fuel boilers. Electricboilers also eliminate the safetyhazards involved withcombustion, venting, and fuelstorage.

Some electric utilities havelower rates for off-peak use thanfor on-peak use. Electric thermal-storage systems heat water withoff-peak electricity and store thisheat for on-peak use. The storedheat is provided by an electricboiler, a hydronic heat pump, or

electric heating elements installedin a large, insulated storage tank.

2.3 Fossil-fuel boilersGas- and oil-fired boilers are byfar the most popular heat sourcesfor hydronic heating systems.Boilers are classified by thematerials and design of their heatexchanger. Many gas and oilboiler designs are identical exceptfor the burner—either oil or gas.The two dominant designs arecast-iron-sectional boilers andcopper water-tube boilers.

2.3.1 Energy-efficient gascombustionBoiler efficiency is measured inlaboratories by a standard testprocedure that yields an AnnualFuel Utilization Efficiency(AFUE). AFUE measures theefficiency of the combustionprocess and considers cyclelosses and losses through thecabinet of the boiler. The mostimportant advance made bynewer, energy-efficient boilersover older, conventional modelsis the reduction of bothcombustion air and dilution air.Draft-producing blowers regulatecombustion air to the heatexchanger, forcing combustionby-products through tighterspaces, where the heat is removed

13


more effectively than inatmospherically vented boilers.Dilution air—chimney-balancingair that comes from the home—isreduced or eliminated because theboiler’s blower regulates theventing of combustion products.

Technicians should usecombustion-test equipment toverify the safe and efficientoperation of new gas and propaneboilers. Modern combustion testequipment measures excesscombustion air, carbon monoxide,and flue-gas temperature toestimate efficiency and evaluatecombustion safety.

Conventional boilers in existingbuildings have AFUEs of 68 to 80percent. Mid-efficiency boilershave AFUEs of 82 to 87 percent.And, high-efficiency gas boilershave AFUEs ranging from 90 to96 percent. The boiler efficiencyhas increased because of anumber of important innovations:

• Electronic ignition (nopilot light);

• Vent dampers to limitairflow through theheat exchanger whenthe burner is off;

• Heat exchangers madesmaller, lighter, and ofbetter materials;

• Fans that controlcombustion air more

precisely thanatmospheric draft; and

• Water vapor condensedfrom flue gases,yielding latent heat forextra efficiency in gasboilers (AFUEs of over90 percent).

The continuous pilot lights inconventional gas boilers useabout 3 percent of the boiler’stotal gas consumption.Intermittent ignition devices(IIDs) eliminate this waste andare featured on all new mid- andhigh-efficiency gas boilers. Theleast advanced type of mid-efficiency gas boiler has an IIDand a vent damper that closes offthe flue when the burner is notoperating.

The more advanced mid-efficiency boilers have adraft-assisting fan in the flue justabove the heat exchanger. The fandraws combustion air into theheat exchanger as it pulls exhaustgases out at a controlled rate. Thisgreatly reduces excesscombustion air that conveys heatfrom the combustion process upthe chimney. The draft-assistingfan also reduces or eliminatesdilution air that increases homeair leakage by creating a slightvacuum. These mid-efficiencyboilers are vented vertically

14


through existing chimneys(assuming the chimney isproperly lined) or horizontally,through an exterior wall, using amanufactured stainless-steel vent.

Mid-efficiency furnaces andboilers have had problems withcondensation in their chimneysand heat exchangers. Thecondensing boilers (highefficiency) have the advantage ofbeing designed to reclaim thelatent heat from the condensationand to resist corrosion.

Natural-gas combustionproduces large amounts of watervapor. About 12 percent of thetotal heat in the gas is tied up inthe water vapor’s latent heat. Thisaccounts for the high-efficiencyboilers’ dramatic increase ofAFUE from the mid-80 to themid-90-percent range. The watervapor condenses in a corrosion-resistant section of the heatexchanger and flows to a drain.The combustion gases are so coolthey exit through horizontalplastic pipe passing through anearby outside wall.

2.3.2 Energy-efficient oilcombustionHigh-efficiency oil boilers haveAFUEs of 80 to 87 percent,compared to conventional boilers

Oil combustion evolution

Older oil gun-type burner design shown at left has beenreplaced by the flame-retention oil burner that swirls the oil-airmixture into a tighter, hotter tube of flame.

Older designFlame-retention burner

with AFUEs of 68 to 80 percent.The high-efficiency oil boilerscontain a number of importantimprovements over older models:

• Flame-retention oilburners provide betterfuel-air mixing and amore efficient flame;

• Combustion gases travelmore slowly throughthe combustionchamber and heatexchanger compared toolder models;

• Interruptable ignitersshut off once the flameis established instead ofoperating during theentire cycle;

• Solid-state igniters andcontrols reduce theburner’s electricalconsumption; and

15


• Solenoid valves provideimmediate oil shutoff,reducing oil drips afterthe flame isextinguished.

Oil burners can only reach theirrated efficiency when adjustedusing a combustion analyzer.Technicians use combustionanalyzers to test the newlyinstalled boiler and during yearlyor bi-yearly service calls.

2.3.3 Cast-iron and steelboilersCast-iron sectional boilers have aseries of boxlike cast-ironsections connected by watertightfittings. Hot combustion gasesflow through spaces between the

sections, heating the cast-iron andwater. Cast-iron boilers are mass-produced and are the mostcommon type of boiler forresidential use, owing to theiravailability and reasonable cost.

Wet-base cast-iron boilers havebecome very popular because oftheir improved efficiency. Wet-base boilers have a cast-iron heatexchanger that surrounds thecombustion chamber. The moretraditional dry-base heatexchangers sit above thecombustion chamber and miss theopportunity to collect heat thatradiates sideways and downwardsfrom the flame.

Steel fire-tube boilers have asteel tank with internal tubing.The tubes, which carry thecombustion gases, may beoriented vertically or horizontally.

Both cast-iron and steel fire-tube boilers are high-massboilers, weighing from 300 to 600pounds (136 to 272 kilograms)when full of water. The boilers’metal and water requireconsiderable energy to reheat totheir operating temperatureduring each heating cycle. Muchof this heat is lost up the chimneybetween cycles, a majordisadvantage in using high-mass

Cast-iron sectional boiler

Cast-iron sectional boilers are available with gas or oil burners.Their efficiency has improved because of better burners andmore efficient venting, along with the elimination of the pilotlight.

forced-draftburner

cast-ironsections

16


boilers. This cycling energy wastecan be reduced by:

• Sizing the boileraccurately to a home’smaximum heat loss;

• Installing a vent damperto minimize flue lossesduring the off-cycle;and

• Using a modular boilersystem, composed oftwo or more smallerboilers.

Design and installation flawscan cause corrosion in cast-ironand steel boilers. If return water isbelow 130° F (54° C), acidiccondensation can form on theheat exchanger and ventingsystem and corrode these vitalboiler parts. To protect the boilerfrom condensation, make sure thesystem has a control thattemporarily shuts down thecirculator if the water temperaturegets too low. Radiant floors andother low-temperature heatemitters pose a threat to iron andsteel boilers, if piped directly,because of low return-watertemperature from the heat emitter.Designers use mixing valves orsecondary piping circuits toprovide boilers with protectionagainst low return-watertemperature.

Dissolved oxygen can corrodeiron or steel parts such as heatexchangers or pump housings.The oxygen originally dissolvedin the system’s water isn’t aproblem, but oxygen continuallyintroduced in makeup water orthrough air leaking into thesystem could eventually corrodeand ruin the iron or steel boilers,pumps, and valves. Water leaks ordischarges through the pressurerelief valve cause makeup waterto flow into the system, carrying acontinual influx of oxygen. Aircan be sucked in through air ventsif the circulator drops thesystem’s water pressure belowatmospheric pressure. Finally,plastic pipe without an oxygenbarrier can allow small amountsof oxygen into the system.

