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Variable-air-volume (VAV) systems provide comfort in many dif-ferent building types and climates. This article discusses several

design and control strategies that can signicantly reduce energy use in

multiple-zone VAV systems. Although none of these strategies are new (in

fact, several are required by energy standards or codes), implementa-

tion of them in buildings seems to be surprisingly infrequent. In this age

of striving for higher levels of energy performance, these ought to be

standard practice for every VAV system.

Optimized VAV System Controls

The first key ingredient to make aVAV system truly high-performanceis the use of optimized system controlstrategies.1

Optimal start/stop.Optimal start isa control strategy that uses a buildingautomation system (BAS) to determinethe length of time required to bringeach zone from current temperature

to the occupied setpoint temperature.Then the system waits as long as pos-sible before starting, so that the tem-perature in each zone reaches occupiedsetpoint just in time for occupancy(Figure 1).This strategy reduces the number of

system operating hours and saves en-ergy by avoiding the need to maintainthe indoor temperature at occupied set-

point even though the building is unoc-cupied.

Optimal stop is a control strategy thatuses the BAS to determine how earlyheating and cooling can be shut off foreach zone so that the indoor tempera-ture drifts only a few degrees from oc-cupied setpoint before the end of sched-uled occupancy (Figure 1). In this case,only cooling and heating are shut off;the supply fan continues to operate andthe outdoor-air damper remains open tocontinue ventilating the building.This strategy also reduces the number

of system operating hours, saving en-ergy by allowing indoor temperatures todrift early.

Fan-pressure optimization.As cool-ing loads change, the VAV terminalsmodulate to vary airflow supplied to thezones. This causes the pressure insidethe supply ductwork to change. In manysystems, a pressure sensor is located

About the Author

John Murphy is an applications engineer with

Trane, a business of Ingersoll Rand, in La Crosse, Wis.

By John Murphy,Member ASHRAE

High-PerformanceVAV Systems

This article was published in ASHRAE Journal, October 2011. Copyright 2011 American Society of Heating, Refrigerating and Air-ConditioningEngineers, Inc. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permissionof ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.

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O c t o b e r 2 0 1 1 A S H R A E J o u r n a l 1 9

approximately two-thirds of the distance downthe main supply duct. The VAV air-handling (orrooftop) unit varies the speed of the supply fanto maintain the static pressure in this location ata constant setpoint. With this approach, however,

the system usually generates more static pressurethan necessary.When communicating controllers are used on

the VAV terminals, it is possible to optimize thisstatic pressure control function to minimize ductpressure and save fan energy. Each VAV con-troller knows the current position of its airflow-modulation damper. The BAS continually pollsthese individual controllers, looking for the VAVterminal with the furthest-open damper (Figure2). The setpoint for the supply fan is then reset toprovide just enough pressure so that at least onedamper is nearly wide open. This results in thesupply fan generating only enough static pres-sure to push the required quantity of air throughthis critical (furthest-open) VAV terminal.

At part-load conditions, the supply fan is ableto operate at a lower static pressure, consumingless energy and generating less noise.

Fan-pressure optimization provides the addedbenefit of allowing the building operator to iden-tify and address rogue zones. A rogue zone isone where something is not working properly.Some possible causes include an undersized VAVterminal, a restriction in the duct that does not al-

low the required airflow, a zone temperature set-point that has been reset too low, or a zone sensorinstalled in the sunlight or near a heat source, likea coffee maker.

Whatever the cause, with conventional con-stant duct pressure control, the building operatoronly learns about these problems when someonecomplains about comfort or noise. However, with fan-pressureoptimization, the BAS regularly gathers data from each VAVterminal, providing the opportunity to identify and fix theserogue zones.Figure 3includes a chart that trends the position of VAV

dampers in an actual building. In the morning, most of thedampers open a little after 7 a.m. to warm up the zones, butthen they close down quite a bit, indicating that not much cool-ing is needed on this day. The VAV terminal serving Room204, however, is nearly wide open for most of the day, and ispreventing the duct pressure setpoint from being reset down-ward to reduce fan energy.This suggests that there may be a problem with this zone.

