co2-contrlsAir_Handlers_Guide.pdf

7/25/2019 co2-contrlsAir_Handlers_Guide.pdf

1/24

Analyzing Air HandlingUnit Ef ciencyMichael Rosenberg, PE, CEM


2/24


3/24

onset

Analyzing Air Handling Unit Ef ciency

Introduction ....................................................................................................................................1

Air Handlers and Energy ...............................................................................................................2

Air Handler Case Study .................................................................................................................7

Optimizing Economization Conditions...........................................................................................8

Mixed Air Temperature Control....................................................................................................10

Carbon Dioxide Control................................................................................................................11

Discharge Air and Pre-heat Temperature Control.......................................................................12

Zone temperature Control...........................................................................................................14

Supply Fan Speed Control..........................................................................................................15

Return/Relief Fan Speed Control................................................................................................16

Calibration, Commissioning, and Balancing................................................................................17

References ....................................................................................................................................19

Analyzing Air Handling Unit Ef ciency1-800-LOGGERS
http://www.onsetcomp.com/http://www.onsetcomp.com/http://www.onsetcomp.com/


4/241 Analyzing Air Handling Unit Ef ciency www.onsetcomp.com

onset

Introduction

Operating a heating, ventilation and air conditioning (HVAC) system at optimumef ciency in a commercial setting is complicated, to say the least. There is avery real chance that any number of setpoints, levels and feedbacks at boilers,chillers, pumps, fans, air delivery components, and more can cause costly

inef ciencies.

According to the US Department of Energy, in 2009 the commercial sectorspent $192.3 billion on energy; $80.7 billion of that (41.9%) was spent on HVACoperations. This cost is only going to rise as energy prices climb, and its in theinterest of many facilities engineers, managers and owners to examine theirHVAC systems and consider recommission.

Cost payoff for tightening up can be huge. This guide explores the analysesand steps taken that realized more than $50,000 in annual savings for an of cebuilding in the Midwestern US. There are other rewards for ne-tuning as well,including increased occupant comfort and productivity, greater real estatemarketability, and enhanced company or industry image, both internal and

external.

This guide will focus on air handling units (AHUs) in particular, which, alongwith their associated air distribution systems, bring conditioned (heated, cooled,moisture-controlled) air to building occupants and contents. AHUs have theirown subset of variables, originating from fans, vents, lters, sound attenuators,cooling and heating coils and more. In an of ce setting, the AHU system mightbe responsible for maintaining a comfortable working environment for dozensto thousands of people, along with providing the cooling and humidi cationnecessary to relieve computer data centers of excess heat. In other instancesof public or academic buildings, comfort and air quality must be maintainedfor spaces that are variously occupied and have many people continuouslyentering and exiting.

At the start of any ne-tuning, recommissioning, or veri cation project, youneed to collect data. Portable data loggers are small, rugged, reusabledevices that are low cost and allow for time-stamped collection of data for

just about any component of an AHU system. The battery-powered loggeritself typically has one or more internal sensors, as well as several data portsfor incorporating external sensors. With data loggers, you can easily monitortemperature, relative humidity, motor on/off, CO 2, and more, without having toinvest in costly permanent metering. When the monitoring period is complete, data is downloaded easily to acomputer via wireless or cellular networks, to an IOS device if using a Bluetooth

Low Energy (BLE) logger, or manually by plugging the logger into the USB port.Then data can be analyzed and formatted using the logger software or mobileapp, or it can be exported to a spreadsheet application.

In this publication, Michael Rosenberg, a Senior Integration Engineer withGlenmount Global Solutions, will guide you through an intimate exploration ofthe air handling system of an of ce building, and you will learn how the datagathered helped provide insight into optimizing overall air handling operation.

According to the USDepartment of Energy, in2009 the commercial sector

spent $192.3 billion onenergy; $80.7 billion of that(41.9%) was spent on HVACoperations.
http://www.onsetcomp.com/http://www.onsetcomp.com/http://www.onsetcomp.com/http://www.onsetcomp.com/http://www.onsetcomp.com/


5/24

onset

Analyzing Air Handling Unit Ef ciency 21-800-LOGGERS

onset

Air Handlers and Energy

Air handling units are components in a mechanical system used to conditionand circulate air as part of a heating, ventilating and air conditioning (HVAC)system. An air handler typically contains a fan, heating coils, cooling coils, a ltersection, sound attenuators, dampers, actuators and controls, depending on theparticular design. Air handlers are usually connected to a system of ductworkthat distributes the conditioned air through the building. The intake side of theAHU typically has a return path for the air to be re-conditioned and mixed withoutside air. Several types of air handler designs are available, including variableair volume, dual duct (hot deck/cold deck), multi-zone, and make-up air units thatprovide 100% outdoor air. The basic design type reviewed in this best practicesguide is a variable volume AHU.