Wet-base boiler section

Another significant energy improvement tocast-iron boilers is the wet-base boiler.These boilers have cast-iron sections thatwrap around the sides and bottom of thecombustion chamber.

17


2.3.4 Low-mass andcondensing boilersCopper water-tube boilers arepopular for central heating, poolheating, hot tubs, and domestichot water. The copper iscorrosion-resistant, but evencopper boilers can corrode undera steady diet of oxygen. Copperwater-tube boilers and other low-mass boilers can be quicklydestroyed by thermal shock ifwater stops circulating while theburner is firing. The hydronicsystem’s design must assurecontinuous circulation duringfiring and emergency burnershutoff.

The water vapor, formed by thecombustion process, representsabout 12 percent of the burner’sheat. Several manufacturers makeboilers that condense water out ofthe flue gases, reclaiming thisheat. The condensing boiler’scorrosion-resistant heatexchanger needs cool return waterto actually condense the heat-laden water out of the flue gases.If the return water is above 130°F (54° C), there will be littlecondensation and the boiler’sefficiency will be about the sameas a mid-efficiency boiler.Condensing boilers are oftenpaired with radiant floors because

Providing combustion air to boilers in confined mechanical rooms

Linking the confined space to agarage or the home’s interior throughhigh and low vents increases theavailability of combustion air. Thisapproach fulfills code requirements.

Providing the confined mechanicalroom with fan-powered combustionair is superior to passive ventsbecause the fan can overpowermost wind effects.

Sealed combustion providesthe best safety and reliabilityof any combustion-airmethod.

18

Selecting Heat Emitters

radiant floors return low-temperature water. The lower thereturn-water temperature, thehigher the condensing boiler’sefficiency.

Manufacturers onceunderestimated the acidiccondensate’s corrosiveness andfire’s ability to burn lightweightmetal, and consequently many ofthe early condensing boilersfailed. New condensing boilers,using stainless steel orelectroplated coatings, are morereliable. Oil-fired condensingboilers aren’t common becausecondensing flue gases doesn’traise their efficiency nearly asmuch as boilers designed fornatural gas and propane. Oildoesn’t have as much hydrogenas natural gas so there isn’t asmuch water in the oil boiler’s fluegases. Oil boilers’ flue gasescontain sulfur, as well, creatingvery corrosive sulfuric acidcondensate.

Low-mass direct-vent boiler

Low-mass direct-vent boilers are usually variations of thewater-tube boiler.

copper water-filled heatexchanger

combustion-airinlet

burner

draft fan vent

combustion air

exterior wall

19


3.0 Selecting heatemittersThe heat emitters discussed heregive off heat by convection andradiation—radiation providinggenerally better comfort. Eachheat emitter has a Btuh (kW)rating depending on its length orsize, the flow rate through it, andthe supply-water temperature.Baseboard convectors are themost common type of heat emitterused in North America. Panelradiators are the most commontype in Europe.

3.1 BaseboardconvectorsResidential baseboard convectors,for years the mainstay of theindustry, are usually 6 to 8 inches(15 to 20 centimeters) high. Ofthe heat emitters discussed here,baseboard convectors are the leastexpensive and most commonlyused. Their element is a 1/2-inch(1.3-cm) or 3/4-inch (1.9-cm)copper tube fitted withrectangular aluminum fins.Baseboard convectors come insections 2 to 10 feet (0.61 to 3.0meters) long. The heatingcapacity of baseboard convectorsgenerally ranges from 300 to 700Btuh per linear foot (3.4 to 8.0

kW per meter), depending on thedesign, circulating watertemperature, and water flow rate.

A minimum of 6 inches (15 cm)of clearance should be maintainedin front of baseboard convectorsto allow for air circulation.Baseboard convectors shouldnever be installed where they willinterfere with a swinging door.

Baseboard convection isadvantageous because moving airreduces condensation on glass andcounteracts cold downdrafts fromwindows. Rising air from theconvectors blankets exteriorwalls, raising their radianttemperature. However, baseboardconvectors may createtemperature-induced airstagnation when they circulatehigh-temperature water. And, theymay not provide acceptablecomfort when used as the onlyheat emitter for rooms with highceilings. Baseboard convectorsare susceptible to mechanicaldamage and their steel housingsrust in damp environments, suchas bathrooms.

Baseboard convectors can benoisy, especially when the systemis cycling frequently andcirculating high-temperaturewater. This noise usually can be

20


eliminated by purchasing good-quality convectors and limitingthe water temperature with aneffective control system.

Baseboard convectors vary inquality, cost, and design. Onestandard design includes a returnpipe that doubles back inside theenclosure, allowing bothconnections to be made on oneend. Another design has two tiersof finned tube, providing aboutdouble the heat output of the one-tiered design or the same heatoutput with half the length.

3.2 Fan coilsFan coils can provide equivalentheating capacity to other heatemitters, while circulating coolerwater and using less wall space.They can be used with lowertemperature heat sources, such ascondensing boilers and hydronicheat pumps. Since fan coils usecopper pipe that doesn’t readilyoxidize, they can be used withboth open and closed systems.

Fan coils require an electricalconnection to power their fans.Dust accumulation on coils cansignificantly hinder fan-coilperformance, particularly whenthe fan coil operates at a lowtemperature. Some units areequipped with filters that must be

changed or cleaned regularly.Others need coil cleaning on aregular basis.

Kick-space heaters are specialtyfan coils that fit under cabinets orinto stair risers. Room air issucked through the top of thekick-space heater’s grill andchanges direction in the fan. Theair is then heated by the coil andpushed out the bottom of the grill.These heaters are especially proneto dust accumulation and must beinstalled with adequate access forcleaning.

Larger fan coils, called airhandlers, connect to supply andreturn ducts like a forced-airfurnace. These hydronic airhandlers may have an A-coil forcooling or a heat-recovery

Hydronic baseboard convectors

Baseboard convectors are the traditional hydronic heat emitter.They are inexpensive and readily available. Avoid using them inbathrooms, kitchens, and other high-traffic or moisture-proneareas.

21


Kick-space heater

Kickspace heaters are small fan coils designed for kitchens andlarge bathrooms. They preserve space and are installed under acabinet, out of harm’s way.

blower

coilgrill

ventilator for whole-houseventilation.

Any fan coil equipped with adrip pan and drain tube can beused for summer cooling, whenprovided cold water from ahydronic heat pump or chiller.Combined with radiant floors, fancoils are being used to coolmultifamily buildings,commercial buildings, and homes.

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3.3 Panel radiatorsPanel radiators have been usedfor years in Europe but have onlyrecently become popular inAmerica. Panel radiators aremade from steel or aluminumpanels welded or pressed togetherinto flat sections. Radiators arevulnerable to corrosion andmineral build-up and so waterquality is very important whenusing open hydronic systems.

Panel radiators are eitherdesigned primarily for radiationor for a combination of radiationand convection. The latter canprovide a greater heat output thanthe former, given approximatelyequal space. The difference indesign between radiators andradiator/convectors is the additionof steel fins mounted vertically tothe back of the panel or betweentwo panels.

Panel radiators come in a widevariety of sizes and shapes,making them the most versatiletype of heat emitter andespecially well-suited for retrofit.

3.4 Radiant floorsRadiant floors give hydronicheating the ability to produceboth the best comfort and thehighest energy efficiency

Hydronic air handler

A hydronic air handler is the missing linkto providing a highly effective heating,cooling, and ventilation system. Heat thehome with radiant slabs. Then providecooling and ventilation air through the airhandler, delivered from ceiling registersthat are very effective for cool-air delivery.