During the first few months of operation, the building op-erator can review this trend periodically, identify any po-tential rogue zones, and add them to his to be fixed list. Ifthe BAS has the capability to temporarily exclude a rogue

zone from the control sequence, it would allow that VAV

Scheduled System Operation

Zone Temperature

Occupied HoursZoneTemperature

Drift Below

OccupiedSetpoint

OccupiedHeatingSetpoint

UnoccupiedHeatingSetpoint

6 a.m. Noon 6 p.m.

Optimal Start Optimal Stop

Figure 1:Optimal start/stop.

Communicating BAS

Supply Fan

Static PressureSensor

DamperPositions

VAV

TerminalUnits

Figure 2:Fan-pressure optimization.

terminal to operate as normal and attempt to control zonetemperature, but would not have a vote when determiningthe optimized duct static pressure setpoint. After a while,all rogue zones would be identified and the fan-pressureoptimization sequence should be operating without one

dominant, or rogue, zone. At that time, a technician can bedispatched to fix all the rogue zones and re-include them inthe sequence.

Supply-air-temperature reset. In a VAV system, it istempting to raise the supply-air temperature (SAT) at part-load conditions to save compressor and/or reheat energy.Increasing the SAT reduces compressor energy because it al-lows the compressors to unload or cycle off. In addition, SATreset makes an airside economizer more beneficial. Whenthe outdoor air is cooler than the SAT setpoint, the compres-sors are shut off, and the outdoor- and return-air dampersmodulate to deliver the desired supply-air temperature. A

warmer SAT setpoint allows the compressors to be shut off

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ZoneVAVDamperPosition Room 204

Average

6 a.m. 8 a.m. 10 a.m. 12 p.m. 2 p.m. 4 p.m. 6 p.m.

Figure 3:Identifying rogue zones.

sooner and increases the number of hours whenthe economizer is able to provide all the neces-sary cooling.

For zones with low cooling loads, when thesupply airflow has been reduced to the minimum

setting of the VAV terminal, raising the supply-air temperature also decreases the use of reheatat the zone level. However, because the supply airis warmer, those zones that require cooling willneed more air to satisfy the cooling load. Thisincreases supply fan energy.

Finally, in climates that experience humidweather, warmer supply air means less dehumidi-fication at the coil and higher humidity levels inthe zones. If dehumidification is a concern, usecaution when implementing this strategy.

SAT reset should be implemented so that it min-imizes overall system energy use. This requiresconsidering the trade-off between compressor,reheat, and fan energy, while not ignoring spacehumidity levels. Although there are several dif-ferent approaches to implementing this strategy,many of the papers and articles written on thesubject tend to agree on some general principlesfor balancing these competing issues.1,2,3

First, when it is warm outside, keep the SATcold. This takes advantage of the significant en-ergy savings from unloading the fan. Then, beginto raise the SAT setpoint during mild weatherwhen it can enhance the benefit of the airside

economizer and reduce reheat energy.Although there are several possible control se-

quences, the example in Figure 4demonstratesone way to attempt to balance these competingimpacts of SAT reset.

With this approach, the SAT setpoint is resetbased on the changing outdoor dry-bulb tem-perature. When the outdoor dry-bulb tempera-

Finally, the amount of reset is limited, to 60F (16C) in thisexample, which allows the system to satisfy cooling loads ininterior zones without needing to substantially oversize VAVterminals and ductwork.

A possible drawback of this approach is that if a zone has ahigh cooling load, even when it is cool outside, it is possiblethat the warmer supply air may not provide enough coolingand that zone may overheat. To prevent this from occurring,when SAT reset does takes place, the amount of reset shoulddepend on the cooling need of the worst-case zone. (Zonesthat are expected to have near-constant cooling loads shouldbe designed for the maximum reset SAT.)

As depicted in the example in Figure 4,when it is 50F(10C) outside, the system will attempt to raise the SAT set-point to 60F (16C). However, if there is a zone that is atnear-design cooling load, this air may not be cold enough. Us-

ing the current temperature and VAV damper position for that

Figure 4:Example of supply-air temperature (SAT) reset control.