Signi cant improvements have been made to thedesign of air handlers, their associative distributionsystems, and their control systems. Improvementsin air handler and system designs, controls, and,equally important, operations have providedimprovements in temperature control and energysavings. Figure 1 provides the energy usagepro le of a typical of ce building. Air ventilation,space heating, and cooling represent the energycomponents typically associated with operating an airhandler.

This best practices guide will highlight controlmethods for optimizing energy ef ciency in anof ce building through the use of data logger andbuilding automation system (BAS) trend logs. Thedemonstration of these best practices will be shown

through a case study of an air handler sequence ofoperations that would be considered non-optimized.

Portable data loggers provide a comprehensive toolset for equipment and energydiagnostics. Proper instrumentation of an air handler requires an understandingof the system functionality, how the various components, devices and ow pathsinteract with one another, and the results one wants to obtain.

Data loggers provide for the collection of several types of data, depending on theunit. They are available in a variety of communication types, from popular stand-alone USB data loggers, to wireless data nodes, to Bluetooth Low Energy loggersthat require only an app and mobile device, to web-based monitoring systemsthat permit instantaneous real-time analysis to a computer.

Portable data logger external sensors are typically programmable and scalablethrough the units software, allowing the logger to be used for a wide range ofprocesses and operations. External sensor types include a 4-20mA input cablethat can be connected to any analog output signal on a variable frequencydevice (VFD) or digital control panel, carbon dioxide sensor, differential pressuresensor, and additional temperature and relative humidity input probes. Small,inexpensive, and reusable data loggers and sensors will provide all of the pointsof analysis that an effective energy study requires.

Figure 1 Of ce building energy distribution
http://www.onsetcomp.com/http://www.onsetcomp.com/http://www.onsetcomp.com/http://www.onsetcomp.com/http://www.onsetcomp.com/http://www.onsetcomp.com/



onset

Figure 2 shows a typical air handler in a Midwest building during the monthof August. Performance of the buildings air handlers was monitored by usingone temperature/humidity logger to log general outdoor air conditions, and asingle four-channel logger with external temperature probes. Measurementsof return air temperature (RAT), discharge supply air temperature (DAT) and,two mixed air temperatures (MAT) (to simulate mixed air averaging) wereattained using several different length temperature probes connected to thefour-channel logger.

Figure 2 Four-channel logger deployment for ventilation air control veri cation

OAT

MATx2

DAT

RAT


7/24Analyzing Air Handling Unit Ef ciency 41-800-LOGGERS

onset

The value of using the four-channel loggers along with the low-costtemperature probes is that it saved money, as individual loggers in eachindependent airstream would have been expensive. There is also value in thetime-saving bene t of multi-channel loggers as they enable a single setup anda single data of oad.

Two of the challenges of effective equipment benchmarking and energyanalysis are the period of analysis and the season of the trending period.For example, the building in this case study was evaluated during the winterperiod. Based on that data, it was possible to ascertain some potentialcooling-related operations without actually providing trends during the coolingseason. The trends in the previous gures were from the strictly cooling modeof operation, without any economizer operation.

In another building located in Glendale, CA, portable data loggers were usedextensively to analyze the operations of approximately 400 water source heatpumps. This building was operating an older DOS-based automation systemwith very limited trending capability. In pursuit of LEED certi cation, the energyteam turned to portable data loggers to analyze the operations of their heat

pumps and make decisions on how the new automation system would beused to improve energy ef ciency. The deployment strategy here was similarto the previous example. Several heat pumps were instrumented with four-channel loggers placed in the ceiling plenum near the unit. A temperatureprobe was directed through the ceiling and toward the heat pumps thermostatfor space temperature monitoring. One probe was placed in the dischargeair side of the heat pump and one in the rear of the heat pump where ductoutside air mixes with plenum/return air for reconditioning. The nal inputwas connected to an external amperage probe to measure the load on theheat pumps compressor as it cycles through its cooling mode. An additionaltemperature and relative humidity logger was placed on the roof in a shelteredarea to obtain a reference of outdoor air ambient conditions during thetrending period.



onset

Again, as with the air handler monitoring example above, the value of usingthe four-channel logger model and lower-cost temperature probes allowed foran investment in fewer loggers.

