Study folder final ahsan 500490133

STUDY

FOLDER

WINTER 2015

CLASS NOTES

HOME WORKS

HOME WORKS I AHSAN I 500490133 2

Week -1: Study Chapter 1 and 5 of guide


Written by: Haresh Khemani edited by: Lamar Stonecypher updated: 11/9/2009

In the summer season the evaporator acts as the cooling coil. The air flows over it, gets cooled and it is supplied to the room for producing the cooling effect. The heat absorbed by the air in the room is thrown to the external atmosphere via the condenser. This is the normal working of the window air conditioner.

In many parts of the world the winters are too cool and the summers are too hot. In such climates the window air conditioner can be used for cooling in the summers and for heating in the winter. The window air conditioner used as the heater in the winters is called as the heat pump. Instead of the heat pump one can also use the ordinary electric heater for heating the room, but the electric consumption of the electric heater is much more than the heat pump for the same amount of heat produced inside the room.

Apart from the compressor and the expansion valve, the window air conditioner has two important heat exchangers: the condenser and the evaporator. Both, the condenser and the evaporator are made of number of turns of the copper tubing and are covered with the fins to increase their heat transfer rate. The evaporator is located inside the room while the condenser is located outside the room exposed to the external atmosphere.

In the summer season the evaporator acts as the cooling coil. The air flows over it, gets cooled and it is supplied to the room for producing the cooling effect. The heat absorbed by the air in the room is thrown to the external atmosphere via the condenser. This is the normal working of the window air conditioner.

To use the window AC as the heat pump in the winters the direction of the flow the refrigerant is changed by changing the position of the valve. Due to this the positions of the condenser and the evaporator gets reversed. The condenser becomes evaporator and the evaporator becomes condenser and the machine becomes the heat pump. In heat pump the condenser is located inside the room, while the evaporator is located outside the room.

The refrigerant inside the evaporator located outside the room, absorbs the heat from the atmosphere. This heat is liberated inside the room via the condenser. Thus the same coil that was producing the cooling effect in the summers produces heating effect in the winters.

The next section of the article shows how the direction of the valve is changed to reverse the function of the condenser and the evaporator.


The figure above shows all the parts of the window air conditioner namely, the compressor, condenser, the expansion valve (capillary) and the evaporator. What is different in this air conditioner from the other types of air conditioners is the 4-way reversing valve incorporated within the refrigeration cycle. The reversing valve is the four position valve with four ports. In the figure the air cooling cycle is shown by the dotted line, while the air heating cycle is shown by the continuous line.

In the normal course of running in the summer seasons, when the air conditioner works as the cooler, the indoor unit acts as the evaporator or the cooling coil and outdoor unit acts as the condenser. Due to this the cooling effect is produced inside the room, while the heat is rejected to the atmosphere.

In the winters, when the atmospheric temperature is too low, the position of the reversing valve is changed and the machine works as the heat pump. The indoor unit becomes the condenser and the outdoor unit becomes the evaporator. The outdoor unit absorbs the heat from the atmosphere and releases it inside the room via the condenser to produce the heating effect. The reversing valve changes the direction of the flow of the refrigerant and plays the vital role between the air conditioner and heat pump.

This method of reverse valve can work in the atmospheres where the external temperature during the winter season is around 3 to 4 degree C. If the atmospheric temperature is below this, the frost will be formed on the surface of the evaporator and its working will be hampered. For effective heat transfer in low temperatures, the frost should be removed from the evaporator at regular intervals. In places where there is extremely low temperature, the defrost system can be employed to remove the frost from evaporator automatically. Even the reversing valve can be operated automatically by using the solenoid valve and operating it with the switch.

Book: Basic Refrigeration and Air Conditioning by P. N. Ananthanarayan, Tata Mc-Graw

Hill Publishing Company Limited, Second Edition, page no. 214.


Figure 1 Heat Recovery Ventilator Schematic

(Source: OEE, NRCan)

The ducts bringing fresh air from the outdoors and stale air from inside the home cross by each other in the heat exchange core of the Heat Recovery Ventilator and the heat from one flow of air is transferred to the other. The fresh air is directed into the house while the stale air is exhausted outside. HRV Schematic from Heat Recovery Ventilators published by OEE permission received to use image. Source: "Components of a ventilation system using an HRV", Natural Resources Canada, 2012. Reproduced with the permission of the Minister of Natural Resources Canada, 2014.

Benefits

Reduced Energy Consumption

Reduces ventilation costs by using heat in outgoing exhaust air to warm incoming fresh air in the winter. Improved Indoor Environment

Ensures delivery, distribution and circulation of fresh outdoor air throughout the house.

Controls indoor air humidity levels to prevent moisture problems such as condensation on windows and mold growth.

Helps control odours and humidity levels in bathrooms and kitchens.

Dilutes indoor air contaminants.

Provides better comfort by warming incoming ventilation air.

Filters incoming air. Energy Recovery Ventilators (ERV) can also recover moisture from air to help prevent over-drying of home in the winter. Reduced Environmental Impact

Reduces energy consumption-related green house gas emissions due to heat recovery that offsets energy otherwise used to heat ventilation air.


Liquid

Temperature Density

- t - - -

(oC) (kg/m3)

Alcohol, methyl (methanol) 25 786.5

Alcohol, propyl 25 800

Benzene 25 873.8

Benzil 15 1230

Brine 15 1230

Water, heavy 11.6 1105

Water - pure 4 1000

Water - sea 77oF 1022


Absolute Viscosity [

Liquid

Absolute Viscosity *)

(Pa s)

Air 1.983 x 10-5

Water 1 x 10-3

Olive Oil 1 x 10-1

Glycerol 1 x 100

Liquid Honey 1 x 101

Golden Syrup 1 x 102

Glass 1 x 1040

*) at room temperature

Kinetic Viscosity [

Liquid

Temperature Kinematic Viscosity

(oC) CentiStokes (cSt)

Seconds Saybolt Universal (SSU)

Water, distilled 20 1.0038 31

Water, fresh 15.6 1.13

31.5 54.4 0.55

Water, sea - 1.15 31.5

FURTHER READINGS

ewsletterstem designer

volume 43 2

Background

The following three cases are composite scenarios of various chilled-water system design issues we have witnessed over the years. In each scenario, the design issues are evaporator pressure drop is 5 ft of head [15 kPa]. At the minimum flow rate of 1.2 gpm per ton [0.022 L/s per kWr], the pressure drop will be about 3 ft of head

Issue 2: Loop volume does not meet the chiller unit control requirement. Chiller unit controls often require a certain loop volume to maintain good chilled-water

highlighted and followed by possible solutions.

By sharing the hard-earned HVAC design wisdom that comes from trial and error, we hope to shorten the Engineers Nproviding insights for todays hvac sy

Chilled-Water SysLearning from past mistakes

As with anything in life, you can learn HVAC design the hard way, from painful and costly mistakes, or the easy way, from veteran engineers who can help you avoid those mistakes. The purpose of this EN is to help new engineers with the latter. 2014 Trane, a business of Ingersoll Rand. A

learning curve for new engineers.

Case 1: New Variable Primary Flow

A new system has been designed using an increased system chilled-water temperature difference as recommended in the ASHRAE GreenGuide1 and by Taylor2. The chillers are selected at low design evaporator pressure drops (5 ft of head [15 kPa]) per chiller to minimize pump power and energy. To reduce installed cost and operating costs, the system is tems Design Issues

designed as variable primary flow (VPF). The bypass line (sized the same as the supply manifold) and valve that allow minimum chiller flow rate are located in the mechanical room, as are the air handlers. A differential pressure sensor across the evaporator is used to determine flow rate and to control the bypass valve.(Figure 1).

See any red flags? Lets take a look at the problems that occurred on the job and how to avoid making the same mistakes.

Issue 1: Flow sensing device is not accurate enough at the minimum flow rate. As mentioned, the design ll rights reserved.

56F (13.3C)

DP

40F (

Figure 1. Case 1: New variable primary flow (VPF)

variable-speed drive (VSD)

modulating control valve for minimum chiller flow

VSD[9.0 kPa]. Even if the differential pressure sensor is of high quality, it will be difficult to select one that has the required precision to accurately measure such a small difference between design flow and minimum flow pressure drops.

Solution 1: Select the proper flow sensing technology. Flow meters are generally more expensive, require more calibration, and must be installed per the manufacturer's requirements with a specific amount of straight piping up- and down-stream. Despite these limitations, they're better suited to monitor flow for this system. 1

4.4C)

6 in pipe

AHUs in same equipment room

200-ton helical rotary chillers:

minimum flow rate: 1.2 gpm/ton

design pressure drop: 5 ft of head

Many designers will use 4 in [10 cm] line for this flow rate. Then select a valve to give proper bypass control. While not required, some consulting engineers specify a pressure independent valve for bypass.