A-coil forcooling

blower

hydronic coilfor heating

circulator

available from any central heatingsystem. However, radiant floorheating is also the mostdemanding distribution system interms of design, planning, andinstallation. Radiant floors offerthe following advantages:

• The unbeatable comfortof low-temperatureradiant heat;

23


• The efficiencyadvantage of low watertemperature, 90° F (32°C) to 130° F (54° C)for radiant floorsversus 130° F (54° C)to 180° F (93° C) forradiators andconvectors; and

• The efficiencyadvantage of beingable to use acondensing boiler orhydronic heat pump.

To achieve all three of theseadvantages requires an energy-efficient building shell and awell-designed and well-installedhydronic heating system.

A radiant floor can be hydronicpiping buried in an insulatedconcrete slab or a layered woodfloor that incorporates hydronictubing. A concrete slab givessuperior heat transfer and is themost economical type of radiantfloor. Masonry tile is an idealfinish flooring material toenhance the concrete’s heat-transfer capability. However, bareconcrete or concrete with tile areconsidered too hard andunyielding by many home buyers.

Thin masonry slabs incorporatethe conductivity of masonry witha more conventional wood floor.

Panel radiators

Panel radiators are available in a varietyof styles, including models that warmtowels as an auxiliary feature.

This European-style panel radiatorheats partly by radiation and partly byconvection.

manual control valve

Runtal R

adiators

24


Thin slabs are poured gypsum orconcrete, installed over a woodsubfloor. Gypsum slabs areinstalled by pumping the liquidgypsum mix through a hose ontothe floor. Concrete is poured inthe traditional way out of a truckfrom a batch plant. Pouring thinslabs carries a lot of water intothe home. The slab and thesurrounding building materialsmust have adequate time to drybefore final finishes are appliedand the home is occupied.

Either thin-slab system must beplanned into the home design atan early stage so that preparationsfor cabinets, plumbing fixtures,door thresholds, and stairheadroom can allow for the extrathickness of the slab. Waitinguntil construction has started tochoose a thin slab can cause somemajor headaches, especially forthe carpenters.

If the thin slab will have a woodfinish floor, wood nailers must beinstalled on the wood subfloorbefore the thin slab is poured. Thetop side of the wood nailers islevel with the slab and serves as afastening surface for the woodflooring or underlayment forcarpet.

Radiant floors require differentwater temperatures, depending on

A workman first pours and then levels a thin gypsum slab. Thingypsum or concrete slabs offer the best heat-transfercharacteristics of any wood-supported floor.

Incorporating radiant heat into an insulated concrete slabprovides the best comfort and energy efficiency of any type ofradiant floor.

chute fromconcretetruck

insulationinstalled atedge andunderneathgravel

slurry pumpedthrough hose isself-leveling

workman flattenssurface ofhardeninggypsum with asqueegee

Radiant masonry slabs

Radiant E

ngineering

Radiant E

ngineering

Radiant E

ngineering

25

Designing Distribution and Controls

how effectively they transfer heatfrom the pipes to the floor’ssurface. For example, a concreteslab may only need 85° F (29° C)water, while a wood floor withcarpet may need 130° F (54° C)or more. Striping or concentratedheat lines directly above thetubing has been a challengingdesign problem for radiant woodfloors. Manufacturers havedeveloped heat-transfer plates thatmove heat laterally away from thetubing to make the floortemperature more uniform. Thetubing is pressed into a snugcircular channel in the plate, andthe plate and tubing are attachedeither to the top or bottom of thefloor.

High water temperatures,needed by some radiant floors tomeet comfort requirements, candamage some finish flooringmaterials, especially wood.Highly insulative carpet andcarpet pad usually require highradiant-floor temperatures. Mostexperts now agree that staplingpipe directly to the bottom of awood floor is poor practice. Thetubing often sags away from thefloor in between staples, leavingcontact between the floor and thetubing insufficient for good heat

Radiant floor options for wood floors

A gypsum or concrete thin slab with a tile-floor finish offers thebest heat transfer of any type of wood-supported floor.

Next best for heat transfer is a thin slab containing sleepers tofacilitate fastening of the wood floor finish.

thin slab wood floor

wood underlayment

tile floor thin slab

wood underlayment

floor joist

PEX tubing

carpet and pad

tube and transfer plate

wood composite sheeting

insulation

wood sleeper

A variety of means exist for using a metal heat-transfer plateand layers of wood sheeting to produce an acceptablyconductive radiant floor. The addition of carpet and pad makethe floor less effective as a radiator, requiring higher floortemperatures.

26


transfer and necessitating highwater temperatures.

Whether the floor is concretepoured on gravel, a thin slab, orwood floor with aluminum heat-transfer fins, installing insulationunder the floor is highlyrecommended by the experts.Foam insulation is typicallyinstalled beneath concrete slabsand vertically at their edges.Fiberglass insulation is commonlyinstalled between wood floorjoists to reduce downward heatflow in floors with thin slabs andaluminum heat-transfer fins.

Radiant-heating designers andcontractors estimate a radiantfloor’s heating capacity in Btuhfor each square foot of floor area(watts per square meter [w/m2]).Concrete or gypsum slabs, bare orwith masonry tile, can radiate asmuch as 35 Btuh per square foot(110 w/m2) and wood floors withcarpet radiate as little as 8 Btuhper square foot (25 w/m2). Thelower heat capacity of the woodradiant floors reflects the slowerheat transfer through the lessconductive materials like woodand carpet.

A very energy-efficient homemay only need 8 Btuh per squarefoot (25 w/m2). Nevertheless,

Heat-transfer plates, attached to the underside of the floor,spread heat from the tubing to the wood flooring materials.Insulation is installed underneath to reduce downward heat flow.

Heat-transfer plates and tubing underneathsubfloor

Heat-transfer plates and tubing mounted ontop of subfloor

Heat-transfer plates, attached to the subfloor, spread heat fromthe tubing to the wood flooring materials. Plywood spacers areinstalled next, then wood underlayment or wood finish flooring.

aluminumheat-transferplates

PEX tube

Radiant E

ngineering

Radiant E

ngineering

27


Piping radiant floors

A 4-way mixing valve blends high-temperature water from aconventional boiler with return water from the zones, giving theradiant floors the cooler water they require, while returningwater to the boiler at a high enough temperature to preventcondensation.

A low-mass boiler or hydronic heat pump is connected to aninsulated buffer tank, giving the heat source a steadier load. Thisreduces cycling and protects a low-mass boiler from thermalshock.

4-way mixingvalve

boilerloop

radiant floormanifold

insulated buffer tankheat pump or low-mass boiler


domestichot waterbaseboard

convectors

closely spaced tees

check valveprimary loop

secondary loops

A hot primary loop supplies a variety of water temperatures to itssecondary loops through closely spaced tees and zone circulators.


mixingvalve forinjectionmixing

rooms with carpeted floors, largewindows, and high ceilings maystill need baseboard convectors orradiators, in addition to theirradiant floor, to respond quicklyto changing outdoor temperature.

The efficiency of any hydronicsystem is directly related to theelevation of temperature—coolerwater means higher efficiency.Condensing boilers and hydronicheat pumps are ideal heat sourcesfor radiant floors because theyprocess cooler water without theproblems of non-condensingboilers. Non-condensing boilersmust receive return water warmerthan 130° F (54° C) to avoidcorrosive condensation. Thisreturn-water temperature is higherthan the supply-water temperaturecirculating in most radiant slabs.Using non-condensing boilerswith radiant slabs requires mixingvalves or primary-secondarypiping to maintain this minimum130° F (54° C) return-watertemperature. These piping andcontrol strategies will bediscussed in sections 4.1.3 Valvesand 4.2 Control systems.