Another deployment example is for a data center located in Northern Virginia.The facility had multiple rooftop air handlers (RTUs) that served a largedata center space followed by power distribution units (PDU) located on amezzanine level in the air pathway back to the RTUs. In this case, HOBO

Figure 3 Heat pump deployment and trend log



onset

data loggers were deployed at strategic locations along the air ow pathwayto obtain a measure of the thermal conditions of the air. Those conditionsinclude the supply air plenum, the data center space (where server equipmentadded heat to the air ow), the mezzanine level (where the PDUs added heat)and the mixed air section prior to the air being reconditioned through the RTUchilled water coils. The data loggers installed were combination temperature/humidity models except for the supply air, where temperature probes wereplaced into discharge air registers.

The particular information sought was to establish the type of coolingprocesses: sensible or latent cooling. The data loggers have the internalcapability to provide calculated dew point temperature, which was used toevaluate the absolute humidity levels at each step in the thermal processalong the air ow path.

Figure 4 Temperature, Dew point, Relative Humidity distribution

Figure 4 demonstrates that as drybulb temperature increases along the airpath, and relative humidity experiences an associated reduction, the dewpoint levels, and therefore, absolute humidity levels remain relatively constantindicating a sensible cooling process without dehumidi cation and latentcooling. In this particular example, we demonstrate how HOBO loggers can bedeployed to analyze a thermal process at multiple locations and evaluate thepsychometric conditions as air is heated and cooled.

Our case study that follows used data logger and Building Automated System(BAS) trend data. However, data loggers and the wide range of plug-indevices can be used extensively in analysis applications, as indicated in apotential deployment shown in Figure 5, showing a fully monitored air handler,including temperature, humidity, CO 2, duct static pressure, VFD analog outputusing a 4-20 mA cable, and amperage CT.

In this particular example,we demonstrate how HOBOloggers can be deployed toanalyze a thermal processat multiple locations andevaluate the psychometricconditions as air is heatedand cooled.



onset

Air Handler Case Study

The building evaluated in this case study is a 100,000 sq. ft., three-storyMidwest of ce building with approximately 400 people, and contains threemechanical rooms housing ve air handling units. The mechanical systemsare controlled through a BAS integrated with an original pneumatic controlsystem. Each penthouse unit has exterior walk-in access to the followingsections:

Minimum outside air intake section Mixed air outside air intake section Return air plenum section Relief air discharge section Mixed air section (prior to pre-heat)

Mixed air section (post pre-heat) Discharge air section Filter section Chilled water coil section Supply fan Return fan

OAT & OA%RH

C02

VFD

VFD

4-20 mA

Preheat DuctStatic

DAT

MATx2

RAT &RA%RH

Figure 5 Graphic of case study air handling unit



onset

Figure 6 Air handler graphic

This building is part of a campus system with a central chilled water plantand a central steam plant. Utility charges are assessed based on buildingBTU meters on the chilled water and steam piping; therefore, any reductionin chilled water or steam usage had a direct cost-reduction to operations.Several operational de ciencies were identi ed during a retro-commissioninginvestigation of the buildings AHUs. Those de ciencies included non-optimized economizer control, inoperative demand control ventilation,control of minimum outside air dampers, excessive pre-heat load caused byineffective mixed air temperature, and suboptimal supply and return/relief fancontrol. The charts in this case study were developed through a combination of dataavailable through the buildings BAS and supplemental data loggers. This datashows how relatively straightforward control processes can have an interrelatedimpact on other control processes. The charts and analysis will show how trendlogs can be used to analyze air handler operations and the several interrelatedcontrol processes, and will show their impact on energy. In this particular casestudy the resulting retro-commissioning opportunities provided relatively modestsequencing modi cations at a comparatively low cost, and greater than $50,000in annual energy savings.

Optimizing Economization Conditions

Air-side economizers save energy by using cooler outside air as a means ofconditioning the indoor space. When the outside air is both suf ciently cooland possessing lower energy content than the indoor space, the air handlersdampers can be controlled to take advantage of that lower-energy outdoor air.

There are a few methods to initiate economizer operations, and the twomost common are drybulb temperature and enthalpy. The preferred methodis usually dependent on the particular climate of the building. Dryer, more arid climate economization can be achieved with simpli ed drybulb control,whereas more humid climates are best served by using enthalpy control.