45 50 55 60 65 70 75

Supply-A

irTemperatureSetpoint(F) 61

60

59

58

57

56

55

Reset Based onWorst-Case Zone

Outdoor Dry-Bulb Temperature (F)

ture is warmhigher than 65F (18C) in this examplenoreset takes place and the SAT setpoint remains at its designvalue of 55F (13C). When it is warm outside, the outdoor airprovides little or no cooling benefit for economizing, and the

cooling load in most zones is likely high enough that reheatis not required to prevent over-cooling. Keeping the air coldallows the fan to turn down, taking advantage of the energysavings from reducing airflow. In addition, the colder supply-air temperature allows the system to provide sufficiently dryair to the zones, improving part-load dehumidification.

When the outdoor temperature is cooler, the controls beginto reset the SAT setpoint upward. At mild or cold outdoor tem-peratures, reset enhances the benefit of the economizer, and ifthere is any zone-level reheat, it is reduced or even avoided.At these cooler temperatures, the supply fan has likely alreadyunloaded significantly, so the incremental energy use of hav-

ing to deliver a little more air is lessened.

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zone, the controls can determine that this amountof reset is too much, and either change the SATsetpoint back to 55F (13C), or lower the SATsetpoint a degree or so and see if that eliminatesthe overheating problem.

Ventilation optimization. In a typical VAVsystem, the VAV air-handling (or rooftop) unitdelivers fresh outdoor air to several, individuallycontrolled zones. Demand-controlled ventilation(DCV) involves resetting intake airflow in response to varia-tions in zone population. Although commonly implementedusing carbon dioxide (CO2) sensors, occupancy sensors ortime-of-day (TOD) schedules can also be used.

One approach to optimizing ventilation in a multiple-zoneVAV system is to combine these various DCV strategies at thezone level (using each where it best fits) with ventilation resetat the system level.

In the example system depicted inFigure 5,CO2sensors areinstalled only in those zones that are densely occupied and ex-perience widely varying patterns of occupancy (such as con-

ference rooms, auditoriums, or a lounge area). The VAV con-troller resets the ventilation requirement for that zone basedon the measured CO2concentration.

Zones that are less densely occupied or have a populationthat varies only a little (such as private offices, open plan of-fice spaces, or many classrooms) are probably better suitedfor occupancy sensors. When a zone is unoccupied, the VAVcontroller lowers the ventilation requirement for that zone,typically to the building-related ventilation rate, Ra, requiredby ASHRAE Standard 62.1.

Finally, zones that are sparsely occupied (such as open of-fice areas) or have predictable occupancy patterns (such as

cafeterias or auditoriums) may be best controlled using a time-of-day schedule. This schedule can either indicate when thezone will normally be occupied versus unoccupied, or can beused to vary the ventilation requirement based on anticipatedpopulation of that zone.These various zone-level DCV strategies can be used to reset

the ventilation requirement for their respective zones. This zone-level control is then tied together using ventilation reset at thesystem level (Figure 5). In addition to resetting the zone ventila-tion requirement, the controller on each VAV terminal continu-ously monitors primary airflow being delivered to the zone.The BAS periodically gathers the current ventilation re-

quirement and primary airflow from all the VAV control-

Communicating BASNew OA Setpoint (V

ot)

Per ASHRAE Standard 62.1

CO2 OCC TOD

DDC/VAV Terminals Required Outdoor Airflow (TOD, OCC, CO2)

Actual Primary Airflow (Flow Ring)

SA

VAV Air-Handling UnitWith Flow-Measuring

OA Damper

Reset Outdoor Airflow

SA RA

OA

Figure 5:Ventilation optimization (DCV at zone level + ventilation reset at

system level).

lers and solves the ventilation reset equations (prescribed byASHRAE Standard 62.15) to determine how much outdoor airmust be brought in through the system-level OA intake to sat-isfy all zones served. Finally, the BAS sends this outdoor air-flow setpoint to the VAV air-handling (or rooftop) unit, whichmodulates a flow-measuring outdoor-air damper to maintainthis new ventilation setpoint.

