A S H RA E J O U R N A L

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The following article was published in ASHRAE Journal, January 1998. © Copyright 1998 American Society of Heating, Refrigerating and Air-Conditioning Engineers,Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE.

Outside Air Ventilation Control and Monitoring

By Eric G. Utterson

and

Harry J. Sauer, Jr., Ph.D., P.E.Fellow ASHRAE

ndoor Air Quality (IAQ) is consid-ered acceptable in buildings if there

are no known harmful contaminants inthe air and if a substantial majority ofpeople exposed do not express dissatis-faction.1 Unacceptable IAQ has led to anincreased number of complaints of head-aches, eye and throat irritation, andbreathing difficulties.

One accepted method of preventingIAQ problems is fresh air dilution. This isaccomplished by supplying fresh outsideair into the building and exhausting con-taminated return air. ANSI/ASHRAE Stan-dard 62-1989, Ventilation for AcceptableIndoor Air Quality, a standard written intomany building codes, requires at least 20cfm (10 L/s) of outside air to be suppliedfor every office building occupant.

With variable-air-volume (VAV) sys-tems, maintaining a constant or con-trolled ventilation rate is not achievablewithout some control method. In VAVsystems, the supply air temperature isconstant, and the amount of supply air-flow to individual spaces is modulated tocontrol return air temperature. Similarly,outside airflow changes with the build-ing cooling load. When VAV systemswithout outside air controls are used, thepossibility of having IAQ problemsincreases.

The goals of our research were toinstrument a VAV building on the cam-pus of the University of Missouri-Rolla(UMR) with IAQ measuring devices, toinvestigate methods of ventilation con-trol on VAV systems, to modify thebuilding control system to achieve ven-tilation control, and to observe and ana-lyze IAQ before and after systemchanges. The IAQ measuring devicesused for this study include a total volatileorganic matter (VOM) sensor, a real-time dust sensor and CO2 sensors. Othermeasurements include temperature andrelative humidity at various locations,outside airflow and space pressure.

In addition to these parameters, thebuilding was studied in normal opera-tion, as well as when the economizerdampers were used to maintain the min-imum outside airflow rate. As expected,CO2 and VOM levels decrease when theoutside air ventilation rate increases.

Additionally, methods of controllingoutside air on a VAV building werestudied using computer simulation. Theresults indicate that building perfor-mance can be improved when the econ-omizer is controlled by the ventilationrate when ambient air conditions callfor operation with minimum outdoorair.

Castleman HallConstructed in 1990, Castleman Hall

houses the music, theater and fine artsdepartments for UMR. Located insidethis building are offices, classrooms,music practice rooms, a large theater andstage, and instrument storage rooms. Sixdifferent air handlers are used in thebuilding. Three constant volume (CV)systems serve the theater, basement, andwork room. Three VAV systems servethe alumni center, music wing, and frontlobby. For the purpose of this study, onlythe music wing VAV system is evalu-ated. The music wing of Castleman Hallis displayed as Figure 1.

The HVAC system used for the musicwing is a VAV system with two fans (seeFigure 2). The supply fan speed is con-trolled to maintain a supply duct staticpressure of 1.75 in. of water (435 Pa).The return fan is set to turn at the samespeed (RPM) as the supply fan. There isno direct space pressure control in thisarrangement.

Figure 3 shows the arrangement ofequipment used on the air handler for themusic wing. Included on this figure isthe location of the sensors used in thisstudy. A hot-film anemometer, x-gridpitot tube anemometer, and CO2 sensorare mounted approximately 10 ft (3 m)

About the Authors

Eric G. Utterson is an HVAC Engineerwith AJT and Associates in Huntsville,Ala. During this research, he receivedsupport from ASHRAE through an Un-dergraduate Grant-In-Aid and a Gradu-ate Grant-In-Aid. He received hisbachelor and master degrees in mechan-ical engineering from the University ofMissouri, Rolla. Harry J. Sauer,Ph.D., P.E., is Professor of Mechanicaland Aerospace Engineering at the Uni-versity of Missouri-Rolla.

I

Fig. 1: Castleman Hall music wing.

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before the minimum outside air damperand economizer inlet damper. Particu-late concentration is measured beforeentering the return fan and after return-ing from the space. Finally, both theVOM sensor, a second CO2 sensor and atemperature/relative humidity sensor aremounted after the return fan. Each ofthese devices measure continuously.