Air-side economizers saveenergy by using cooleroutside air as a means ofconditioning the indoor

space.



onset

Between those two methods, the conditions that initiate economization cansometimes be based on a comparison between outdoor air and interiorconditions, or based on a xed outdoor air condition. The control system that was investigated provided two settings to initiatethe economizer sequence: drybulb temperature and enthalpy. The drybulbeconomizer enable was based on a xed outside air temperature of 55F, andthe enthalpy economizer enable was based on a xed outside air enthalpyof 23 BTU/lb. Through various functional tests it was determined that theeconomizer control was based on outside air drybulb temperature only, andthat the enthalpy capability was not active.

As part of the energy study for this project, a temperature bin analysis wasperformed. Figure 7 provides a comparison between the current economizercontrol and the area of lost economization by enabling economizationbased on comparative enthalpy. The bin chart shows that the xed 55Foutside air temperature economizer enable setpoint fails to take advantageof approximately 300 hours of annual economization through the use ofcomparative enthalpy. The chart also shows that economizer control based on

a comparison between outdoor air and return air drybulb temperature wouldresult in inef cient economizer operations due to the outdoor enthalpy in thecoincident outdoor air temperature range of 63-72F being greater than thereturn air enthalpy. Therefore, the most effective economizer control, for thisenvironment, would be based on enthalpy.

Figure 7 Economizer optimization

Considerations in evaluating optimized economizer operations include notonly the economizer initiation setpoints, but also the control of the mixedair dampers to maintain mixed air temperature, sensor calibration, sensorplacement, and balancing the economizer thermal savings with the relief fan



onset

power, economizer lockouts, and unintended heating loads. Considerationstoward dew point temperature of the airstream and temperature of the chilledwater coil are also important. Economizer control can be a complex study ofpsychometrics that is outside of the scope of this guide. Portable data loggersprovide the tools necessary to evaluate effective economizer operations.

Best Practice Tip:1. Optimize the economizer control based on the environment.

In areas where moisture levels in the air are prevalent, use anenthalpy type control. Dryer environments can be achieved withdrybulb.

2. The effects of inef cient economizer, mixed air damper andminimum outside air control have clear impacts on cooling andheating. The case study also demonstrates fan power issuesassociated with outside air damper control, showing a correlationto relief fan speed and power consumption.

3. Maintain an outside air limit on economization. Excessiveventilation air can result in excessive relief fan energy and

lter usage. Using a direct comparison of enthalpy may notalways be the most economical. With regard to the case study,the recommended programming modi cation is to include anenthalpy comparison and raise the drybulb limit setpoint from55F outside air temperature to 65F.

4. Sensor calibration and placement can be the number-one issueaffecting effective economizer control. Regularly calibratethe critical process-related BAS system sensors for effectiveeconomization.

Mixed Air Temperature Control

Mixed air temperature control is intended to supplement economizer controlby adjusting the proportion of outside air with building return air as outside airconditions get colder, providing a means to exit economizer control. Figure8 shows the economizer mixed air damper controlling to maintain a 55Fmixed air temperature setpoint. During this period of trending the outdoor airtemperature ranged between 10F and 30F. The objective of using a 55Fmixed air temperature setpoint is to provide cooler air for cooling space withinternal heat gains, while other zones are provided heat through local variableair volume (VAV) reheat control. We will see through further review of thiscase study how that temperature control philosophy actually increases energyconsumption.

Best Practice Tip:

5. The preferred method of control for the mixed air dampers isbased on an adjustable mixed air temperature setpoint thatshould track with the discharge air temperature setpoint.

Portable data loggersprovide the tools necessaryto evaluate effectiveeconomizer operations.



onset

Figure 8 Mixed air temperature control

Carbon Dioxide Control

Figure 8 above and the following Figure 9 show that the minimum outsideair damper command is 100% open during the entire period of occupiedcontrol. However, our trend log, as shown in Figure 9, indicates that CO 2 levels are never greater than 525 ppm during occupied periods and 420 ppmduring unoccupied periods. Current ASHRAE Ventilation Standard 62-89provides appropriate ventilation guidelines of 20 CFM per person and sets themaximum CO 2 level in commercial buildings at 1000 ppm. Combined with the

xed 55F mixed air temperature control of the mixed air dampers and the fullcapacity of the minimum outdoor air damper, we are starting to see a practiceof unnecessarily cool mixed air temperature and excessive fresh air levels asindicated by relatively low CO 2 levels.