Issue 4: Flow can't be significantly reduced. Chiller design flow rate is 1.5 gpm per ton [0.027 L/s per kWr] and the minimum is 1.2 gpm per ton [0.022 L/s per kWr]. This means the flow rate can only be reduced 20 percent before reaching minimum. That's not much for a system that is supposed to be variable primary flow.

Solution 4: Put the chillers in series!

With chillers in series (Figure 2), the design flow rate per chiller is about 3.0

The series chiller layout has two significant benefits:

The upstream chiller operates at an elevated temperature and increased efficiency. Very often this compensates for any increase in pump energy.

When chillers are piped in parallel in a VPF system, there is a significant flow rate change in the operating chiller when the second chiller is added. When the chillers are piped in series, there is no flow rate transition when the second chiller is enabled. This can greatly simplify system control.

rs

4temperature control and ensure that compressors are not cycled unnecessarily. The loop volume includes the fluid in the evaporator bundle, pipes, and coils. In many close-coupled systemsfor example where the air handlers and chillers are in the same equipment roomthe loop volume will not be adequate. The required loop volume varies by chiller manufacturer, chiller type, and unit control capabilities. Check with the manufacturer for each chiller specified. In this case, the required loop time was two minutes. So the system water volume required is the loop time multiplied by the water flow:

2 min. x 300 gpm = 600 gal [2 min. x 60 sec. per min. x 18.9 L/s = 2270 liters]

Solution 2: Increase the loop volume to the required level. If the loop volume is close to that required, increasing the piping manifolds may be adequate. Otherwise add a buffer tank, preferably on the return side of the system, and on the chiller side of the bypass line. Ensure the system volume with the tank meets the required loop volume. To attain smoother system control, consider making the tank a little larger than the absolute minimum requirement. The additional cost and space will likely be small.

Issue 3: Oversized bypass line and valve do not provide adequate control. The bypass line is the same size as the pipe manifold. Given this fact, if a large valve is installed in this line, control at the minimum flow rate may be a challenge, since initially opening the valve a little allows significant flow to be bypassed.

Solution 3: Size the bypass line and valve properly. In a variable primary flow system, size the bypass line for the largest minimum flow rate. In this case:

200 tons x 1.2 gpm per ton = 240 gpm [15 L/s] 2 Trane Engineers Newsletter volume 432gpm per ton [0.054 L/s per kWr], allowing significant turndown. Design evaporator pressure drop increases since water must flow through both evaporator bundles. This increases design pump power. However, in many systems the flow rate is less than design flow much of the year, so the resulting pump energy increase may be small.

To allow for servicing, engineers often design manual bypass lines into the system.

56F (13.3C) 47.5F (8.6C)

Figure 2. New variable primary flow (VPF) series chille

VSD

manual bypass for servicingbuffer tankpIt may be beneficial to pipe the chillers in series if the system

is designed for variable primary flow,

has a chilled-water design temperature difference of 14F [7.8C] or larger,

has two chillers, and

is not likely to require a future expansion.

If the system T is 10F [5.6C], consider increasing it to 14F [7.8C] or larger.

flow meter

0F (4.4C)

Solution:

install flow meter

increase loop volume by adding a buffer tank

size the bypass line for the largest minimum flow rateroviding insights for todays HVAC system designer

54F (12.2C)DP

VSD

control valve

VPF conversion plan:

additional 30-tons of heat recovery

Value engineering:

pumps

- use secondary pumps, dont move them

- remove primary pumps

$100 DP sensor rather than magnetic flow meter

VSD

Figure 3. Case 2: After value-engineered conversion to VPF

44F (6.7C)

54F (12.2C)

T

44F (6.7C)

Figure 4. Convert to variable primary-variable secondary

VSD

Solution:

keep primary pumps and add VSDs

add DP and T sensors

control primary pump VSD speed to maintain T of 1-2F

if necessary, override temperature control to maintain minimum flow rate

control valve

VSD

DP

VSD

VSDCase 2: Conversion from Primary Secondary to Variable Primary Flow

The system includes two 500-ton chillers in a primary-secondary configuration. The owner wants to convert the system to variable primary flow (VPF) and add a small (30-ton) heat recovery chiller.

This job is "value engineered" to:

use the existing secondary pumps (which can handle the flow and pressure drop requirements),

switch out the high quality differential pressure sensor, which costs $1500, to a $100 sensor, and

connect the heat recovery chiller in parallel with the existing chillers (Figure 3).

Issue 1: The pumps are positioned incorrectly to allow bypass for minimum flow. Water cannot flow from right-to-left in the bypass line, so minimum flow cannot be maintained.

Solution 1: Rather than converting to full variable primary flow, consider converting to variable primary/variable secondary. As its name implies, a variable primary/variable secondary system (Figure 4) employs variable primary (chiller) flow as well as variable secondary (coil) flow. In retrofitting a system, the use of the existing pumps already installed is generally simple. In contrast, converting to primary secondary requires significant piping changes, and the present secondary pumps must be moved and possibly increased in size.

To convert to variable primary/variable secondary:

Contact the chiller provider to ensure that chiller unit controls can tolerate variable evaporator water flow. If not, update the unit controls.

Add variable-speed drives (VSDs) to the primary pumps (make sure the pump motors are compatible).

Add a high quality differential pressure sensor across each evaporator.providing insights for todays HVAC system desig Add a matched pair of temperature sensors to monitor the system return-water temperature and the chiller return-water temperature.

Control the primary pump VSDs to maintain the chiller return-water temperature a degree or two lower than the system return-water temperature. This ensures there is always a little more chilled water being produced than demanded.

If the temperature control results in chiller flow dropping below its minimum flow rate, increase the pump speed to maintain the required minimum chiller flow rate.

Issue 2: It is likely the flow-sensing device is not accurate enough. A $100 differential pressure sensor is very unlikely to provide the accuracy, reliability nerand repeatability required to operate a variable primary flow system. Unfortunately this seems to occur often. We suggest you resist the temptation to reduce costs when value will be lost. A VPF system operates on flow rate, so accuracy is critical.

Solution 2: Specify and install an accurate, reliable and repeatable flow- sensing device. It's very important that the flow-sensing device is accurate, reliable, and repeatable. Another benefit of a higher quality sensor is that, in general, it requires less calibration. The cost of a proper flow-sensing device will often be closer to $1000 than $100. Informative Appendix E of ASHRAE Guideline 22-20083 provides an example of flow measurement accuracy and precision specification language.Trane Engineers Newsletter volume 432 3

consider the following factors:

Many coils in air-handling units have adequate heat transfer area to perform similarly to the data in Table 1. On the other hand, if a coil has limited heat transfer capability (e.g., small, two-row fan-coils), reselecting at a higher T is unlikely to work well.

First, the system flow rate dropped by nearly 38 percent because of the wider T across the coils and reduced water flow rate. This lower flow rate results in lower system pressure drop and pump power.

In many constant flow systems, its difficult to retrofit a separate

54F (12.2C) 44F (6.7C)

Figure 5. Value-engineered conversion heat recovery

Issues:

flow balance between dissimilar chillers

inability to fully load the heat recovery chiller due to mixed return water temperature

Solution:

put heat recovery chiller in

e

VSD

VSD

DP

Table 1. Air-handler coil reselection using low entering-water temperature

original selection reselection

capacity (MBh) [kW] 504 [148] 504 [148]

entering water temperature (F) [C] 44 [6.7] 41 [5.0]

flow rate (gpm) [L/s] 101 [6.37] 63 [4.0]

leaving water temperature (F) [C] 54 [12.2] 57 [13.9]

water T (F) [C] 10 [5.5] 16 [8.0]Issue 3: Poor heat recovery chiller location. Flow and control issues can result when a smaller chiller is piped in parallel with larger chillers. When evaporators are piped in parallel, the chillers are loaded to equal percentages. This does not allow the heat recovery chiller to be preferentially loaded or to deliver the desired hot water temperature and quantity.

Solution 3: Pipe the heat recovery chiller in the sidestream position. The sidestream configuration (Figure 5) -allows the heat recovery chiller to:

Be preferentially loaded, since it receives the warmest return water temperature.

Operate as a heater to make the desired leaving condenser water temperature. The chilled water produced reduces the load on the central chillers.

Meet its needed evaporator flow requirements independent of other chiller operation.

Case 3: Conversion of a Parallel-piped System to Series, and from Conventional to Increased T.This 400-ton system includes two water-cooled chillers piped in parallel. The chillers and air-handling coils were both selected with water temperatures of 5444F. The system serves a school, which does not have a trained plant operator on sitesimplicity is beneficial.