Because Direct Digital Control(DDC) is available on this building,most modifications to the controlscheme are possible through softwarechanges. The control system utilizes afinite time difference proportional feed-back control, proportional integral (PI)feedback control and proportional inte-gral differential (PID) control. Whendirect ventilation control is used on thissystem, the PID controller receives asignal from the outside air duct-mountedanemometer only.

For most of the measurements in Cas-tleman Hall, automatic data acquisitionis accomplished with a PC and dataacquisition board. This system allowsfor eight channels of analog voltageinputs, with a sampling rate of roughlyone sample per second (for all eightchannels). The analog to digital conver-sion provides 16 bits of resolution over a+/− 5 volt range. This device is well-suited for measuring small voltage dif-ferences encountered with pressure sen-sors and the particulate sensor.

Data acquisition was controlled bysoftware that enables the user to pro-gram the sampling time. For most of thisstudy, the computer was programmed tosample every half hour on the half hour.Data was stored directly on the com-puter’s hard drive.

Ventilation Control on VAV BuildingsDirect control of ventilation on VAV

buildings has been successfully tried inthe past. Janu, et al.2 describe a controlscheme which utilizes the economizerset of dampers to control outside airbased on CO2 concentration and theminimum ventilation requirement.Graves3 describes another system ofventilation control that utilizes mixed airplenum pressure and the return airdamper to maintain the minimum venti-lation requirement.

For this study, the economizer is con-trolled to provide a constant ventilationrate by opening and closing the econo-mizer damper. If the economizer can be

used to save energy while maintainingminimum ventilation requirements, thesystem behaves as usual. However, if theoutside airflow rate tends to go higher orlower than the minimum requirementduring non-economizing times, theeconomizer damper is modulated tomaintain minimum required outside air.The logic for this control scenario is dis-played in Figure 4.

When the outside air temperature liesbetween the supply air temperature andthe set point temperature (a temperaturewhich is several degrees below thereturn air temperature to account forpotentially higher humidity in the out-side air), the economizer will open upfully to save cooling costs. When theoutside air temperature is above the setpoint air temperature, the economizerwill modulate to maintain minimum out-

side air. If the outside air temperaturefalls below the supply air temperature,the economizer modulates to maintainminimum outside air and attempts toraise the mixed air temperature to that ofthe supply air temperature.

In addition to making control systemchanges, the dampers must also be mod-ified to enable full control. For Castle-man Hall, the minimum outside airdamper was manually closed, and theeconomizer damper and return airdamper were modulated for control. In anew building, a minimum outside airdamper would not be needed.

When these changes were imple-mented for a sample case, the energysavings occur mainly in the summermonths when large zone load fluctua-tions during the day can result in corre-sponding fluctuations in economizer

Fig. 2: VAV system layout.

Fig. 3: Equipment layout.

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damper position and outside airflow rate. Figure 5 displays atypical ventilation trend for a VAV system. For this samplecase, the economizer set point and supply air temperatureswere 70°F and 55°F (21°C and 13°C), respectively. If outsideair temperature is between 70°F and 55°F (21°C and 13°C),the economizer is fully open. Between 55°F and 45°F (13°Cand 7°C), the economizer modulates to maintain 55°F (13°C)mixed air. At temperatures above 70°F (21°C) and below 45°F(7°C), the economizer damper is closed. Air ventilates solelythrough the minimum outside air damper in this temperaturerange. For this example, the minimum required outside air is9000 cfm (4200 L/s). During non-economizer operation, ven-tilation above the minimum requirement represents wastedcooling or heating energy.

If the economizer control system is modified as outlined ear-lier, the trend of Figure 6 can be obtained. With the direct ven-tilation control, energy is no longer wasted, as was indicated for

the operation of Figure 5. Simulation results indicate that forthis example, between 1.7 and 3.3% of the annual energy billcan be saved utilizing minimum ventilation control.