Best Practice Tip:

6. Coupled with improved control over the economizer mixed airdamper, further reductions in ventilation air ow delivery througheffective CO 2 control will facilitate decreased cooling andheating loads during non-economization and mixed air modes ofoperation.



onset

Figure 9 Carbon dioxide control of minimum outside air damper

Discharge Air and Pre-heat Temperature Control

We observed the control of the economizer dampers maintained a xed 55Fmixed air temperature. By itself, that control sequence may seem acceptablebased on the objective of tempering internal loads. However, through furtherinvestigation and trend log analysis, we observed some additional operationsthat were inef cient and inconsistent with best practices.

The AHUs had a sequence of control to enable a pre-heat valve based onan outside air temperature at 35F. The intent of the pre-heat enable is forfreeze protection. This is an arbitrary value in that with effective mixed aircontrol, mixing the warmer return air temperature with the colder outsideair temperature will result in mixed air temperature greater than freezing. Inthe current sequence that mixed air temperature is 55F. In addition to thearbitrary pre-heat valve outdoor air temperature enable setpoint, the controlof the pre-heat valve is also, arbitrarily, commanded fully open without anytemperature control. During cooling mode the discharge air temperature setpoint for the airhandlers is a xed 58F. Similar to the pre-heat valve, the chilled water valveon the AHUs was also provided with an outdoor air temperature disable.Although our investigation was conducted during a heating mode of operation,we were still able to evaluate some cooling mode conclusions, ironically,through the operation of the pre-heat valve. The chart in Figure 10 showsthat with the chilled water valve closed and the pre-heat valve full-open, themeasured discharge air temperature was generally 62-65F with a xedmixed air temperature of 55F . Controlling the mixed air to 55F and then

raising the discharge air temperature with steam heat is not intuitive andenergy inef cient.



onset

Figure 10 Pre-heat temperature control

Best Practice Tip:

7. Maintain an adjusted discharge air temperature setpoint, alsoknown as discharge air temperature reset. As load requirementsresult in an increasing discharge air temperature, the mixed airtemperature setpoint will track the discharge air temperature,resulting in more ef cient mixed air temperature control.

8. Coupled with the improvements to mixed air temperature controland minimum outside air damper control, the modi cations to thepre-heat control is intended to minimize the use of pre-heatingenergy toward either freeze protection or conditions of extremecold ambient conditions in which the mixed air temperature is notstrictly maintainable through damper control.

A discharge air temperature reset strategy was based on outside airtemperature or zone feedback. The discharge air temperature reset isnecessary for successful operation of the mixed air control strategy. Thiscontrol strategy will force the mixed air damper control to a more closedposition during colder ambient conditions and reduce air handler heating loadsand relief fan power consumption.

In one discharge air temperature reset control scheme, the BAS polledVAV zone damper positions as an indication of building load. Alternatively,if the buildings control system lacks the capability to provide feedback onzone loading, the supply temperature is often reset based on either returnair temperature or outside air temperature. An example might be of anold building with a direct digital control (DDC) controlling the major HVAC

equipment but maintaining VAV control through an older pneumatic system.The goal of discharge air reset is to minimize the simultaneous coolingand heating or, in this case, the use of heating to offset inef cient outsideair damper control. However, resetting the discharge air temperature notonly impacts the cooling and heating energy consumption, but also has thepotential to impact supply fan power consumption or indoor air humidityproblems, or result in a fan power consumption penalty. In optimizing thedischarge air temperature reset logic consideration toward minimizing the



onset

overall heating energy, cooling energy and fan power consumption must beevaluated.

Zone Temperature Control

The zone thermostats for this case study provided tenants with local

temperature override capability. The typical zone temperature setpoint onthe thermostats was 72F. However, the zones were provided the ability toadjust the local temperature by +/- 5.0F. With full local adjustment at thethermostats, the maximum cooling setpoint places an excessive cooling loadon the system and forces the air handling unit to work harder to maintain ductstatic pressure. On the heating side, the effective temperature setpoint riskedthe greater energy usage through the VAV hot water reheat valves. The widerange of control over the zone thermostats makes effective control of the airhandling units dif cult to maintain.

An extremely important point with regards to VAV operations is that VAVsare, essentially, part of the air handling system. Inef ciencies in the operationof VAVs can impact the air ow, heating and cooling requirements of the

air handler, and fan energy. With integrated DDC control in buildings, theoperational status of VAVs is used to drive reset sequence of operationssuch as static pressure and discharge air temperature control. Without tighttemperature control, a routine temperature calibration check and validationof proper functionality, the operation of the network of VAVs can have untoldimpact on the operation of the air handling unit.