The school officials would prefer air-cooled chillers to eliminate cooling tower maintenance requirements. They have decided to use 50 percent glycol to keep the fluid from freezing in the winter. No changes are to be made to the airside of the system. In addition, energy usage reductions are desired.

For this scenario, lets examine some common misconceptions that might foil a new engineer.4 Trane Engineers Newsletter volume 432Misconception 1: Existing coils cannot be reselected at higher temperature differences.

Solution 1: Coils are heat transfer media that can be selected at many conditions. Table 1 shows a coil originally selected for a 10F [5.6C] chilled water T reselected to produce the same cooling capacity. This can be done if the entering water temperature is lower and results in a lower flow rate and higher chilled-water T. This change has no effect on the airside of the system, but a substantial impact on the waterside.

The advantages of series chillers (Figure 6) to produce the higher chilled-water T was discussed earlier in Case 1.

When making supply water temperature and flow rate changes to existing systems,

contvalv

VSD

Tprovidin What is the coldest (or lowest) chilled-water supply temperature possible before condensation forms within the pipe insulation? Often with existing pipe insulation, supply water temperature reduced to 40F [4.4C] can be successful, but perhaps not 36F [2.2C].

If coil control valves were oversized at a 10F [5.6C] T, they are even more oversized at a higher T and lower flow rate. Valves may need to be changed to allow good control.

Misconception 2: Use of three-way valves as the method of bypass can use more pump energy.

Solution 2: Yes, but this is mitigated by a number of factors.

"sidestream position

load using either condenser- leaving or chilled-water temperatures

rol

VSDg insights for todays HVAC system designer

bypass, control valve and controller Figure 6. Retrofit VPF series chillers Retrofit changes: new chillers in series

send 41F water to existing coils to increase T

new pump with VFD

leave 3-way valves equal to chiller minimum flow rate

add differential pressure sensor at remote AHU to control pump speed

add 50 percent glycol to the system for freeze protection

44F (6.7C)57F (13.9C)

50% glycol

DP

Source: Dow Chemical Company. 2008. HVAC Application Guide: Heat Transfer Fluids for HVAC and Refrigeration Systems www.dow.com/heattrans

Concentration required for freeze protection vs. burst protection

ethylene glycol propylene glycol

concentration volume (% volume) concentration volume (% volume)

temperatureF (C)

freeze protection

burst protection

freeze protection

burst protection

20 (-7) 16 11 18 12

10 (12) 25 17 29 20

0 (-18) 33 22 36 24

-10 (-23) 39 26 42 28

-20 (-29) 44 30 46 30

-30 (-34) 48 30 50 33

-40 (-40) 52 30 54 35

-50 (-46) 56 30 57 35

-60 (-51) 60 30 60 35

Freeze vs. Burst Protection

As the temperature drops below the glycol solution's freeze point, ice crystals begin to form. Because the water freezes first, the remaining glycol solution is further concentrated and remains a fluid. The resulting ice crystals and fluid combination make up a flowable slush. The fluid volume increases as this slush forms and flows into available expansion volume.

Freeze protection indicates the concentration of antifreeze required to prevent ice crystals from forming at the given temperature. Burst protection indicates the concentration required to prevent damage to equipment (e.g., coil tubes bursting). Burst protection requires a lower concentration of glycol, which results in less degradation of heat transfer.

indoors and still uses an outdoor air-cooled condenser.into the system. Leaving enough three-way valves to allow the required minimum chiller flow rate can be a simple, cost-effective way to convert a system from constant to variable flow.

Minimum pump speed can be set to attain the minimum flow rate with all converted two-way valves closed.

Using three-way valves as a method of bypass is a good compromise between simplicity and performance.

Misconception 3: More glycol is better. When antifreeze is added to a system, fluid viscosity and pump power are often considered. However, these impacts may be small compared to the effect on chiller and coil heat transfer capability and capacitywhich can be degraded by 30 percent or more.

Solution 3: Add only the level of antifreeze necessary. In general, use the smallest amount of antifreeze to adequately protect the system. This minimizes first cost of the antifreeze itself, plus minimizes the first cost and operating cost impact on the heat transfer surfaces and pumps. When adding antifreeze to an existing system, ensure the chiller and coil capacities are adequate given the level of antifreeze.

Final Thoughts

While it's often more memorable to experience issues in the field, it's cheaper and much less painful to learn from others!

We can all listen to those with experience, ask probing questions, and attempt to learn. By doing so the industry can be elevated, deliver higher performing systems that result in more satisfied building owners and occupants. providing insights for todays HVAC system designerFor a chilled-water VAV system, since the cooling coil is typically shut off during sub-freezing weather, burst protection is usually sufficient. Freeze protection is mandatory in those cases where no ice crystals can be permitted to form (such as a coil loop that operates during very cold weather).

When an air-cooled chiller is used, an alternative approach is to use a packaged condensing unit (condenser and compressor) located outdoors, with a remote evaporator barrel located in an indoor equipment room. The two components are connected with field-installed refrigerant piping. This configuration locates the part of the system that is susceptible to freezing (evaporator) Trane Engineers Newsletter volume 432 5

Trane believes the facts and suggestions presented here to be accurate. However, final design and application decisions are your responsibility. Trane disclaims any responsibility for actions taken on

This article is based on a seminar presented by Mick Schwedler, Trane and Jason Atkisson, Affiliated Engineers,during the 2013 ASHRAE annual meetings in Denver. The acknowledgment of Mr. Atkisson in this newsletter in no way implies the endorsement of a specific system provider or manufacturer by either Affiliated Engineers or Mr. Atkisson. You can find this and previous issues of the Engineers Newsletter at www.trane.com/engineersnewsletter. To comment, e-mail us at [email protected].

References[1] American Society of Heating, Refrigerating

and Air-Conditioning Engineers. 2010. ASHRAE Green Guide: The Design, Construction, and Operation of Sustainable Buildings, 3rd ed. Atlanta, GA: ASHRAE.

[2] Taylor, S. 2011. Optimizing Design & Control of Chilled Water Plants; Part 3: Pipe Sizing and Optimizing . ASHRAE Journal. 53(12):22-34.

[3] American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2008. ASHRAE Guideline 22: Instrumentation for Monitoring Central Chilled Water Plant Efficiency, Appendix E. Atlanta, GA: ASHRAE.

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Application manuals. Comprehensive reference guides that can increase your working knowledge of commercial HVAC systems. Topics range from component combinations and innovative design concepts to system control strategies, industry issues, and fundamentals. The following are just a few examples. Please visit www.trane.com/bookstore for a complete list of manuals available to order.

Central Geothermal Systems discusses proper design and control of central geothermal bidirectional cascade systems that use borefields. This manual covers central geothermal system piping, system design considerations, and airside considerations. (SYS-APM009-EN, February 2011)

Chilled-Water VAV Systems discusses the advantages and drawbacks of the system, reviews the various components that make up the system, proposes solutions to common design challenges, explores several system variations, and discusses system-level control. (SYS-APM008-EN, updated May 2012)

Water-Source and Ground-Source Heat Pump Systems examines chilled-water-system components, configurations, options, and control strategies. The goal is to provide system designers with options they can use to satisfy the building owners desires. (SYS-APM010-EN, updated November 2013) Trane, A business of Ingersoll Rand

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October Chilled-WaterTerminal Systems6 Trane Engineers Newsletter volume 432 ADM-APN051-EN (May 2014)

the material presented.For more information, contact your local Trane office or e-mail us at [email protected]

ANSI/ASHRAE Addendum f toANSI/ASHRAE Standard 90.2-2001

Energy-EfficientDesign of Low-RiseResidential BuildingsApproved by the ASHRAE Standards Committee January25, 2003; by the ASHRAE Board of Directors January 30,2003; and by the American National Standards InstituteApril 10, 2003.

This standard is under continuous maintenance by a Stand-ing Standard Project Committee (SSPC) for which the Stan-dards Committee has established a documented programfor regular publication of addenda or revisions, includingprocedures for timely, documented, consensus action onrequests for change to any part of the standard. Thechange submittal form, instructions, and deadlines aregiven at the back of this document and may be obtained inelectronic form from ASHRAEs Internet Home Page, http://www.ashrae.org, or in paper form from the Manager ofStandards. The latest edition of an ASHRAE Standard andprinted copies of a public review draft may be purchasedfrom ASHRAE Customer Service, 1791 Tullie Circle, NE,Atlanta, GA 30329-2305. E-mail: [email protected]. Fax:404-321-5478. Telephone: 404-636-8400 (worldwide), ortoll free 1-800-527-4723 (for orders in U.S. and Canada).