28


4.0 Designing thedistribution systemand control systemThe distribution system’s purposeis to carry water between theboiler and the heat emitters. Acontrol system’s purpose is toregulate heat flow to match heatloss by controlling water flow andtemperature. The more closely theload (home) can be matched tothe source (boiler or heat pump)the greater potential for optimalcomfort and efficiency. Hydronicdistribution and control systemshave evolved dramatically in thepast 10 years in their ability todeliver comfort and efficiency.

Heating systems cycle on andoff during the heating seasonbecause they are sized to meet themaximum load during very coldweather. This cycling isaccomplished by the thermostatin the simplest systems. Anaquastat is a simple on-off watertemperature control. Since theideal water temperature changeswith outdoor temperature, resetcontrols were developed tochange water temperature inresponse to outdoor-temperaturechanges.

The system may be zoned tooffer different temperaturesetpoints and schedules todifferent parts of the home. Thetraditional way to zone a hydronicsystem is with zone valvescontrolled by thermostats. Thethermostat is located in arepresentative area of the zone.When the thermostat calls forheat, the zone valve opens the

Automatic radiator control valves

Automatic radiator control valves meter flow to heat emittersbased on temperature sensors in these valves. The aboveschematic shows a reverse-return piping system, which givesheat emitters piping loops of approximately equal lengths.

Radiator valve mounted onpanel radiator.

Note: Valve should never be mounteddirectly above radiator or convector.

fan coils

baseboard convectors

panel radiators

boiler

Reverse return pipinggives the heat emitterwith the shortestsupply pipe thelongest return.

radiator controlvalves

29


piping circuit between the heatsource and the heat emitters inthat zone. Zone valves haveswitches inside them that activatecirculators and boilers after thevalves open.

It’s now common to use aseparate circulator for each zone,instead of zone valves with asingle circulator, because ofcirculator improvements andreduced prices. The mostcommon piping arrangement forusing separate circulators isprimary-secondary piping. Thispiping system employs a primaryor boiler loop, and branching off,a number of secondary loopscontaining the home’s heatingzones. The boiler loop has its owncirculator, but it doesn’t providewater to any heat emittersdirectly. Each secondary circuithas a circulator that moves warmwater to and from the heatemitters.

One of the most importantissues to consider in designing thedistribution and control system iswhether your system will uselow-temperature water. Low-temperature water is used withradiant floors and fan coils. If ahome can accommodate fan coilsand radiant floors, then it makessense to consider heating the

water with a hydronic heat pumpor a condensing boiler. If a non-condensing boiler is chosen, as itoften is, then the boiler must beprotected from cool return wateras described in sections 2.3.3Cast-iron and steel boilers and3.4 Radiant floors.

4.1 Hydronicdistribution systemsThere are three common types ofdistribution circuitry: series,parallel, and primary-secondary.Series (all heat emitters in thesame loop) is very simple andeconomical, but the watertemperature is lower at the inletof every successive heat emitter,and this must be factored intodesign. Parallel is a very commonpiping method featuring theability for zoning. Reverse-returnparallel seeks to equalize headloss of parallel circuits toaccommodate the one or twocirculators moving the water.

The primary or boiler circuitreaches out from the boiler andprovides shorter, more localizedconnections between the boilerand secondary circuits, instead oflooping each zone to and from theboiler itself. The boiler-circuitcontrols ensure that the boilerwater temperature is warm

30


enough to prevent condensationin conventional boilers. However,the most important benefit ofprimary-secondary piping is itsversatility in providing differentwater temperatures and flow ratesto the different types of heatemitters in the zones. Forexample, primary-secondarypumping can provide a particularhome with the following differentwater temperatures and flowrates:

• Bathroom radiant panel,140° F (60° C) water at1.5 gpm (0.09 L/s);

• Radiant slab in greatroom, 100° F (38° C)water at 2 gpm (0.13 L/s);

• Baseboard convectors ingreat room, 180° F(82° C) water at 1.5gpm (0.09 L/s);

• Fan coil in garage, 130°F (82° C) water at 2.5gpm (0.16 L/s); and

• Radiant masonry slabwith carpet inbedrooms, 110° F (82°C) water at 2 gpm(0.13 L/s).

Each secondary piping circuithas a supply and return pipeconnected to the boiler circuit bytwo tees. The tees are spaced asclosely as possible to minimizepressure differences leading toinadvertent flow between the

primary and the secondarycircuits.

4.1.1 Piping choicesPipe distributors offer severalchoices for piping hydronicheating and cooling systems. Thefour most common types arecopper, polyethylene,polybutylene, and rubber tubing.Steel piping was commonly useduntil the 1960s, when copperbecame dominant. Now plasticpiping has gained market sharefrom copper because of itsreasonable cost and easyinstallation.

Copper piping has been widelyused for hydronic systems formany years because of its manyvirtues, including corrosionresistance, mechanical strength,low flow resistance, and thermalconductivity. Copper pipe andfittings are joined by the well-known technique of softsoldering. Because of its highthermal conductivity, copper isstill the first choice for heatexchangers and coils. Althoughcopper is still used fordistribution piping, itsconductivity can be adisadvantage because of heat lossand water-temperature reductionin uninsulated copper tubing.

31


diffuse through them, creating thepossibility of corrosion for ironand steel system components.Consequently, most of the plastictubing now sold for hydronicsystems has an oxygen-diffusionbarrier. This barrier is most oftena plastic film applied to the outersurface of the pipe or analuminum foil tube sandwichedbetween two plastic-tubinglayers.

The most popular and time-tested type of plastic piping iscalled PEX tubing. PEX is cross-linked polyethylene, composed ofvery large molecules arranged ina three-dimensional matrix. PEXjoints, like tees, are made withgrooved plastic or metal fittingsand compression rings. PEXattaches to other systemcomponents, like manifolds andheat emitters, by way of fittingsthat adapt it to standard pipethreads. PEX is kink resistant andhas the unique ability amongplastic pipe to recover fromkinking, after being heated to275° F (135° C).

The sandwiched PEX-aluminum-PEX tubing known asPEX/AL/PEX is easy to installbecause it retains its shape whenbent into loops in radiant floors.The aluminum oxygen-diffusion

Joining PEX tubing

tubing

crimp rings

crimpingpliers

control valve

compression nut

compression ring

radiator

tubing

Joints in PEX tubing are made by ribbed fittings with crimprings, or with specially made compression fittings that join PEXto pipe threads.

Ribbed Fitting

Compression Fitting

Plastic tubing is less conductivethan copper, making it a betterchoice for piping distributionsystems. Plastic tubing is ideal forradiant floors and ceilingsbecause joints aren’t necessary,since plastic tubing is sold in coils250 feet (76 meters) or longer.Plastic pipe has gained marketshare in recent years and mayreplace copper as the commodityof choice for piping hydronicdistribution systems.

Plastic pipe and rubber tubingallow small amounts of oxygen to

32


barrier, located within this tubing,is less likely to be damaged thanbarriers attached to the tubing’souter surface.

Polybutylene tubing issometimes used in hydronicsystems, although its maximumpressure and temperature ratingsare less than PEX. Rubber-basedtubing has been used extensivelyfor attachment to the underside ofwood floors, because of itsflexibility for being threaded intospaces between floor joists.Because of its tendency to sag,rubber-based tubing must besupported every few inches.Rubber-based tubing doesn’t havethe long-proven reliability ofPEX and some failures haveoccurred.

4.1.2 CirculatorsA hydronic circulator or pumpconsists of an electric motorcoupled to an impeller by a shaft,supported by bearings. Theimpeller turns in a chamber,called the volute, where it addsmechanical energy to the water.Head is the term used to describethe mechanical energy in footpounds per pound of water orjoules per kilogram (ft-lb/lb orfeet of head or J/kg). The pipingsystem removes this head through

the hydraulic resistance of itscomponents, like valves, fittings,and heat emitters. The circulator’sability to add mechanical energyshould be matched to the head-removing characteristics of thepiping system.