As an example of VAV impact on air handling units, another buildinginvestigated in the same Midwest climate as the current case study had thesame type of zone thermostats. This building had several courtrooms, manyof which were frequently unoccupied. In several of those courtrooms the localthermostat adjustment had been set for the maximum level of cooling, drivingthe VAVs serving those courtrooms into full cooling. Although those VAVsaccounted for 10% of the full design capacity of the associate air handlingunit(s), the static pressure sensor for the air handler supply fan VFD controlhad typically been located very close to those VAVs. With those VAVs infull cooling continuously, the system static was impacted, resulting in thesupply fan VFD operating at a higher speed and power usage than would beotherwise necessary.

Best Practice Tip:

9. Modify the override capability in the zones to prevent wideswings in VAV operations and uncontrollable air handler fanspeeds. Establish rules for zone temperature setpoint overrides

by the buildings tenants. Balance comfort considerations withenergy ef cient operations. Remember, a single VAV can be theproverbial tail wagging the dog, signi cantly impacting energyof an entire air handling system.

Supply Fan Speed Control

An extremely importantpoint with regards toVAV operations is thatVAVs are, essentially,part of the air handling

system. Inef ciencies inthe operation of VAVs canimpact the air ow, heatingand cooling requirementsof the air handler, and fanenergy.



onset

Figure 11 shows the control of the supply fan speed to maintain a xed designduct static pressure setpoint. The period of trending shown is during coldoutside air temperatures with VFD speed operating in the range of 90%-100%.This operating condition was typical of most of the buildings air handling units.

Figure 11 Discharge air static pressure control

It is not uncommon for the static pressure setpoint for supply fan control of anair handler to be this high. The static pressure setpoint is typically arbitrary,based on an expected design condition and established during the test andbalance (T&B) process. During the T&B process, the duct static pressuresetpoint is determined based upon pressure and air ow requirements toestablish the air handler and air delivery system for design conditions, i.e.,cooling.

The air ow requirements for the individual VAVs during full cooling modeare typically greater than during minimum air ow or heating mode. Duringpart load operations, the supply fan VFD is designed to modulate to areduced speed and power consumption as VAVs control to a reduced air ow.Fan savings will be realized, but they will still be controlling to maintainthe elevated duct static pressure setpoint established for design coolingconditions.

The operating conditions show that the supply fan speed is higher than wouldbe otherwise expected during a heating mode of control. The high supplyfan VFD speeds in Figure 11 indicate that VAV boxes may not be functioning

properly, temperature sensor may be off calibration, or zone temperaturesetpoints are too broad to prevent effective VAV modulation.

During the T&B process,the duct static pressuresetpoint is determinedbased upon pressure andair ow requirements to

establish the air handlerand air delivery systemfor design conditions, i.e.,cooling.



onset

Best Practice Tip:

10. For additional fan savings, the duct static pressure setpointshould be incrementally adjusted downward or upward. Thiscontrol sequence is known as static pressure reset. Thissequence of control will regularly adjust the static pressure

setpoint based on the demand requirements of the network ofVAVs.

11. Balancing system air ow requirements to match the zone levelair ow requirements should be the objective of an ef cient airdelivery system. Static pressure reset will help facilitate thisobjective.

Return/Relief Fan Speed Control

In this case study the supply fan and relief fan air ows are measuredthrough pressure across the fan inlet and discharge with a calculated air ow.The return fan is controlled to maintain a xed air ow offset between themeasured supply fan air ow equivalent to the design outside air ow rate. Themerit of this method of control is debatable; nonetheless, high motor powerconsumption at higher supply fan speed will also result in excessive return fanoperating speeds and motor power consumption as shown in Figure 12. Anymeans implemented to reduce supply fan speeds and motor usage will have acorrelating reduction in return fan speeds and power usage.

Figure 12 Supply and return fan operating characteristics

Another observation from the trending charts is that an increased rateof outside air into the building has an associative increase in the powerconsumption on the relief fan, as shown in Figure 13. Therefore, anyopportunity to reduce building ventilation rates will not only have the bene t ofthermal savings, but also fan energy associated with purging the building ofoutside air.



onset

Figure 13 Relationship between outside air and relief fan kW

Calibration, Commissioning, and Balancing Previously in this case study it was noted that outside air temperature was thedriving factor for several processes of control: chilled water valve enable; pre-heat valve enable; and economizer (both drybulb and, indirectly, enthalpy).As part of this case study a newly calibrated wireless temperature node wasplaced near the outdoor air temperature device associated with the buildingmanagement system (BMS). Both the BMS outside air and logger outside airtemperatures were logged for 24 hours and a deviation in the measurementswas noted up to 20%.