Copyright 2003 American Society of Heating,Refrigerating and Air-Conditioning Engineers, Inc.

ISSN 1041-2336

SPECIAL NOTE

This American National Standard (ANS) is a national voluntary consensus standard developed under the auspices of the AmericanSociety of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). Consensus is defined by the American National StandardsInstitute (ANSI), of which ASHRAE is a member and which has approved this standard as an ANS, as substantial agreement reachedby directly and materially affected interest categories. This signifies the concurrence of more than a simple majority, but not necessarilyunanimity. Consensus requires that all views and objections be considered, and that an effort be made toward their resolution.Compliance with this standard is voluntary until and unless a legal jurisdiction makes compliance mandatory through legislation.

ASHRAE obtains consensus through participation of its national and international members, associated societies, and publicreview.

ASHRAE Standards are prepared by a Project Committee appointed specifically for the purpose of writing the Standard. TheProject Committee Chair and Vice-Chair must be members of ASHRAE; while other committee members may or may not be ASHRAEmembers, all must be technically qualified in the subject area of the Standard. Every effort is made to balance the concerned interestson all Project Committees.

The Manager of Standards of ASHRAE should be contacted for:a. interpretation of the contents of this Standard,b. participation in the next review of the Standard,c. offering constructive criticism for improving the Standard,d. permission to reprint portions of the Standard.

ASHRAE INDUSTRIAL ADVERTISING POLICY ON STANDARDSASHRAE Standards and Guidelines are established to assist industry and the public by offering a uniform method

of testing for rating purposes, by suggesting safe practices in designing and installing equipment, by providing proper definitions of this equipment, and by providing other information that may serve to guide the industry. The creation of ASHRAE Standards and Guidelines is determined by the need for them, and conformance to them is completely voluntary.

In referring to this Standard or Guideline and in marking of equipment and in advertising, no claim shall bemade, either stated or implied, that the product has been approved by ASHRAE.

DISCLAIMER

ASHRAE uses its best efforts to promulgate Standards and Guidelines for the benefit of the public in light of availableinformation and accepted industry practices. However, ASHRAE does not guarantee, certify, or assure the safety orperformance of any products, components, or systems tested, installed, or operated in accordance with ASHRAEs Standardsor Guidelines or that any tests conducted under its Standards or Guidelines will be nonhazardous or free from risk.

ASHRAE STANDARDS COMMITTEE 2002-2003

Thomas E. Watson, ChairVan D. Baxter, Vice-ChairCharles G. ArnoldDean S. BorgesPaul W. CabotCharles W. Coward, Jr.Brian P. DoughertyAbdelhakim ElmahdyArthur D. HallstromMatt R. HarganRichard D. HermansStephen D. Kennedy

David E. KnebelFrederick H. KohlossWilliam J. Landman

Merle F. McBrideRoss D. Montgomery

Cyrus H. NasseriDavor Novosel

Dennis A StankeMichael H. Tavares

Steven T. TaylorDavid R. Tree

Terry E. Townsend, COMaureen Grasso, ExO

Claire B. Ramspeck, Manager of Standards

ASHRAE Standard Project Committee 90.2 Cognizant TC: TC 9.6, System Energy Utilization

SPLS Liaison: Michael H. Tavares

Stephen Skalko, Chair* Harold W. Heiss Wayne R. Reedy*

David C. Bixby* Jonathan Humble* Steven Rosenstock*

Paul W. Cabot* Stephen D. Kennedy* Max H. Sherman

Drury B. Crawley Ted J. Kesik Martha G. Van Geem*

Charles R. Foster, III* Merle F. McBride* Iain S. Walker*

Allan Fraser* Richard A. Morris* Lawrence R. Wethje

John F. Hogan Ronald G. Nickson* Raymond J. Wojcieson*

*Denotes members of voting status when the document was approved for publication

(This foreword is not part of this standard. It is merelyinformative and does not contain requirements necessaryfor conformance to the standard. It has not been pro-cessed according to the ANSI requirements for a stan-dard and may contain material that has not been subjectto public review or a consensus process.)

FOREWORD

The following proposal represents work conducted by theSSPC 90.2 Envelope Subcommittee Reflective Roof TaskGroup, a task group organized to address new research infor-mation concerning high-albedo roof benefits in low-rise resi-dential construction. These proposed changes allow aresidential structure to consider the use of high-albedo roofsin hot and humid climates in order to reduce air-conditioningenergy use; this, in turn, contributes to reducing the heatisland effect in or near urban centers.

Unless otherwise noted, underlining indicates additionand strikethrough indicates deletion.

ANSI/ASHRAE ADDENDUM f to 90.2-2001

Section 5.3.1.1 (Existing text to remain. Add the followingexception.)

Exception to 5.3.1.1: For roofs where the exterior surfacehas either of the following:a. a minimum total solar reflectance of 0.65 when

tested in accordance with ASTM E90369 orE191870 and a minimum thermal emittance of0.75 when tested in accordance with ASTME40871 or C137172 or

b. a minimum solar reflectance index (SRI) of 75calculated in accordance with ASTM E198073

for medium wind-speed conditions,the U-factor of the proposed ceiling shall be permit-ted to be adjusted using Equation 5-3.1 for demon-strating compliance.

Uceiling_adj = Uceiling_proposed Multiplier (5-3.1)

where

Uceiling_adj = adjusted ceiling U-factor for use indemonstrating compliance

Uceiling_proposed = U-factor of the proposed ceiling, asdesigned

Multiplier = ceiling U-factor multiplier from Table5.3.1

Section 5.3.1.2 (Existing text to remain. Add the followingexception.)


tested in accordance with ASTM E90369 orE191870 and has a minimum thermal emittanceof 0.75 when tested in accordance with ASTME40871 or C137172 or


for medium wind-speed conditions,

the U-factor of the proposed ceiling shall be permit-ted to be adjusted using Equation 5-3.1 for demon-strating compliance.


whereUceiling_adj = adjusted ceiling U-factor for use in

demonstrating complianceUceiling_proposed = U-factor of the proposed ceiling, as

designedMultiplier = ceiling U-factor multiplier from Table

5.3.1Section 5.5.1.1 (Existing text to remain. Add the followingexception.)







demonstrating complianceUceiling_proposed = U-factor of the proposed ceiling, as designedMultiplier = ceiling U-factor multiplier from Table 5.3.1Section 5.5.1.2 (Existing text to remain. Add the followingexception.)







demonstrating complianceUceiling_proposed = U-factor of the proposed ceiling, as

designedMultiplier = ceiling U-factor multiplier from Table

5.3.1ANSI/ASHRAE Addendum f to ANSI/ASHRAE STANDARD 90.2-2001 1

(Add the following table)

Section 8.8.3.1 (Add new text and add note)

8.8.3.1 Exterior Absorptivity. Since the colors are sub-ject to change over the life of the building, the exterior absorp-tivity of all walls and roofs shall be 0.5 regardless of color, andthe exterior absorptivity of roofs shall be 0.2 regardless ofcolor. If unconditioned spaces such as garages are not mod-eled, walls between them and conditioned space shall betreated as exterior walls with an absorptivity of zero.

Note: For low absorptivity roofs, the reference house mayemploy Exceptions 5.3.1.1 or 5.3.1.2 or 5.5.1.1 or 5.5.1.2.

Add the following references to Section 10:69. ASTM E 903-96, Standard Test Method for Solar

Absorptance, Reflectance, and Transmittance of Materi-als Using Integrating Spheres, West Conshokocken, PA,American Society of Testing and Materials.

70. ASTM E1918-97, Standard Test Method for MeasuringSolar Reflectance of Horizontal and Low-Sloped Sur-faces in the Field, West Conshokocken, PA, AmericanSociety of Testing and Materials.

71. ASTM E408-71 (Reapproved 1996), Standard TestMethods for Total Normal Emittance of Surfaces UsingInspection-Meter Techniques, West Conshokocken, PA,American Society of Testing and Materials.

72. ASTM C 1371-98, Standard Test method for Determina-tion of Emittance of Materials Near Room TemperatureUsing Portable Emissometers, West Conshokocken, PA,American Society of Testing and Materials.

73. ASTM E 1980-98, Standard Practice for CalculatingSolar Reflectance Index of Horizontal and Low-SlopedOpaque Surfaces, West Conshokocken, PA, AmericanSociety of Testing and Materials.

Add the following after Section 10:

(This bibliography is not part of this standard. It ismerely informative and does not contain requirementsnecessary for conformance to the standard. It has notbeen processed according to the ANSI requirements for astandard and may contain material that has not beensubject to public review or a consensus process.)

BIBLIOGRAPHY

ASHRAE. 1998. Energy Savings of Reflective Roofs,ASHRAE Technical Data Bulletin, Volume 14, Number2, January. American Society of Heating, Refrigeratingand Air-Conditioning Engineers, Atlanta, GA.