There are two main types ofcirculators available for hydronicsystems: the wet-rotor circulatorand the mechanically coupledthree-piece circulator. Both typesof circulators are available witheither cast-iron volutes for use inclosed-loop hydronic systems, orwith stainless-steel or bronzevolutes for open-loop hydronicsystems.

Most hydronic circulators arein-line circulators, designed to beinstalled in a straight piece ofpipe. However, end-suctioncirculators that turn the waterflow 90° from intake to dischargeare also available. Thecirculator’s weight generallyshould be supported by vibration-absorbing hangers.

Wet-rotor circulators aresmaller, quieter, and work wellfor piping circuits with low flowand head requirements. Wet-rotorcirculators are cooled andlubricated by the system’s waterand have no external pump sealsto leak. Newer types of wet-rotor

33


some newer circulators haveinternal controls to exercise themin the off-season. Opening a wet-rotor circulator to service themotor, shaft, or impeller involveslosing some water andintroducing some air to the pipingsystem.

Three-piece circulators aregenerally more powerful thanwet-rotor circulators and canovercome more pressure loss athigher flow rates. Three-piececirculators have a longer servicelife and their motors, bearings,and shafts may be servicedwithout opening the pipingsystem to the atmosphere.However, they are noisier andheavier than wet-rotor circulatorsand require periodic oiling. Wornseals can leak water and allow airinto the piping system.

The circulator must be designedand installed to keep the pressurein the piping above the vaporpressure of the water in thesystem. Vapor pressure is thepressure required to preventliquid from boiling at a specifictemperature. If the water beginsboiling at the suction side of thecirculator (the system’s low-pressure point), it can wear outthe circulator quickly through aprocess called cavitation.

motor

inlet

eye of theimpeller

impeller vane

outlet

Wet-rotor circulator

Wet-rotor circulators work well as zone circulators. Somecontain speed controls and electronic chips for controlling flowrate according to demand.

circulators have variable-speeddrives and offer improvedpumping efficiency and roomtemperature control when used toinject heat from the boiler loopinto a zone circuit.

Wet-rotor circulators are proneto having stuck impellers after anextended shutdown. Externalcontrols can be programmed torun the circulators for shortperiods during shutdown and

volute

34


Locating the pressure tank nearthe circulator’s inlet and using thelowest water temperature possibleare other effective ways toprevent cavitation, a notoriouspump killer.

The circulator’s performance isdescribed by the same type ofgraph as the head loss of a pipingcircuit. This makes sense becausethe circulator adds head and thecircuit subtracts head, dependingon the system’s flow rate ingallons per minute. Comparingthe curve representing thecircuit’s head loss with the curveof the circulator’s head gain onthe same graph allows thedesigner to determine the actualoperating head loss and flow rate.The location of the intersection ofthe two curves determines thisoperating point and also gives thedesigner an indication ofefficiency. The peak efficiencyoccurs when the head-loss curveintersects the pump curve near thepump curve’s “knee,” ormidpoint.

4.1.3 ValvesHydronic valves range fromsimple plumbing devices tosophisticated servants of thecontrol system. Simple controlvalves serve the hydronic systemby controlling flow or by

Pump curve and system curves

The pump curve describes the circulator’s capacity to addmechanical energy or head. The system curves shown heredefine a region where the system curve should ideally intersectthe pump curve to achieve maximum efficiency.

isolating parts of the system sothat they can be serviced withoutshutting down the entire system.Globe valves are used whenever athrottling effect is needed in thesystem. Gate valves are used toisolate parts of the system andpose less hydraulic resistance thanglobe valves. Ball valves may beused for either isolation orthrottling.

A check valve limits flow to onedirection to preventthermosiphoning while a circuit isoff. Thermosiphoning is heatedwater flowing upward through thepiping and cooler water flowingdownward because of the densitydifference between the warm and

Flow rate in gallons per minute

Hea

d in

feet

(ft-

lbs/

lb) Pump curve

knee: zoneof highestefficiency

system curves

35


cool water. Check valves are alsoused to prevent operating pumpsfrom inducing flow in inactivepiping circuits. This isparticularly important in multi-circulator systems.

Radiator valves regulateindividual heat emitters. They areavailable as manual valves or asthermostatic radiator valves(TRVs). Some heat emitters areequipped with manual radiatorvalves at the factory. Thethermostats on TRVs may beintegral or remote for attachmentto a nearby wall. It’s veryimportant to avoid locating thethermostatic element of a TRV

directly above the heat emitterbecause this location will causethe valve to close very soon afterit opens.

Zone valves divide the hydronicsystem into zones where thecontrol system and the zonevalves maintain differenttemperatures according tooccupancy and need. Commonzone valves are opened andclosed by 24-volt control circuitsby way of an electric motor or aslower-acting heat motor. Bothtypes are effective and have goodservice records. Zone valves haveswitches inside them to energize arelay for controlling thecirculator. When a hydronicsystem has three or more zonevalves and a single powerfulcirculator, a differential-pressurebypass valve is used to preventnoise from high flow rates in asingle active zone. The circulator,serving a single zone calling forheat, could also create highpressure against closed zonevalves, causing them to fail. Thedifferential-pressure bypass valveallows part of the flow to bypassthe active zone’s piping.Manifolds, centralized pipingstations designed for three ormore zones, may be equippedwith zone valves, called telestats,

Zone valves

Zone valves, powered by a 24-volt controlcircuit, provide on-off flow control using asingle circulator.

36


for each zone. Zone valves mayalso connect a continuouslycirculating zone to the boiler loophot-water injection.

Mixing valves control supplyand return water temperatures inresponse to signals from thecontrol system. The two types ofmixing valves used with boilersto maintain an acceptable return-water temperature arethermostatic mixing valves andfour-way mixing valves. Thesemixing valves blend hot supplywater with cooler return water tosupply a low-temperature radiantfloor, and at the same time returnblended water back to the boilerat a temperature above 130° F(54° C) to prevent condensationin the boiler. Mixing valves arealso used to inject heat from theprimary loop into a secondaryloop in response to the controlsystem. Mixing valves can alsoreturn cool water to a condensingboiler while supplying heatingzones with a variety of watertemperatures.

4.1.4 Expansion tanksThe hydronic system’s waterexpands as it’s heated andcontracts as it’s cooled. Sincewater is incompressible, thesystem needs a device that canabsorb the water’s expansion and

contraction. A diaphragm-typeexpansion tank serves thispurpose in a modern hydronicsystem. This tank is divided intotwo chambers: one charged withair at a pressure of about 5 poundsper square inch (34 kilopascals)below the relief-valve setting, andone filled with the system’s water.As water is heated, it expands intothe tank, compressing the air onthe other side of a rubberdiaphragm that divides the tank.As the system’s water cools,water flows out of the expansiontank and returns to the system.

Older expansion tanks didn’thave a rubber diaphragm betweenthe air and water. This allowed the

0

5

10

0

5

10

0

5

10Four-way mixing valve

Valve partially open: Mixedwater supplied to thesecondary loop with mixedwater returning back to theboiler.

Valve closed:Secondary looprecirculates itswater.

Valve open: Boiler water goescompletely to secondary loop.Return comes unmixed back toboiler.

boilerloop

boilerloop

boilerloop

secondaryreturn

secondarysupply

boiler return

37


air to mix with the water,resulting in the water eventuallydissolving all the tank’s air andcarrying it around the systemuntil it was ejected through airvents. This process created anexpansion tank that was filledwith water and no longer fulfilledits purpose. This waterloggedexpansion tank forced water outthe relief valve, in turn bringingmany gallons of makeup waterinto the system. This waterbrought more dissolved air, whichproduced more rust. Many boilersdied this way. Replacing an olderexpansion tank with a diaphragm-type expansion tank is a goodway to save an existing boiler.