Occupied standby mode.When an occupancy sensor isused in combination with a time-of-day schedule, the sensorcan be used to indicate if the zone is unoccupied although theBAS has scheduled it as occupied. This combination can be

used to switch the zone to an occupied standby mode.In this mode, all or some of the lights in that zone can be

shut off, the temperature setpoints can be raised or loweredby 1F to 2F (0.5C to 1C), and the ventilation requirementfor that zone can be reduced, typically to the building-relatedventilation rate,Ra, required by Standard 62.1.

In addition, for a VAV system the minimum airflow settingof the VAV terminal can be lowered to avoid or reduce theneed for reheat. This minimum airflow setting is typically setto ensure proper ventilation. However, when nobody is in theroom and with the ventilation requirement reduced, the mini-mum airflow setting can be lowered significantly during this

occupied standby mode. This reduces both reheat and fan en-ergy use.

When the occupancy sensor indicates that the zone is againoccupied, these settings are switched back to normal occupiedmode.

Variable airflow during heating.The conventional wayto control a VAV reheat terminal has been to reduce primaryairflow as the zone cooling load decreases. When primary air-flow reaches the minimum setting, and the cooling load con-tinues to decrease, the reheat coil is activated to warm the airand avoid overcooling the zone. When this occurs, the airflow-modulation damper typically maintains a constant heating air-

flow.

Optimal start (Section 6.4.3.3.3) and zone-

level demand-controlled ventilation (Sec-

tion 6.4.3.9) are mandatory requirements of

ASHRAE/IES Standard 90.1-2010.6Fan-pres-

sure optimization(Section 6.5.3.2.3), supply-

air-temperature reset (Section 6.5.3.4), andsystem-level ventilation reset(Section 6.5.3.3)

are prescriptive requirements of the standard.

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Figure 6:Control of a VAV reheat terminal to vary airow during heating.

100%

0%

MaximumPrimaryAirflow

MaximumPrimary Heating

Airflow

MinimumPrimary Cooling

Airflow

DesignHeating Load

Space Load DesignCooling Load

50%

20%

Deadband

90F

Maximum Limit

PercentAirflowto

Space

DischargeAirTemperatureSetpoint

90F

55F

Heating Coil Activated

DischargeAir

TemperatureSetpoint

Figure 6depicts an alternate methodto control a VAV reheat terminal. Whenthe zone requires cooling, the control se-quence is unchanged; primary airflow isvaried between maximum and minimum

cooling airflow as needed to maintainthe desired temperature in the zone.When primary airflow reaches the

minimum cooling airflow setting, andthe zone temperature drops belowthe heating setpoint, the heating coilis activated to warm the air to avoidovercooling the zone. As more heatis needed, the controller resets thedischarge-air temperature setpoint up-ward to maintain zone temperature atsetpoint (orange dashed line in Figure6), until it reaches a defined maximumlimit90F (32C) in this example. The discharge tempera-F (32C) in this example. The discharge tempera-32C) in this example. The discharge tempera-C) in this example. The discharge tempera-. The discharge tempera-ture is limited to minimize temperature stratification whendelivering warm air through overhead diffusers.

When the discharge-air temperature reaches this maximumlimit and the zone requires more heating, primary airflow isincreased while the discharge-air temperature setpoint re-mains at this maximum limit. The result is that the airflow-modulation damper and hot-water valve will modulate opensimultaneously.

By actively controlling the discharge-air temperature, it canbe limited so that temperature stratification and short circuit-ing of warm air from supply to return are minimized when the

zone requires heating. This improves occupant comfort andresults in improved zone air-distribution effectiveness, whichavoids wasteful over-ventilation.Note: Section 6.5.2.1 of Standard 90.1-2010 all ows this

alternate control strategy (as long as the maximum heatingpr imary airflow is less than 50% of maximum cool ing pr imaryairflow) as an exception to comply wi th the Standards pre-scri ptive limitati on on simultaneous heating and cooli ng.