Akbari, H., and S. Konopacki. 1999. Calculations for Reflec-tive Roofs in Support of Standard 90.2. A TechnicalNote Prepared for the Reflective Roofs Task Group,June.

Akbari, H., and S. Konopacki. 1999. Reflective Roofs TaskGroupASHRAE SSPC 90.2, Progress ReportJune,1999 ASHRAE Annual Meeting, Seattle, WA. A ReportPrepared to the Reflective Roofs Task Group, June.

TABLE 5.3.1Ceiling U-Factor Multiplier

HDD 65 (HDD18)Ceilings with

AtticsCeilings without

Attics

0-360 (0-200) 1.50 1.30

361-900 (201-500) 1.30 1.30

901-1800 (501-1000) 1.20 1.30

1801-2700 (1001-1500) 1.15 1.30

2701-3600 (1501-2000) 1.10 1.20

> 3600 (> 2000) 1.00 1.002 ANSI/ASHRAE Addendum f to ANSI/ASHRAE STANDARD 90.2-2001

POLICY STATEMENT DEFINING ASHRAES CONCERNFOR THE ENVIRONMENTAL IMPACT OF ITS ACTIVITIES

ASHRAE is concerned with the impact of its members activities on both the indoor and outdoor environment. ASHRAEsmembers will strive to minimize any possible deleterious effect on the indoor and outdoor environment of the systems andcomponents in their responsibility while maximizing the beneficial effects these systems provide, consistent with acceptedstandards and the practical state of the art.

ASHRAEs short-range goal is to ensure that the systems and components within its scope do not impact the indoor andoutdoor environment to a greater extent than specified by the standards and guidelines as established by itself and otherresponsible bodies.

As an ongoing goal, ASHRAE will, through its Standards Committee and extensive technical committee structure,continue to generate up-to-date standards and guidelines where appropriate and adopt, recommend, and promote those newand revised standards developed by other responsible organizations.

Through its Handbook, appropriate chapters will contain up-to-date standards and design considerations as the material issystematically revised.

ASHRAE will take the lead with respect to dissemination of environmental information of its primary interest and will seekout and disseminate information from other responsible organizations that is pertinent, as guides to updating standards andguidelines.

The effects of the design and selection of equipment and systems will be considered within the scope of the systemsintended use and expected misuse. The disposal of hazardous materials, if any, will also be considered.

ASHRAEs primary concern for environmental impact will be at the site where equipment within ASHRAEs scopeoperates. However, energy source selection and the possible environmental impact due to the energy source and energytransportation will be considered where possible. Recommendations concerning energy source selection should be made byits members.

Chapter 7: Heating, Ventilation, Air Conditioning (HVAC) 103

CHAPTER 7: HEATING, VENTILATION, AIR CONDITIONING (HVAC)

When thinking about energy efficiency, one of the most important decisions to be made regarding a new home is the type of heating and cooling system to install. Equally critical to consider is the selection of the heating and cooling contractor. The operating efficiency of a system depends as much on proper installation as it does on the performance rating of the equipment.

Improper design and improper installation of the HVAC system have negative impacts on personal comfort and on energy bills. Improper design and installation of a HVAC system can dramatically degrade the quality of air in a home. Poorly designed and poorly installed ducts can create dangerous conditions that may reduce comfort, degrade indoor air quality, or even threaten the health of the homeowners.

104 Chapter 7: Heating, Ventilation, Air Conditioning (HVAC)

TYPESOFHEATINGSYSTEMSKeys to obtaining design efficiency of a system in the field include:

Sizing the system for the specific heating and cooling load of the home being built; Proper selection and proper installation of controls; Correctly charging the unit with the proper amount of refrigerant; Sizing and designing the layout of the ductwork or piping for maximizing energy efficiency; and Insulating and sealing all ductwork.

Two types of heating systems are most common in a new home: forced-air or radiant, with forced-air being used in the majority of the homes. The heat source is either a furnace, which burns a gas, or an electric heat pump. Furnaces are generally installed with central air conditioners. Heat pumps provide both heating and cooling. Some heating systems have an integrated water heating system.

FORCED-AIR SYSTEM COMPONENTS

Most new homes have forced-air heating and cooling systems. These systems use a central furnace plus an air conditioner, or a heat pump. Figure 7-1 shows all the components of a forced-air system. In a typical system, several of these components are combined into one unit. Forced-air systems utilize a series of ducts to distribute the conditioned heated or cooled air throughout the home. A blower, located in a unit called an air handler, forces the conditioned air through the ducts. In many residential systems, the blower is integral with the furnace enclosure.

Figure 7 1 Components of Horizontal Flow Forced-Air Systems


Most homes in Kentucky have a choice of the following approaches for central, forced-air systems; fuel-fired furnaces with electric air conditioning units, electric heat pumps or a dual fuel system that combines both a fuel-fired furnace with an electric heat pump. The best system for each home depends on the cost and efficiency of the equipment, annual energy use, and the local price and availability of energy sources. In most homes, either type of system, if designed and installed properly, will economically deliver personal comfort.

RADIANT HEATING SYSTEMS

Radiant heating systems typically combine a central boiler, water heater or heat pump water heater with piping, to transport steam or hot water into the living area. Heating is delivered to the rooms in the home via radiators or radiant floor systems, such as radiant slabs or underfloor piping.

Advantages of radiant heating systems include:

Quieter operation than heating systems that use forced-air blowers. Increased personal comfort at lower air temperatures. The higher radiant temperatures of the

radiators or floors allow people to feel warmer at lower air temperatures. Some homeowners, with radiant heating systems, report being comfortable at room air temperatures of 60F.

Better zoning of heat delivered to each room. Increased comfort from the heat. Many homeowners, with radiant heating systems, find that the

heating is more comfortable.

Disadvantages of radiant heating systems include:

Higher installation costs. Radiant systems typically cost 40% to 60% more to install than comparable forced-air heating systems.

No provision for cooling the home. The cost of a radiant heating system, combined with central cooling, would be difficult to justify economically. Some designers of two-story homes have specified radiant heating systems on the bottom floor and forced-air heating and cooling on the second floor.

No filtering of the air. Since the air is not cycled between the system and the house, there is no filtering of the air.

Difficulty in locating parts. A choice of dealers may be limited.

HEAT PUMP EQUIPMENT

Heat pumps are designed to move heat from one fluid to another. The fluid inside the home is air and the fluid outside is either air (air-source), or water (geothermal). In the summer, heat from the inside air is moved to the outside fluid. In the winter, heat is taken from the outside fluid and moved to the inside air.

AIR-SOURCE HEAT PUMPS

The most common type of heat pump is the air-source heat pump. Most heat pumps operate at least twice as efficiently as conventional electric resistance heating systems in Climate Zone 4. They have typical lifetimes of 15 years, compared to 20 years for most furnaces.


Heat pumps use the vapor compression cycle to move heat (see Figure 7-2). A reversing valve allows the heat pump to work automatically in either heating or cooling mode. The heating process is:

1. The compressor (in the outside unit) pressurizes the refrigerant, which is piped inside.

2. The hot gas enters the inside condensing coil. Room air passes over the coil and is heated. The refrigerant cools and condenses.

3. The refrigerant, now a pressurized liquid, flows outside to a throttling valve where it expands to become a cool, low pressure liquid.

4. The outdoor evaporator coil, which serves as the condenser in the cooling process, uses outside air to boil the cold, liquid refrigerant into a gas. This step completes the cycle.

5. If the outdoor air is so cold that the heat pump cannot adequately heat the home, electric resistance strip heaters usually provide supplemental heating.

Periodically in winter, the heat pump must switch to a "defrost cycle," which melts any ice that has formed on the outdoor coil. Packaged systems and room units use the above components in a single box.

At outside temperatures of 25F to 35F, a properly sized heat pump can no longer meet the entire heating load of the home. The temperature at which a properly sized heat pump can no longer meet the heating load is called the balance point. To provide supplemental backup heat, many builders use electric resistance coils called strip heaters. The strip heaters, located in the air-handling unit, are much more

Figure 7 2 Air Conditioner Vapor Compression Cycle


expensive to operate than the heat pump itself. The strip heaters should not be oversized, as they can drive up the peak load requirements of the local electric utility.

A staged, heat pump thermostat can be used in concert with multistage strip heaters to minimize strip heat operation. To overcome this problem, some houses use a dual-fuel system that heats the home with natural gas or propane when temperatures drop below the balance point.

Air-source heat pumps should have outdoor thermostats, which prevent operation of the strip heaters at temperatures above 35F or 40F. Many mechanical and energy codes require controls to prevent strip heater operation during weather when the heat pump alone can provide adequate heating.