Expansion tanks need to besized to meet the expansion needsof each individual system, whichrange from 3 to 10 gallons (11 to38 liters).

4.1.5 Air eliminatorsAir causes a host of problems inhydronic systems, including:reduced heat transfer in the boilerand heat emitters because oftrapped air bubbles, corrosion ofiron and steel, reduced flow rates,and pump cavitation. To preventthese problems hydronic systemsuse both manual and automaticair vents.

Manual air vents are installed atthe top of heat emitters and atmanifolds. Manual air vents alsomay be necessary where pipingturns downward after an upwardor horizontal run. Most manualvents open by screw or push-button. The installer’s skill atpiping and filling the system hasa large effect on how much airremains in the system initially.

Automatic air vents arestrategically located near theconnection to the expansion tankto purge air from the pipingsystem. The simplest type ofautomatic air vents use a float toopen the vent’s orifice and expelthe air. The more sophisticatedtypes are designed to catch small

Expansion tank / air vent

air vent

air eliminator

diaphragm-type expansiontank

The expansion tank, air eliminator, andair vent are often connected together andlocated near the circulator’s inlet.

38


bubbles and combine them intolarger bubbles before expellingthem. The very smallest bubblesare the most difficult for the ventto catch because they are carriedby the moving water and don’trise quickly like larger bubbles.Newer air vents calledmicrobubble resorbers aredesigned to pluck these tinybubbles out of the water stream.

4.1.6 ManifoldsMany modern hydronic heatingsystems, especially radiant panelsystems, use manifold stations,consisting of both a supply and areturn manifold. Manifoldstations are installed inside a wallor floor cavity in central locationsfitted with access doors. A supplypipe from the boiler feeds thesupply manifold and a return pipeconveys water from the radiantfloor sections or other heatemitters back to the boiler. Eachpiping circuit is joined at one endto the supply manifold and at theother to the return manifold,connecting these piping circuitsin parallel to each other.

Manifolds are available with avariety of optional features.Manifolds are often fitted withzone valves, called telestats, thatare controlled by room

thermostats. Some manifoldstations are equipped withpressure gauges, flow meters, andbalancing valves that can adjustthe flow rate in each circuitseparately. Balancing valves arehandy for fine-tuning heatdelivery to the home’s zones.

4.2 Control systemsHydronic control systems performseveral important functions:

Hydronic supply and return manifolds

air vent manual flowcontrols

telestat valves

thermometer

flow meters

PEX loopsupply ends

main supply

main return

thermometer

PEX loopreturn ends

Manifolds for radiant floors: one end of a loop of PEX tubing isattached to the supply manifold while the other is attached to thereturn manifold. Manifolds are often located remote from theboiler.

39


• Minimize boiler or heat-pump short cycling,

• Optimize system watertemperatures forcomfort and efficiency,and

• Protect boilers fromcorrosion and thermalshock.

A hydronic control systemshould be easy to use, easy todiagnose, and reliable. The newercontrol systems provideautomatic diagnostics, visualreadouts of system functioning,and the virtual elimination ofmanual controls.

4.2.1 Aquastats and resetcontrolsAn aquastat controls thetemperature setpoint of the boilerwater. The aquastat establishes ahigh limit temperature beyondwhich the aquastat shuts theburner off. This high limit isrequired for safety and to preventoverheating. Aquastats may alsoestablish a low limit to ensurethat the boiler is capable ofheating domestic water ondemand. A low limit is necessaryfor boilers that have a tanklesscoil, a pipe coil inserted into theboiler.

Many conventional aquastatssense heat through their housings,strapped directly to a supply pipe

near the boiler. Other aquastatssense temperature with a remotebulb, connected to the housing.The remote bulb is filled with afluid that responds to changingtemperature, operating a bellows,which in turn operates a switch.The remote bulb is either strappedto a supply pipe near the boiler orinserted in a well inside of a teein the pipe.

A more advanced controlstrategy, called reset, adjusts theaquastat’s setpoint according tooutdoor temperature. The resetcontrol is an electronic aquastatequipped with an outdoortemperature sensor. The resetcontrol adjusts the watertemperature higher for coldweather and lower for warmweather. The heat emitters mayneed 180° F (82° C) water to heatthe home when the temperatureoutdoors is 0° F (–18° C), butonly 130° F (54° C) when thetemperature outdoors is 50° F(10° C). The reset control’spurpose is to control supply waterto the heat emitters so that theyemit heat to the home atapproximately the same rate thatheat is being lost. This resetstrategy minimizes the tendencyof boilers—necessarilyoverpowered for their heating

40


load for most of the year—topulse heat into the building,causing temperature fluctuationsand energy waste.

4.2.2 Electronic controlsElectronic controllers forhydronic systems have improvedrapidly in their controlcapabilities and reliability in thepast several years in response tohydronic’s resurgence. Electroniccontrols can perform morefunctions and can regulatehydronic systems more preciselythan simpler controls. Electronics’greater precision and capabilitieshelp to maximize comfort andminimize energy use.

Electronic controllers combinemeasurement, timing, and otherinputs with outputs throughelectronic logic andelectromagnetic relays to controlcirculators, valves, and boileroperation. Electronic controllersgive digital readouts of systemstatus and give warnings ofsensor failure and other systemfaults. Electronic controllers canreset boiler water temperature,prevent warm-weather boileroperation, control the position ofmixing valves, activate multipleboilers in stages, control thespeed of injection pumps, and

activate and deactivate loads inorder of priority.

Modern electronic controllersmeasure temperature, processinformation, and switch power tothe boiler, circulators, zonevalves, and mixing valves.Thermistors are electronic sensors

Simple reset control

Multiple-function electronic controller

reset controller

aquastat

outdoorsensor

electronic controller

zone circulators

boiler water sensor

The circulator operates continuously, circulating water at atemperature determined by the reset controller according tooutdoor temperature.

Electronic controllers combine a number of functions, includingcontrolling zone circulators, water-temperature reset, warm-weather shutoff, and boiler-cycle control.

41


used to measure indoortemperature, outdoor temperature,and water temperature. Usingthermistors allows the electroniccontroller to regulate boiler andsystem operation intelligently,based on a number of space andwater temperatures.

Zones may be operated by zonevalves and a central circulator, oreach zone may have its own on-off circulator, or a zone maycirculate constantly with heatinjected as needed by a pump orvalve. Domestic water is oftenjust another zone, heated by thecentral boiler and equipped withits own circulator. A multi-zonerelay center can control severalzones along with domestic-hot-water circuit, giving this circuitpriority whenever there’s a callfor hot water. The electroniccontroller can coordinate zoneoperation to allow a relativelysmall boiler to heat a relativelylarge load. Electronic controllersoften provide a post-purge controlthat leaves a circulator on afterthe heat source turns off toconvey the residual heat intospace- or water-heating zones.And, electronic controllers cananticipate large loads and startboilers early to fill the demand.

The multi-zone relay centeruses a familiar strategy ofcombining a number of controlcomponents into one box, wherewiring connections can be easilyand conveniently made.Centralizing zone controls on ornear piping manifolds helps tocentralize important piping andwiring components, making thesystem simpler, more space-efficient, and easier totroubleshoot.

4.2.3 Modular boiler systemsLarge homes with hot tubs,swimming pools, and multi-headed shower spas may need upto 500,000 Btuh (146 kW) duringmaximum demand. Minimumdemand may be only one-tenth orless of maximum. When thehome’s combined space- andwater-heating load exceeds150,000 Btuh (44 kW), designersoften consider specifying amodular boiler system. Servingwidely variable loads with asingle large boiler can be bothinefficient and difficult toregulate. The greater the variationin load, the more energy amodular boiler system will save.