Cold-Air Distribution

Another key ingredient of some high-performance VAVsystems, especially chilled-water VAV systems, is lowering

the supply-air temperature.1,4

Supplying air at a colder temperaturebetween 45F to 52F(7C to 11C) for exampleallows the system to deliver a lowersupply airflow rate. This can significantly reduce fan energy use,and it can also allow fans, air-handling units, and VAV terminalsto be downsized, which reduces installed cost. Sometimes, duc-twork is downsized also, which further reduces installed cost.

Another potential benefit is that delivering colder air meansthat the air is drier, which can lower indoor humidity levels inclimates that experience humid weather.

Although supplying air at a colder temperature reducesfan energy use, it requires the use of a colder chilled-water

temperature (which impacts chiller efficiency), increases

reheat energy, and results in fewer hours when the airsideeconomizer can provide all the necessary cooling. Althoughthe lower humidity levels that result from colder air may beappreciated in some applications, this extra dehumidifica-tion results in an increased latent load on the chiller. Thisincreased latent load is partially offset by a reduction in thesensible load due to fan heat (reduced fan power). Theseimpacts partially offset the fan energy savings. Therefore,whole-building energy simulation should be used to deter-mine the impact of a lower supply-air temperature on theoverall energy use of a VAV system.The first tip to maximizing energy savings in a cold-air VAV

system is to reset the SAT setpoint upward during mild weath-er. As explained previously, this helps maximize the benefit ofthe airside economizer and reduces reheat energy use, whilestill achieving fan energy savings during warm weather.The second tip is to try raising the zone temperature set-

point by one or two degrees. Since people are comfortableat warmer temperatures when humidity is lower, the lowerhumidity levels that occur with cold-air systems provide theopportunity to slightly raise the zone cooling setpoint. Thisfurther reduces airflow and fan energy use, and reduces cool-ing energy a little too.

Next, while lowering the supply-air temperature can allow

the ducts to be downsized to reduce installed cost, if a goal ofthe project is to maximize energy savings, consider designingthe system for the colder supply-air temperature but not down-sizing the ductwork as much as possible. Keeping the ducts alittle larger reduces fan energy and allows SAT reset to be usedwithout concern for any zones with near-constant cooling loads.It also improves the ability of the air distribution system to re-spond to any future increases in load, since it will be capable ofhandling an increased airflow rate if needed. Use caution whenoversizing VAV terminals to ensure that they are able to prop-erly operate across the entire range of expected airflows.

Challenges.Of course, cold-air VAV systems are not with-

out challenges. Design engineers typically express two con-

55F

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2 6 A S H R A E J o u r n a l O c t o b e r 2 0 1 1

cerns with this approach: 1) minimizingcomfort issues related to cold air dump-ing on the occupants, and 2) avoidingcondensation on components of the airdistribution system. A great resource

for anyone designing a cold-air systemis the ASHRAE Cold Air Distri butionSystem Design Guide, which discussesin detail how to avoid these problems.4

Linear slot diffusers with a high in-duction ratio are good for any VAV sys-tem, but work very well for a cold-airVAV system. They induce large quan-tities of air from the space to mix withthe cold supply air, so the mixture dropsslowly into the occupied zone at nearlyroom temperature. On the other hand, many conventional dif-fusers allow cold air to drop directly into the occupied zone,which can result in occupant comfort complaints, especiallyat reduced airflows.

Since the surfaces of the air distribution components in alow-temperature system are colder than in a conventional sys-tem, there is often concern about condensation. To minimizethe risk of condensation:

Figure 7:Example energy analysis for a large ofce building.

6 Million

4 Million

2 Million

AnnualHVACEne

rgyUse(kBtu/yr)

Houston Los Angeles Philadelphia St. Louis

Pumps

Fans

Heating

Cooling

1 | 2 | 3 1 | 2 | 3 1 | 2 | 3 1 | 2 | 3

1 Baseline Chilled Water VAV | 2Active Chilled Beams | 3 High Performance Chilled Water VAV

Insulate all the cold surfaces, including supply ducts, VAVterminals, and diffusers. In addition, a properly sealed vaporretarder should be included on the warm side of the insulationto prevent condensation within the insulation.