The proper airflow across the coil is essential for the efficient operation of a heat pump. During installation, the airflow rate must be checked to ensure that it meets the manufacturers recommendations.

AIR-SOURCE HEAT PUMP EFFICIENCY

The heating efficiency of a heat pump is measured by its Heating Season Performance Factor (HSPF), which is the ratio of heat provided in Btu per hour to watts of energy used. This factor considers the losses when the equipment starts up and stops, as well as the energy lost during the defrost cycle.

New heat pumps manufactured after 2005 are required to have an HSPF of at least 7.7. Typical values for the HSPF are 7.7 for minimum efficiency, 8.0 for medium efficiency, and 8.2 for high efficiency. Variable speed heat pumps have HSPF ratings as high as 9.0, and geothermal heat pumps have HSPFs over 10.0. The HSPF averages the performance of heating equipment for a typical winter in the United States, so the actual efficiency will vary in different climates.

To modify the HSPF for a specific climate, a modeling study was conducted and an equation was developed that modifies the HSPF, based on the local design winter temperature. In colder climates, the HSPF declines and in warmer climates, it increases. In Climate Zone 4, the predicted HSPF is approximately 15% less than the reported HSPF.

GEOTHERMAL HEAT PUMPS

Unlike an air-source heat pump, which has an outside air heat exchanger, a geothermal heat pump relies on fluid-filled pipes, buried beneath the earth, as a source of heating in winter and cooling in summer, Figures 7-3, 7-4. In each season, the temperature of the earth is closer to the desired temperature of the home, so less energy is needed to maintain comfort. Eliminating the outside equipment means higher efficiency, less maintenance, greater equipment life, no noise, and no inconvenience of having to mow around that outdoor unit. This is offset by the higher installation cost.


There are several types of closed loop designs for piping:

In deep well systems, a piping loop extends several hundred feet underground. Shallow loops are placed in long trenches, typically about 6 feet deep and several hundred feet

long. Coiling the piping into a "slinky" reduces the length requirements. For homes located on large private lakes, loops can be installed at the bottom of the lake, which

usually decreases the installation costs and may improve performance.

Proper installation of the geothermal loops is essential for high performance and the longevity of the system. Choose only qualified professionals, who have several years experience installing geothermal heat pumps similar to that designed for your home.

Geothermal heat pumps provide longer service than air-source units do. The inside equipment should last as long as any other traditional heating or cooling system. The buried piping usually has a 25-year warranty. Most experts believe that the piping will last even longer because it is made of a durable plastic with heat-sealed connections, and the circulating fluid has an anticorrosive additive.

Geothermal heat pumps cost $1,300 to $2,300 more per ton than conventional air-source heat pumps. The actual cost varies according to the difficulty of installing the ground loops as well as the size and features

Figure 7 3 Deep Well Loop

Figure 7 4 Shallow Trench Loop

Geothermal Heat Pumps


of the equipment. Because of their high installation cost, these units may not be economical for homes with low heating and cooling needs. However, their lower operating costs, reduced maintenance requirements, and greater comfort may make them attractive to many homeowners.

GEOTHERMAL HEAT PUMP EFFICIENCY

The heating efficiency of a geothermal heat pump is measured by the Coefficient of Performance (COP), which measures the number of units of heating or cooling produced by a unit of electricity. The COP is a more direct measure of efficiency than the HSPF and is used for geothermal heat pumps because the water temperature is more constant. Manufacturers of geothermal units provide COPs for different supply water temperatures. If a unit were installed with a COP of 3.0, the system would be operating at about 300% efficiency.

FURNACE EQUIPMENT

Furnaces burn fuels such as natural gas, propane, and fuel oil to produce heat and provide warm, comfortable indoor air during cold weather. Furnaces come in a variety of efficiencies. The comparative economics between heat pumps and furnaces depend on the type of fuel burned, its price, the homes design, and the outdoor climate. Recent energy price increases have improved the economics of more efficient equipment. However, due to the long-term price uncertainty of different forms of energy, it is difficult to compare furnaces with various fuel types and heat pumps.

FURNACE OPERATION

Furnaces require oxygen for combustion and extra air to vent exhaust gases. Most furnaces are non-direct vent unitsthey use the surrounding air for combustion. Others, known as direct vent or uncoupled furnaces, bring combustion air into the burner area via sealed inlets that extend to outside air.

Direct vent furnaces can be installed within the conditioned area of a home since they do not rely on inside air for safe operation. Non-direct vent furnaces must receive adequate outside air for combustion and exhaust venting. The primary concern with non-direct vent units is that a malfunctioning heater may allow flue gases, which could contain poisonous carbon monoxide, into the area around the furnace. If there are leaks in the return system, or air leaks between the furnace area and living space, carbon monoxide could enter habitable areas and cause severe health problems.

Most new furnaces have forced draft exhaust systems, meaning a blower propels exhaust gases out the flue to the outdoors. Atmospheric furnaces, which have no forced draft fan, are not as common due to federal efficiency requirements. However, some furnace manufacturers have been able to meet the efficiency requirements with atmospheric units. Atmospheric furnaces should be isolated from the conditioned space. Those units located in well ventilated crawl spaces and attics usually have plenty of combustion air and encounter no problem venting exhaust gases to the outside.

However, units located in closets or mechanical rooms inside the home, or in relatively tight crawl spaces and basements, may have problems. Furnace mechanical rooms must be well sealed from the other rooms of the home (see Figure 7-5). The walls, both interior and exterior, should be insulated. Two outside-air ducts sized for the specific furnace should be installed from outside into the room, one opening near the floor and another near the ceiling, or as otherwise specified in your localitys gas code.


MEASURES OF EFFICIENCY FOR FURNACES

The efficiency of a gas furnace is measured by the Annual Fuel Utilization Efficiency (AFUE), a rating that takes into consideration losses from pilot lights, start-up, and stopping. The minimum AFUE for most furnaces is now 78%, with efficiencies ranging up to 97% for furnaces with condensing heat exchangers. The AFUE does not consider the units electricity use for fans and blowers, which can easily exceed $50 annually. An AFUE rating of 78% means that for every $1.00 worth of fuel used by the unit, approximately $.78 worth of usable heat is produced. The remaining $.22 worth of energy is lost as waste heat and exhaust up the flue. Efficiency is highest if the furnace operates for longer periods. Oversized units run intermittently and have reduced operating efficiencies.

Furnaces with AFUEs of 78% to 87% include components such as electronic ignitions, efficient heat exchangers, better intake air controls, and induced draft blowers to exhaust combustion products. Models with efficiencies over 90%, commonly called condensing furnaces, include special secondary heat exchangers that actually cool flue gases until they partially condense, so that heat losses up the exhaust pipe are virtually eliminated.


A drain line must be connected to the flue to catch condensate. One advantage of the cooler exhaust gas is that the flue can be made of plastic pipe rather than metal and can be vented horizontally through a side wall.

There are a variety of condensing furnaces available. Some rely primarily on the secondary heat exchanger to increase efficiency, while others, such as the pulse furnace, have revamped the entire combustion process.

A pulse furnace achieves efficiencies over 90% using a spark plug to explode gases, sending a shock wave out an exhaust tailpipe. The wave creates suction to draw in more gas through one-way flapper valves, and the process repeats. Once such a furnace warms up, the spark plug is not needed because the heat of combustion will ignite the next batch of gas. The biggest problem is noise, so make sure the furnace is supplied with a good muffler, and do not install the exhaust pipe where any noise will be annoying.

Because of the wide variety of condensing furnaces on the market, compare prices, warranties, and service. Also, compare the economics carefully with those of moderate efficiency units. Condensing units may have longer paybacks than expected for energy efficient homes due to reduced heating loads. Table 7-1 compares the break-even investment for high efficiency gas furnaces in Code and in ENERGY STAR homes.

Table 7-1 Economic Analysis of Gas Furnaces

Type of Treatment AFUE 0.95

Energy Savings*($/yr) Compared to AFUE 0.80 Break-even Investment ($)

Code Home 42 477 ENERGY STAR Home 31 352

*For a system in Lexington, KY See Chapter 2 for information on break-even investment.

ELECTRIC INTEGRATED SYSTEMS

Several products use central heat pumps for water heating, space heating, and air conditioning. These integrated units are available in both air-source and geothermal models. To be a viable choice, integrated systems should:

Have a proven track record in the field; Cost about the same, if not less, than comparable separate heating and hot water systems; Provide at least a five-year warranty; and Be properly sized for both the heating and hot water load.


Make sure the unit is not substantially more expensive than a separate energy efficient heat pump and electric water heater. Units within $1,500 may provide favorable economic returns.