Much of the energy needed toheat up the boiler is lost after theburner turns off with each cycle.So, the less a boiler cycles, the

42


greater its efficiency. Using twoor more boilers offers a greaterseasonal efficiency. The modularboiler system can also eliminatethe need for separate boilers fordomestic hot water, spaceheating, and pool heating andthereby eliminate their separateelectrical service, controls, fuelsupplies, and flues.

Modular boiler systems employspecially designed controlsystems. These controls performthe following tasks:

• Operate the correctnumber of boilers tomatch the current load,

• Change the order inwhich the boilers arefired to give eachboiler approximatelyequal operating time,

• Control watertemperature based onload and outdoortemperature,

• Interrupt space heatingin times of highdemand for domestichot water, and

• Provide safety shutoff.The design of the piping,

circulators, and valves mustprevent water heated in activeboilers from circulating throughinactive ones, which wastes heat.Preventing circulation throughidle boilers is accomplished by

installing a small circulator oneach boiler, designed only tocirculate water through that boilerand into the primary piping of aprimary-secondary piping system.

T-stat

Electronic controller for hydronic systems

boiler4-way mixingvalve

outdoorsensor

circulator

transformer

roomthermostat

Electronic controllers for hydronic systems come in a widevariety of models for controlling mixing valves, circulators,water temperatures, space temperatures, and one or moreboilers.

supply sensor

returnsensor

43

Cooling, Domestic Hot Water, and Installation Issues

5.0 Domestic hotwater, cooling, andinstallation issuesWater-heating tanks and coils arepart of many modern hydronicsystems. The most common wayto couple domestic hot water andheating is with an external tankthat has an internal water-heatingcoil. Hydronic cooling is onlybriefly discussed here because itsuse is not common in single-family and small multifamilybuildings.

5.1 Domestic hot waterWater heaters can be classified asdirect and indirect. Direct waterheaters heat their water directlywith a fuel source, like gas, oil, orelectricity. Indirect water heatersuse a central boiler, furnace, orheat pump to heat domestic water.

The two most common types ofindirect water heaters are thetankless coil and the indirectlyfired storage water heater.Tankless coils are found in single-family and multifamily buildingsand often have a separate storagetank near the boiler. A tanklesscoil is a piping coil inserted into areservoir in one part of the boiler.A tankless coil can waste energywhen it forces the maintenance of

Tankless coil

Indirectly fired storage water heater

A tankless coil is a pipingcoil installed inside a cast-iron secton of the boiler.

An indirectly fired storage water heater uses a coil circulatingboiler water to heat domestic hot water.

domestic hot water out

domestic water in

boiler water in

boiler water out

storage tank

thermostat

tankless coil

cast-ironsections

gasket

Radiant Engineering

44


high water temperature in a largeboiler for water heating during thesummer months. For this reason,the tankless-coil design isconsidered obsolete by manyprogressive designers andinstallers. However, well-insulated storage tanks may becombined with existing tanklesscoils to erase much of thisefficiency disadvantage.

An indirectly fired storage waterheater is a storage tank with animmersed heating coil, heated byanother appliance, usually aboiler. It is the most common typeof water heater for newinstallations. Boiler watercirculates through the coil,heating the water inside the tank.

Well-engineered, indirect waterheaters may have significantadvantages over direct waterheaters. First, they eliminate theadditional chimney and chimneylosses of direct units. Indirectwater heaters are safer becausethey don’t burn fuel or needcombustion air. Indirectly firedstorage water heaters may beeither more or less efficient thandirect water heaters.

An indirectly fired storage waterheater can beat a direct waterheater’s efficiency, if this waterheater is well-insulated and

coupled to a boiler that is loadednear its capacity with space-heating and water-heatingresponsibilities. However,indirectly fired storage waterheaters also can be less efficientbecause they require an additionalheat exchanger, with its additionallosses. This depends on theboiler’s efficiency and howclosely the boiler is matched toboth its space- and water-heatingloads.

Domestic hot water is usuallygiven priority by the hydronicsystem’s controls. Space-heatingloads are temporarily interruptedduring the short time periodswhen the water heater issupplying hot water. This controlstrategy can be suspended duringvery cold weather when heatingzones need priority to maintaintheir setpoints. When a home hasa hot tub, multiple-headedshower, or swimming pool, it isessential to plan separately forthese loads, adding boiler capacityas necessary to meet maximumdemand. When the total heatingand water heating load exceeds150,000 Btuh (44 kW), consider amodular boiler system.

45


5.2 Hydronic coolingHydronic heat pumps can be usedfor both hydronic space coolingand heating. The biggestchallenge for hydronics is thatwarm air is best delivered near thefloor, while cool air is bestdelivered near the ceiling.Another concern is condensationon the surfaces of convectors orfan coils. Valence coolingconvectors, designed forinstallation near the ceiling,contain pans for drainingcondensate. These coolingconvectors are very similar tobaseboard convectors, but theydon’t work too well for heating.Radiant ceilings work well forcooling, too, but havingcustomers’ heads closer to aradiant heat source than their feetdoesn’t produce very good wintercomfort. Hydronic radiators andbaseboard convectors aren’tusually considered appropriate forcooling.

Fan coils equipped with drippans work well for both heatingand cooling and are used in manylarge residential and officebuildings. Radiant floors havegood potential for radianthydronic cooling, but they mustbe bare concrete or concrete with

tile. Keeping the chilled watertemperature above the air’s dewpoint is also a key to employingany type of hydronic radiantcooling. If a radiant floor werecarpeted, the cooler watertemperature needed tocompensate for the carpet’sinsulating effect would likelycondense water vapor out of theair, wetting the carpet.

5.3 Putting it alltogetherFor new installations, the houseplans should include one or moresheets portraying the design of thehydronic heating system. Thesedrawings and specificationsshould include:

• A schematic drawingshowing the position ofthe system’scomponents;

• Specifications for theboiler or other heatsource;

• Specifications for thepump(s);

• Specifications for theheat emitters;

• An electric controlschematic;

• Piping diagrams forradiant floors, showingmanifold location,heating pipe, piping

46


lengths for each circuit;and

• Cross sections of theradiant floor types

These documents shouldbecome a permanent record to beused for adjustment, repair,planning additions, and otheractivities that require anunderstanding of the hydronicheating system.

When replacing a hydronicheating system or a boiler,consider the following:

• Compare the warrantiesof competing heatsources,

• Ask about the heatsources’ service andrepair records,

• Confirm that parts andservice will beavailable in the future,

• Insist on proper design,planning, andinstallation,

• Insist that deficiencies ingas service andelectrical supply becorrected as part of theinstallation, and

• Establish a maintenanceschedule for the newfurnace or boiler, andstick to it.

A Tale of Two Systems

Fan coil providesoccasional quick heatto a garage workshop.

Fan coil provides extrakick to heat great roomduring cold weather.

Towel rack radiator heatsbathroom.

Two panel radiatorsheat the kitchenand dining area.

Domestic hot-water circuithas its owncirculator

Zone valves controlledby separatethermostats providevarying heat loads andschedules.

Common-areaconvectors

Bedroomconvectors

Differential bypassvalve protectszone valves andother systemcomponents fromexcessivepressure from thesystem’s largecirculator.

Air handler provides centralcooling, ventilation for airtighthome, and extra heat for coldweather and night-setbackrecovery.

domestic hot water

Baseboard convectors counteract draftsand prevent condensation on windowwalls in living and dining areas.

Three-way valveis controlled byreset controllerbased on outdoortemperature.

Radiant floors inhalls, kitchen,dining, living room,and bedrooms.