Figure 14 outside air temperature comparison

The outdoor air temperature is the driving factor for several sequences. Evenwith a perfect sequence of operations, a deviation of this critical sensor willhave obvious rami cations to ef cient operations of all the off-the-building



onset

air handlers. Additional consideration toward device placement must alsobe evaluated. For example, placement of the static pressure transmitter in aturbulent zone in the main supply air trunk can have an impact on effectiveand ef cient supply fan control. As another example, placement of themixed air temperature sensor in contact with a chilled water coil can impactprocess control through the radiation of coldness of the coil onto the sensor,

impacting the measured value of the mixed air temperature.

Best Practice Tip:

12. Regular calibration of this and other critical process-relatedcontrol variables must be performed regularly.

It is important to minimize the potential for ow variation due to varying staticpressure requirements in the different ow paths as the economizer systemmodulates from full return to full relief. The particular dampers installed inthis case study lacked the linearity necessary to achieve consistent staticpressures through all combinations of mixed air damper positions from fulleconomization to minimum outside air mode. The result of the non-linearrelationship between damper command and air ow percentage mismatch wasresulting in fan starvation, during mixed air control mode, and excessive fanspeeds in order to maintain duct static pressure. Damper linearity issues are only one example of an air handler componentlevel issue that can have an impact on effective temperature and pressurecontrol in an air handler.

Best Practice Tip:

13. Validating the most effective sequence of control for an airhandling unit requires a rigorous commissioning process. Proper

air balancing is also a critical step in achieving energy ef cientoperations.

This case study demonstrates that an air handlers sequence of operationscan be highly interrelated with regards to operations and energy consumption.In an expansion of this type of investigation it can be shown that operationalor control de ciencies can have an integrated response on other systemcontrol sequences. In this case study it was examined how VAV operationshave a direct impact on air handler operations. An expanded investigation willdemonstrate the interrelated impact of air handler operations on central plantpumping and chillers and boiler operations.

Understanding the overall process of control of the entire system sequence

of operations provides an overall system corrective strategy. This requiresa thorough commissioning or retro-commissioning process with functionalperformance testing and extensive trending analysis. Trending multipleprocesses and evaluating the interactive impacts requires an integratedapproach toward retro-commissioning. Portable data loggers are cost-effective, reusable, valuable tools in such an endeavor, and can be used indata-gathering and veri cation.

Understanding the overallprocess of control of theentire system sequenceof operations provides anoverall system corrective

strategy. This requires athorough commissioningor retro-commissioningprocess with functionalperformance testing andextensive trending analysis.



onset

ReferencesTaylor, Steven T., P.E., 2010. Economizer high limit controls and why enthalpyeconomizers dont work. ASHRAE Journal. Nov 2010.

Acronyms/AbbreviationsAHU air handling unitASHRAE American Society of Heating, Refrigeration and Air ConditioningEngineersBAS building automation systemBMS building management systemBTU british thermal unitCT current transducerDAT discharge air temperatureDDC direct digital controlMAT mixed air temperatureOAT outside air temperaturePDU power distribution unitRAT return air temperature

RTU rooftop air handlersT&B test and balanceVAV variable air volumeVFD variable frequency drive



onset

Choosing an Occupancy and Light On/Off Data Logger 5 Important Considerations

This paper provides guidance on features to consider whenchoosing an occupancy and light on/off data logger, includingcalibration, LCD display, logger accuracy and range, speed ofdeployment, and time-saving software. Learn how to select theright logger for identifying ideal locations in your building wherechanges in lighting could result in cost savings up to 80%.

Utility Incentive Programs: How to Get More MoneyQuickly and Easily

Utility Incentive Programs: How to Get More Money Quicklyand Easily, is aimed at making the process of applying for andreceiving energy ef ciency incentives and rebates faster, easier,and more rewarding. Authored by Carbon Lighthouse, an energy

rm that makes it pro table for commercial and industrial buildingsto eliminate their carbon footprint, the paper discusses the twomain types of incentive and rebate programs, how utility ef ciencyprogram managers think, and how to use data to get moreincentive dollars for your projects.

Using Data Loggers to Improve Chilled Water PlantEf ciency

Chilled water plant ef ciency refers to the total electrical energyit takes to produce and distribute a ton (12,000 BTU) of cooling.System design, water quality, maintenance routines, cooling towerdesign, and cooling coil load all affect chiller water plant ef ciencyand the expense of operating the system.

Data Logger Basics

In todays data-driven world of satellite uplinks, wireless networks,and the Internet, it is common to hear the terms data logging anddata loggers and not really have a rm grasp of what they are.