UNVENTED FUEL-FIRED HEATERS

Unvented heaters that burn natural gas, propane, kerosene, or other fuels are not recommended. While these devices usually operate without problems, the consequences of a malfunction are life threateningthey can exhaust carbon monoxide directly into household air. Unvented heaters also can cause serious moisture problems inside the home.

Most devices come equipped with alarms designed to detect air quality problems. However, many experts question putting a family at any risk of carbon monoxide poisoning; they see no rationale for bringing these units into a home (Figure 7-6).

Examples of unvented units to avoid include:

Vent-free gas fireplaces. Use sealed combustion, direct vent units instead. Room space heaters.

Choose forced draft, direct-vent models instead (Figure 7-7).

Figure 7-6 Unvented Heater


AIRCONDITIONINGIn summer, air conditioners and heat pumps work the same way to provide cooling and dehumidification. They extract heat from inside the home and transfer it outside. Both systems typically use a vapor compression cycle. This cycle circulates a refrigerant, a material that increases in temperature significantly when compressed and cools rapidly when expanded. The exterior portion of a typical air conditioner is called the condensing unit and houses the compressor, the noisy part that uses most of the energy, and the condensing coil.

An air-cooled condensing unit should be kept free from plants and debris that might block the flow of air through the coil or damage the thin fins of the coil. Ideally, the condensing unit should be located in the shade. However, do not block air flow to this unit with dense vegetation, fencing or overhead decking.

The inside mechanical equipment, called the air-handling unit, houses the evaporator coil, the indoor blower, and the expansion, or throttling valve. The controls and ductwork for circulating cooled air to the house complete the system.

Figure 7-7 Direct Vent Heater


AIR CONDITIONERS

Air conditioners use the vapor compression cycle, a 4-step process (see Figure 7-8).

1. The compressor (in the outside unit) pressurizes a gaseous refrigerant. The refrigerant heats up during this process.

2. Fans in the outdoor unit blow air across the heated, pressurized gas in the condensing coil; the refrigerant gas cools and condenses into a liquid.

3. The pressurized liquid is piped inside to the air-handling unit. It enters a throttling or expansion valve, where it expands and cools.

4. The cold liquid circulates through evaporator coils. Inside air is blown across the coils and cooled while the refrigerant warms and evaporates. The cooled air is blown through the ductwork. The refrigerant, now a gas, returns to the outdoor unit where the process repeats.

If units are not providing sufficient dehumidification, the typical homeowners response is to lower the thermostat setting. Since every degree the thermostat is lowered increases cooling bills 3% to 7%, systems that have nominally high efficiencies, but inadequate dehumidification, may suffer from higher than expected cooling bills. In fact, poorly functioning "high" efficiency systems may actually cost more to operate than a well-designed, moderate efficiency unit.

Figure 7-8 Air Conditioner Vapor Compression Cycle


Make certain that the contractor has used Manual J techniques to size the system so that the air conditioning system meets both sensible and latent (humidity) loads at the manufacturers claimed efficiency.

THE SEER RATING

The cooling efficiency of a heat pump or an air conditioner is rated by the Seasonal Energy Efficiency Ratio (SEER), a ratio of the average amount of cooling provided during the cooling season to the amount of electricity used. Current national legislation mandates a minimum SEER 13.0 for most residential air conditioners. Efficiencies of some units can exceed SEER 19.0.

Similar to the HSPF, a modeling study was conducted and an equation was developed that modifies the SEER, based on the local design summer temperature. In warmer climates, the SEER declines. In Climate Zone 4, the predicted SEER is approximately 5% less than the reported SEER.

VARIABLE SPEED UNITS

The current minimum standard for air conditioners is SEER 13. Higher efficiency air conditioners may be quite economical. Table 7-2 examines the economics of different options for a sample home. In order to increase the overall operating efficiency of an air conditioner or heat pump, multispeed and variable speed compressors have been developed. These compressor units can operate at low or medium speeds when the outdoor temperatures are not extreme. They can achieve a SEER of 15 to 17. The cost of variable speed units is generally about 30% higher than standard units. Variable speed units offer several advantages over standard, single-speed blowers, such as:

They usually save energy; They are quieter, and because they operate fairly continuously, start-up noise is far less (often the

most noticeable sound); and They dehumidify better. Some units offer a special dehumidification cycle, which is triggered by a

humidistat that senses when the humidity levels in the home are too high.

Table 7-2 Air Conditioner Economics

Type of Treatment Energy Savings* ($/yr) Break-even Investment ($)

SEER 14 (3 tons) -compared to SEER 13 20 227

SEER 15 (3 tons) -compared to SEER 14 32 363

*For a system in Lexington, KY See Chapter 2 for information on break-even investment.


PROPER INSTALLATION

Too often, high efficiency cooling and heating equipment is improperly installed, which can cause it to operate at a substantially reduced efficiency. A SEER 13 air conditioning system that is installed poorly with leaky ductwork may operate at 25% to 40% lower efficiency during hot weather. Typical installation problems are:

Improper charging of the systemthe refrigerant of the cooling system is the workhorseit flows back and forth between the inside coil and the outside coil, changing states, and undergoing compression and expansion. A system can have too little or too much refrigerant. The HVAC contractor should use the manufacturers installation procedures to charge the system properly. The correct charge cannot be ensured by pressure gauge measurements alone. In new construction, the refrigerant should be weighed in. Then, use either the supercharge temperature method or, for certain types of expansion valves, the subcooling method, to confirm that the charge is correct.

Reduced air flowif the system has poorly designed ductwork, constrictions in the air distribution system, clogged or more restrictive filters, or other impediments, the blower may not be able to transport adequate air over the indoor coils of the cooling system. Reduced air flow of 20% can drop the operating efficiency of the unit by about 1.7 SEER points; thus, a unit with a SEER 13.0 would only operate at SEER 11.3.

Inadequate air flow to the outdoor unitif the outdoor unit is located under a deck or within an enclosure, adequate air circulation between the unit and outdoor air may not occur. In such cases, the temperature of the air around the unit rises, thereby making it more difficult for the unit to cool the refrigerant that it is circulating. The efficiency of a unit surrounded by outdoor air that is 10 degrees warmer than the ambient outside temperature can be reduced by over 10%.

HVACSYSTEMSFor proper operation, a HVAC system must be properly designed, sized and installed. A proper HVAC system will provide an improved indoor environment and minimize the cost of operation. In the planning process for an energy efficient home, everything should be done to reduce the heating and cooling load on the home before the HVAC system is designed.

SIZING

When considering a HVAC system for a residence, remember that energy efficient and passive solar homes have less demand for heating and cooling. Substantial savings may be obtained by installing smaller units that are properly sized to meet the load. Because energy bills in more efficient homes are lower, higher efficiency systems will not provide as much annual savings on energy bills and may not be as cost effective as in less efficient homes.

Not only does oversized equipment cost more, but also it can waste energy. Oversized equipment may also decrease comfort. For example, an oversized air conditioner cools a house but may not provide adequate dehumidification. This cool, but clammy air creates an uncomfortable environment.


Many contractors select air conditioning systems based on a rule, such as 600 square feet of cooled area per ton of air conditioning (a ton provides 12,000 Btu per hour of cooling). Instead, use a sizing procedure such as:

Calculations in Manual J published by the Air Conditioning Contractors Association; Similar procedures developed by the American Society of Heating, Refrigeration, and Air

Conditioning Engineers (ASHRAE); or Software procedures developed by electric or gas utilities, the U.S. Department of Energy or

HVAC equipment manufacturers.

The heating and cooling load calculations rely on the outside winter and summer design temperatures (see the appendix for a definition) and the size and type of construction for each component of the building envelope, as well as the heat given off by the lights, people, and equipment inside the house. If a zoned heating and cooling system is used, the loads in each zone should be calculated. Table 7-3 compares the size of heating and cooling systems for the homes in Table 2-2. The more efficient home reduces the heating load 35% and the cooling load 26%. Thus, the $600 to $1,000 savings from reducing the size of the HVAC equipment offset the additional cost of the energy features in the more efficient home.

Table 7-3 Equipment Sizing Comparison

Type of House Code Home HERS=98 ENERGY STAR Home

HERS=85

Exceeds ENERGY STAR Home

HERS=70

HVAC System Sizing

Heating (BTU/hour) 52,200 38,800 25,700 Cooling (BTU/hour) 31,700 25,700 19,800 Estimated tons of cooling* 3.0 2.5 2.0 Square feet/ton 667 800 1,000

*Estimated at 110% of calculated size. There are 12,000 Btu/hour in a ton of cooling.