Circulator moves water at low temperaturecontinuously through radiant floorsthroughout the heating season.

Conventional boileror insulated buffertank coupled to low-mass boiler orhydronic heat pump.

Indirectly firedstorage waterheater commandspriority except invery cold weather.

A-coil for cooling

hydronic coilfor heating

Primary loop

return

supply

closely spaced teesair handler

Sophisticated hydronic system for a modest home (new or retrofit)

Complex high-performance hydronic system for a new luxury home

48

Glossary

GlossaryAir eliminator – A device that captures air bubbles from water and ejects it fromthe system.Air handler - A steel cabinet containing a blower with cooling and/or heating coilsconnected to ducts, which transports indoor air to and from the air handler.Air vent – A manual or automatic device that releases air from a hydronic pipingsystem.Annual fuel utilization efficiency (AFUE) - A laboratory-derived efficiency forheating appliances which accounts for chimney losses, jacket losses, and cyclinglosses, but not distribution losses or fan/pump energy.Aquastat - A heating control that switches the burner or the circulator in ahydronic heating system.Atmospheric – Used to describe burners that vent gases by the difference inpressure between warm gases and cooler air outside the vent.Blower door - A device that consists of a fan, a removable panel, and gauges usedto measure and locate air leaks.British thermal unit (Btu) - The quantity of heat required to raise the temperatureof one pound of water one degree Fahrenheit.Btuh - British thermal units per hour.Burner - A device that facilitates the burning of a fossil fuel like gas or oil.Cavitation - Damage to a circulator from by boiling water at its inlet caused by toolow pressures.Circulator – The pump in a hydronic system.Check valve – A valve that limits water flow to one direction.Closed-loop system – A piping system that is completely closed to the atmosphere.Coil - A snakelike piece of copper tubing surrounded by rows of aluminum finswhich clamp tightly to the tubing to aid in heat transfer.Combustion air - Air that chemically combines with a fuel during combustion toproduce heat and flue gases, mainly carbon dioxide and water vapor.Combustion efficiency – Efficiency of a combustion boiler, determined bymeasuring temperature and either oxygen or carbon dioxide.Compressor - A motorized pump that compresses the gaseous refrigerant andsends it to the condenser where heat is released.

Glossary

49

Condensate - Vapor condensed back to a liquid.Condense - When a gas turns into a liquid as it cools, we say it condenses.Condensation is the opposite of evaporation. When a gas condenses into a liquid itreleases heat.Condenser – A coil where the refrigerant condenses from a vapor into a liquid,releasing heat.Conductance - The property of a material to conduct some energy form like heator electricity.Conduction - Heat flow from molecule to molecule in a solid substance.Convection - The transfer of heat caused by the movement of a fluid like water orair. When a fluid becomes warmer it becomes lighter and rises.Demand - The peak need for electrical energy. Some utilities levy a monthlycharge for demand.Density - The weight of a material divided by its volume, usually measured inpounds per cubic foot.Distribution system - A system of pipes or ducts used to distribute energy.Efficiency - The ratio of output divided by input.Electro-mechanical controls – Controls that use mechanical movement fromsprings, bi-metal elements, or pressure diaphragms to open and close electricalcontacts.Evaporator - The heat transfer coil of an air conditioner or heat pump that coolsthe surrounding air as the refrigerant inside the coil evaporates and absorbs heat.Fan-assisted combustion – A combustion system that moves air and combustiongases through the heat exchanger and into an atmospheric chimney using a fan.Fan coil – A forced-air convector utilizing a coil with a fan to blow room airthrough it.Feet of head – Measurement of a fluid’s mechanical energy similar to pressure,measured in foot pounds per pound, which simplified becomes feet of head.Finned tube – A pipe, usually copper, with fins, usually aluminum , attached toimprove heat transfer.Flow rate – Volume of fluid flow per unit of time. For example: gallons per minute(gpm).Flue - A channel for combustion gases.

50

Glossary

Head – The mechanical energy of a fluid measured in foot-pounds of energy perpound of fluid or “feet of head” for short.Head loss – The quantity of mechanical energy removed by friction as watermoves through piping, fittings, and heat emitters.Heat emitter – A hydronic device that releases heat by radiation and/orconvection. Includes radiators, convectors, and fan-coils.Heat motor – A slowly rotating motor driven by heat from an electric current.Heat motors power some hydronic zone valves.Heat-transfer plate – An aluminum plate designed to house tubing and to conductthe tubing’s heat laterally through building components.Heating load - The maximum rate of heat conversion needed by a building duringthe very coldest weather.High limit - A bimetal thermostat that turns the heating element of a furnace off ifit senses a dangerously high temperature.Hydraulic resistance – The ability of a piping system to dissipate head.Hydronic - A heating system using hot water or steam as the heat-transfer fluid(Webster’s Dictionary). A water heating system (common usage).Infrared - Pertaining to heat rays emitted by the sun or warm objects on earth.Injection mixing – A method of controlling water temperature in a hydronicsystem by injecting hot water into a circuit using a variable-speed circulator.Load – Heating requirement to maintain a given setpoint temperature.Make-up air - Air supplied to a space to replace exhausted air.Manifold - A section of pipe with multiple openings.Natural ventilation - Ventilation using only natural air movement, without fans orother mechanical devices.Open-loop system – A piping system that is exposed to the atmosphere or one thatcarries fresh water.Oxygen-diffusion barrier – A coating or film that reduces the oxygen diffusion ofa plastic pipe.Panel radiator – A wall-mounted heat emitter that emits most of its heat asinfrared radiation.Parallel piping – Branching piping paths that begin at a single point and end atanother single point.

Glossary

51

Pressure - A force encouraging movement by virtue of a difference in somecondition between two areas.Pump curve – A chart showing the pump’s head energy versus the fluid flowthrough the pump.R-value - A measurement of thermal resistance.Radiant temperature - The average temperature of objects in a home like walls,ceiling, floor, furniture, and other objects.Radiation - Heat energy, which originates on a hot body like the sun, and travelsfrom place to place through the air.Radiator valve – A valve that regulates hot-water flow through a heat emitter.Relay - An electromagnet switch operated by a control circuit to energize or de-energize a motor or valve.Reset controller - Adjusts fluid temperature or pressure in a central heating systemaccording to outdoor air temperature.Sealed-combustion - A venting system that draws combustion air from outdoorsand has an exhaust system sealed from indoors.Series piping – An assembly of pipes and other components that forms a singleun-branching loop.Setpoint temperature – The temperature setting of a control that represents thedesired temperature in some part of a system.Specific heat - The ratio of a material’s heat storage capacity to the heat storagecapacity of water.Thermal conductance - General term applied to both K-value and U-value,meaning heat flow rate.Thermal mass – The ability of a building component to store heat according to itsmass and specific heat.Thermal resistance - Same as R-value, expressing ability to retard heat flow.U-value - The amount of heat that will flow through a square foot of buildingcross-section with multiple slabs of materials.Vent connector - The vent pipe carrying combustion gases from the appliance tothe chimney.Venting - The removal of combustion gases by a chimney.Wet-base boiler – A boiler with a heat exchanger that goes underneath the burner.Zone valve – A valve that controls hot-water flow to an individual zone of abuilding.

52

Resource list

References1999 Standard Guidelines for the Installation of Residential Radiant Panel

Heating Systems, Radiant Panel Association, P.O. Box 717, Loveland, CO 80539Modern Hydronic Heating by John Siegenthaler, Delmar Publishers, Albany,

New YorkRadiant Engineering, Bozeman, Montana: Conversations with principals, Dale

Pickard and Bob Knebel.Radiant Floor Heating, Hydronics Institute Division of GAMA, 35 Russo Place,

P.O. Box 218, Berkeley Heights, NJ 07922.

Documents

Boiler, Piping and Controls Design