If possible, use an open ceiling plenum for return air. Thisresults in a conditioned plenum, which means a lower dewpoint and less risk of condensation.

During humid weather, maintain positive building pressur-ization to reduce or eliminate the infiltration of humid outdoorair.

During startup, slowly ramp the SAT setpoint downwardto slowly lower surface temperatures while lowering the dewpoint inside the building.

Putting It All TogetherAlthough this article focused on the airside of VAV systems,

many opportunities exist to reduce energy on the equipment orplant side of the system. Some examples include:

High-efficiency rooftop equipment, water chillers, and air-handling unit fans;

Air-to-air energy recovery; Evaporative condensing; Low-flow, low-temperature chilled-water systems; Waterside heat recovery; Central geothermal; and Solar heat recovery for reheat.

The impact of any of these strategies on overall operat-ing costs depends on climate, building use, and utility costs.

Therefore, whole-building energy simulation should be usedto determine if a specific strategy makes sense for a given ap-plication. As an example, Figure 7contains the results froma whole-building energy simulation of a large office build-ing, comparing a typical VAV system to a high-performancechilled-water VAV system.The baseline building uses a conventional chilled-water

VAV system, designed for 55F (13C) supply air, and mod-eled according to Appendix G of Standard 90.1-2007. Thehigh-performance VAV system is designed for 48F (9C)

supply air (ductwork has not been downsized) and uses the op-

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2 8 A S H R A E J o u r n a l O c t o b e r 2 0 1 1

timized VAV control strategies mentioned in this article, and alow-temperature, low-flow water-cooled chiller plant.

For this example, the building with the high-performanceVAV system uses about 20% less energy than the baselineVAV system in Houston, Philadelphia, or St. Louis. The build-

ing uses about 10% less in Los Angeles, which has milderweather and lots of hours for airside economizing. As a com-parison, the high-performance VAV system uses 5% to 10%

less energy than an active chilled beam system that was mod-eled for this same building.

Finally, over the past several years, a series of AdvancedEnergy Design Guides, have been jointly developed by theU.S. Department of Energy, ASHRAE, the American Institute

of Architects, the Illuminating Engineering Society, and theU.S. Green Building Council.7These guides include climate-specific recommendations that can be used to achieve 30% (or

in some cases 50%) energy savings overconventional design. Seven guides arecurrently available in this series, cover-ing building types from office buildingsto schools to warehouses.

Most of these guides include severaloptions for HVAC systems. In several ofthem, VAV systems are one of the optionscovered that can help the overall buildingachieve the stated energy-savings thresh-old. For example, in the recently pub-lished guide for small- and medium-sizedoffice buildings, a high-performancerooftop VAV system is included as oneof the options that can be used to achieve50% energy savings. In the guides forK-12 school buildings and small health-care facilities; rooftop VAV and chilled-water VAV systems are included as op-tions for achieving 30% energy savings.These guides, and the whole-building

energy simulations that were used to

confirm the climate-specific recommen-dations contained in them, provide vali-dation that VAV systems can be used inhigh-performance buildings.

References1. Murphy, J., B. Bakkum. 2009. Chilled-

Water VAV Systems. La Crosse, Wis.: Trane.

2. Energy Design Resources. 2009. Ad-vanced Var iable Air Volume System DesignGuide. Sonoma, Calif.

3. Wei, G., M. L iu, D. Claridge. 2000.Optimize the supply air temperature reset

schedule for a single-duct VAV system,Proceedings of the Twelfth Symposium onImproving Bui ldi ng Systems in Hot and HumidClimates.

4. ASHRAE. 1996. Cold Air D istributionSystem Design Guide.

5. ANSI /ASHRAE Standard 62.1-2010,Ventilati on for Acceptable Indoor Air Quali ty.

6. ANSI/ASHRAE/IESNA Standard 90.1-2010,Energy Standard for Buil dings ExceptLow-Rise Residential Buil dings.

7. ASHRAE. Advanced Energy DesignGuideseries. www.ashrae.org/technology/page/938.


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