Oversimplified rules-of-thumb would have provided an oversized heating and cooling system for the more efficient home. The typical rule-of-thumb in Kentucky has been to allow for 600 square feet per ton of air conditioning. Since the home has 2,000 square feet of conditioned space, HVAC contractors could well

Do not rely on rule of thumb methods to size HVAC equipment.


provide 3.5 to 4 tons of cooling (2,000 600 = 3.33, then round up.) The oversized unit would have cost more to install. In addition, the operating costs would be higher. The oversized unit would suffer greater wear and may not provide adequate dehumidification.

Proper sizing includes designing the cooling system to provide adequate dehumidification. In a mixed-humid climate, it is important to calculate the latent load. The latent load is the amount of dehumidification needed for the home. If the latent load is ignored, the home may become uncomfortable due to excess humidity.

The Sensible Heating Fraction (SHF) designates the portion of the cooling load for reducing indoor temperatures (sensible cooling). For example, in a HVAC unit with a 0.75 SHF, 75% of the energy expended by the unit goes to cool the temperature of indoor air. The remaining 25% goes for latent heat removaltaking moisture out of the air in the home. To accurately estimate the cooling load, the designer of a HVAC system must also calculate the desired SHF and thus, the latent load.

Many homes in Climate Zone 4 have design SHFs of approximately 0.7. This means that 70% of the cooling will be sensible and 30% latent. Systems that deliver less than 30% latent cooling may fail to provide adequate dehumidification in summer. It takes 15 minutes for most air conditioners to reach peak efficiency. During extreme outside temperatures (under 32F in winter and over 88F in summer), the system should run about 80% of the time. Oversized systems cool the home quickly and often never reach their peak operating efficiency.

TEMPERATURE CONTROLS

The most basic type of control system is a heating and cooling thermostat. Programmable thermostats, also called setback thermostats, can be big energy savers for homes. These programmable thermostats automatically adjust the temperature setting when people are sleeping or are not at home. Be certain that the programmable thermostat selected is designed for the particular heating and cooling equipment it will be controlling. This is especially important for heat pumps, as an improper programmable thermostat can actually increase energy bills.

A thermostat should be located centrally within the house or zone. It should not receive direct sunlight or be near a heat-producing appliance. A good location is often 4 to 5 feet above the floor in an interior hallway near a return grille. The interior wall, on which it is installed, should be well sealed at the top and bottom to prevent circulation of cool air in winter or hot air in summer. Some homeowners have experienced discomfort and increased energy bills for years because air from the attic leaked into the wall cavity behind the thermostat and caused the cooling or heating system to run much longer than needed.

ZONED HVAC SYSTEMS

Larger homes often use two or more separate heating and air conditioning units for different floors or areas. Multiple systems can maintain greater comfort throughout the house while saving energy by allowing different zones of the house to be at different temperatures. The greatest savings come when a unit serving an unoccupied zone can be turned off.


Rather than install two separate systems, HVAC contractors can provide automatic zoning systems that operate with one system. The ductwork in these systems typically has a series of thermostatically controlled dampers that regulate the flow of air to each zone. Although somewhat new in residential construction, thermostats, dampers, and controls for zoning large central systems have been used for years in commercial buildings.

If your heating and air conditioning subcontractors feel that installing two or three separate HVAC units is necessary, have them also estimate the cost of a single system with damper control over the ductwork. Such a system must be carefully designed to ensure that the blower is not damaged if dampers are closed to several supply ducts. In this situation, the blower still tries to deliver the same air flow as before, but now through only a few ducts. Back pressure created against the blades of the blower may cause damage to the motor. There are three primary design options:

1. Install a manufactured system that uses a dampered bypass duct connecting the supply plenum to the return ductwork. Installing the bypass damper is the typical approach. When only one zone is open, the bypass damper, which responds automatically to changes in pressure in the duct system, will open to allow some of the supply air to take a shortcut directly back to the return, thus decreasing the overall pressure in the ductwork (Figure 7-9).

2. Create two zones and oversize the ductwork so that when the damper to one zone is closed, the blower will not suffer damage. This approach is only recommended for two zones of approximately equal heating and cooling loads.

3. Use a variable speed HVAC system with a variable speed fan for the duct system. Because variable speed systems are usually more efficient than single-speed systems, they will further increase savings.


COOLING EQUIPMENT SELECTION

Tables 7-4 and 7-5 show equipment charts for two sample air conditioning units. Each system provides a wide range of outputs, depending on the blower speed and the temperature conditions. The SHF (Sensible Heating Fraction) is the fraction of the total output that cools down the air temperature. The remainder of the output dehumidifies the air and is the latent cooling. Note that both systems provide about 36,000 Btu/hour of cooling.

Consider System A (Table 7-4) with 80F return air and SEER 15:

At low fan speed, System A provides 35,800 Btu/hour, 0.71 SHF, and thus 29% latent cooling (dehumidification).

At high fan speed, System A provides 38,800 Btu/hour, but a 0.81 SHF, and only 19% latent cooling. This is not enough dehumidification in many Kentucky homes.

Table 7-4 Sample Cooling System A Data, SEER 15

Total Air Volume (cfm)

Total Cooling Capacity (Btu/h)

Sensible Heating Fraction (SHF) Dry Bulb (F)

75F 80F 85F 950 35,800 0.58 0.71 0.84

1,200 37,500 0.61 0.76 0.91 1,450 38,800 0.64 0.81 0.96

Consider System B (Table 7-5) with 80F return air and SEER 13:

At low fan speed, System B provides 32,000 Btu/ hour, 0.67 SHF and 33% dehumidification. At high fan speed, System B provides 35,600 Btu/hour, 0.76 SHF and 24% dehumidification.

Table 7-5 Sample Cooling System B Data, SEER 13

Total Air Volume (cfm)

Total Cooling Capacity (Btu/h)

Sensible Heating Fraction (SHF) Dry Bulb (F)

75F 80F 85F 950 32,000 0.56 0.67 0.78

1,200 34,100 0.58 0.71 0.84 1,450 35,600 0.61 0.76 0.90

Thus, System A, while nominally more efficient than B, provides less dehumidification and potentially less comfort.


VENTILATIONANDINDOORAIRQUALITYAll houses need ventilation to remove stale interior air and excessive moisture and to provide oxygen for the inhabitants. There has been considerable concern recently about how much ventilation is required to maintain the quality of air in homes. While it is difficult to gauge the severity of indoor air quality problems, building science experts and most indoor air quality specialists agree that the solution is not to build an inefficient, leaky home.

Research studies show that standard houses are as likely to have indoor air quality problems as energy efficient ones. While opening and closing windows offers one way to control outside air for ventilation, this strategy is rarely useful on a regular, year-round basis. Most building researchers believe that no house is so leaky that the occupants can be relieved of concerns about indoor air quality. The researchers recommend mechanical ventilation systems for all houses.

The amount of ventilation required depends on the number of occupants and their lifestyle, as well as the design of the home. The ANSI/ASHRAE standard, Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings (ANSI/ASHRAE 62.2-2007) recommends that houses have 7.5 natural cubic feet per minute of fresh air per bedroom + 1, plus additional air flow equal to (in cubic feet per minute) 1% of the house conditioned area, measured in square feet. In addition, the standard requires exhaust fans in the kitchen and bathrooms that can be operated when needed.

For example, consider a 2,000 square foot home, with 3 bedrooms, and assume an occupancy of 4 people. The amount of ventilation recommended by ASHRAE would be 50 cfm:

7.5 cfm x (3 + 1) + 1% x 2,000 = 30 cfm + 20 cfm = 50 cfm

Increasing the number of occupants or increasing the square footage of the home would increase the necessary ventilation requirements.

Older, drafty houses can have natural air leakage of 1.0 to 2.5 ACHnat. Standard homes built today are tighter and usually have rates of from 0.35 to 0.75 ACHnat. New, energy efficient homes have rates of 0.30 ACHnat or less. The problem is that air leaks are not a reliable source of fresh air and are not controllable.

The ENERGY STAR rating system includes a consideration of homes that are tightly constructed. If the home has a measured natural air leakage rate below 0.35 ACHnat, the HERS score will not improve unless mechanical ventilation is provided. If the measured natural air leakage rate is below 0.25, the software will provide a warning that additional ventilation air should be provided and the amount needed.

Air leaks are unpredictable, and leakage rates for all houses vary. For example, air leakage is greater during cold, windy periods and can be quite low during hot weather. Thus, pollutants may accumulate during periods of calm weather even in drafty houses. These homes will also have many days when excessive infiltration provides too much ventilation, causing discomfort, high energy bills, and possible deterioration of the building envelope.

Concerns about indoor air quality are leading more and more homeowners to install controlled ventilation systems for providing a reliable source of fresh air. The simplest app

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