Most people have a vague idea that data logging involveselectronically collecting information about the status of somethingin the environment, such as temperature, relative humidity, orenergy use. Theyre right, but thats just a small view of what datalogging is.

Addressing Comfort Complaints With Data LoggersThis paper provides facility managers, HVAC contractors, andothers with valuable tips on how low-cost data loggers can beused to validate temperature-related comfort complaints.

Monitoring Green Roof Performance with WeatherStations

Data logging weather stations are the ideal tools for documentinggreen roof performance. A weather station can measure weatherparameters such as rainfall, stormwater runoff, temperature,relative humidity, wind speed, solar radiation, and a host of non-weather parameters such as soil moisture on a continuous basis(say every ve minutes, hourly, or an interval appropriate to thesituation).

Using Data Loggers Beyond Equipment Scheduling

While data loggers are a great tool for identifying equipment-scheduling opportunities in buildings, their usefulness far exceeds

just that one function. This paper discusses how the use ofinexpensive data loggers and some spreadsheet analysis canprovide all the evidence needed to make powerful building-speci c cases for saving money by replacing failed air-handlereconomizers. It also describes how information from data loggerscan be used to accurately calculate the energy savings that canbe realized from variable frequency drives (VFDs) on pumps andfans, supply air resets, and boiler lockouts

Analyzing Air Handling Unit Ef ciency with DataLoggers

Operating a heating, ventilation and air conditioning (HVAC)system at optimum ef ciency in a commercial setting iscomplicated, to say the least. There is a very real chance that anynumber of setpoints, levels, and feedbacks at boilers, chillers,pumps, fans, air delivery components and more can cause costlyinef ciencies.

Finding Hidden Energy Waste with Data Loggers: 8Cost-Saving Opportunities

The rst step to reducing building energy costs is identifyingenergy waste. Statistics on utility bills or name plates onequipment, while useful, are not enough to identify what practicesand equipment are contributing to high energy use. Portable dataloggers can be used to obtain critical energy use information ina wide range of commercial building types from manufacturingplants to of ce buildings.

Monitoring HVAC Performance with Data Loggers

Building operators and managers have the dif cult job of providingcomfortable working conditions and coaxing aging mechanicalequipment to operate at peak performance while minimizingenergy costs.

Other informational resources available from Onset:

Access our full resources library at: www.onsetcomp.com/ learning
http://www.onsetcomp.com/http://www.onsetcomp.com/http://-/?-http://www.onsetcomp.com/learninghttp://-/?-http://www.onsetcomp.com/learninghttp://-/?-http://www.onsetcomp.com/


24/24

onset

Copyright 2015, Onset Computer Corporation. All information in this document is subject to change without notice. Onset and HOBO are registered trademarks of Onset Computer Corporation.All other trademarks are the property of their respective owners. All rights reserved. Printed in the USA. Lit. No. MKT1122-0815

About the Author

Michael Rosenberg has over 12 years experience in mechanical engineering design, project management, energymanagement and systems commissioning. His expertise is focused on analyzing integrated system control and assistingclients in optimizing system operations through the most ef cient operational modes of control. He has degrees inMechanical Engineering and Business and is a licensed professional engineer in the states of Illinois and California. He alsoholds certi cations as a Certi ed Energy Manager through the AEE and is a LonMark Certi ed Professional.

Contact Us

Our goal is to make your data logging project a success. Our product application specialists are available to discuss your

needs and recommend the right solution for your project.

About Onset

Onset is a world leader in data loggers. Our HOBO data logger products are used around the world in a broadrange of monitoring applications, from verifying the performance of green buildings and renewable energy systemsto agricultural and coastal research.

Based on Cape Cod, Massachusetts, Onset has been designing and manufacturing its HOBO data loggers on sitesince the companys founding in 1981.

Onset headquarters, Cape Cod, MA

Sales (8am to 5pm ET, Monday through Friday) Email [email protected] Call 508-759-9500 In US call toll free 800-564-4377 Fax 508-759-9100

Technical Support (8am to 8pm ET, Monday through Friday) Email [email protected] Call 508-759-9500

In US call toll free 877-564-4377

Onset Computer Corporation470 MacArthur Blvd.Bourne, MA 02532
http://www.onsetcomp.com/http://www.onsetcomp.com/http://-/?-http://-/?-http://-/?-http://-/?-http://www.onsetcomp.com/

Documents

co2-contrlsAir_Handlers_Guide.pdf