IAQ Measurements With and WithoutDirect Ventilation Control

The main purpose of instrumenting this building with IAQmeasuring devices was to determine and examine any changesthat occur after the building control system has been modified.Data collection took place over a two-week period. In the firstweek, the building control system was modified to maintain3200 cfm (1500 L/s) (34% of the peak supply air required duringthis period) of outside air by modulating the economizer damp-ers. With the limited accuracy and stability of the electronicmanometer at very low flow rates, full-range energy-savingeconomizer control could not be utilized.

During the second week, the system was restored to its reg-ular control scheme with one change: the economizer was notallowed to open. This allowed for an IAQ comparison with theventilation controlled system data. Outside air quantities dur-ing normal operation tend to be around 1800 cfm (850 L/s)(17% of the peak supply air).

The data displayed on Figure 7 were from the week inwhich outside air ventilation was controlled to 3200 cfm(1500 L/s). In general, for CO2, the concentration tracks build-ing occupancy. After lunch and dinner breaks, there are corre-sponding drops in CO2 concentration. During the evening,CO2 concentration builds up because the building is occupiedlate into the night. The VOM reading is less predictable thanthe CO2. However, many of the VOM peaks occur at the sametime as CO2 peaks. Because initial testing determined that CO2concentrations of 1000 ppm did not register on the VOM sen-sor, it is apparent that the VOM sensor is detecting a possiblepollutant. Particulate trends do not seem to follow any occu-pancy-related trend. When the air handler is off, the particulatesensor does not detect particulate.

Figure 8 (which displays data from a week in which outside airventilation was not directly controlled) displays trends similar toFigure 7. The main difference between Figure 7 and 8 is the mag-nitude of the peak concentration levels. The peaks during thedirect outside air control case (Figure 7) were lower than in thenon-outside air control case (Figure 8). This is significant because

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Fig. 4: Control scheme for economizer controlled ventilation.

Fig. 5: Computer simulated typical ventilation trend.

Fig. 6: Ventilation controls the economizer (computersimulation results).

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the scheduled occupancy during both peri-ods was the same.

A closer look at CO2 concentration onFigure 9 reveals another noteworthytrend. The outside air duct CO2 concen-tration was higher during the non-out-side air control case. This was notexpected because the ambient level ofCO2 does not vary by amounts seen onthese figures. Further investigationrevealed that this resulted from a reverseairflow condition in the common outside

air plenum that serves both the musicwing system and the alumni center sys-tem.

Apparently, the alumni center HVACsystem (also a VAV system) wasexhausting through its minimum outsideair intake. Figure 11 displays pictures ofthe outside air intakes. Air streamersillustrate the air exhausting from thealumni center intake. Because both out-side air intakes are located within 10 ft (3m) of each other, some return air from

the alumni center vents into the musicwing system. This certainly can cause apotential IAQ problem. This flow can beattributed to interactions among the sixair handlers with many open connectingcorridors acting as common return ple-num.

If the outside air concentration wassubtracted from the return air concentra-tion, the return air CO2 concentration was7.3 +/− 2.9 ppm higher on average (95%confidence) when ventilation was not

Fig. 7: Data from Sept. 25 to Oct. 1, economizer controlsventilation to 3200 cfm (1500 L/s).

Fig. 8: Data from Oct. 9 to Oct. 15, economizer closed afterMonday.

Fig. 9: Direct comparison of IAQ with and without ventilation control.

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controlled. This trend was expected because the higher ventila-tion rate associated with the direct outside air control case pro-vides more dilution. This indicates that IAQ improvement isachievable with the modified control system.

Comparison of the two VOM trends produces less conclu-sive results. VOM spikes do not appear to be as time depen-dent as CO2 (Figure 9). Comparing data from the periodbetween Tuesday and Friday (both outside air controlschemes), more VOM was detected during the non-outside aircontrol case (average 3.3% versus average 2.5% of full scale).This indicates that some IAQ improvement was achieved.

Particulate trends remain puzzling. During the occupied time,the concentration initially rose to levels that are unacceptablebased on Standard 62-1989 and then decreased during the after-noon. Particulate concentration was indicated as zero during thehours when the system was not in operation and there was noairflow past the sensor. As with VOM, particulate concentrationwas lower on average during the direct outside air control situ-ation (average 0.24 vs average 0.26 mg/m3).

Space Pressure MeasurementSpace pressure was measured in the interior room. The out-

side pressure port was shielded to minimize the effects ofwind. During this test, space pressure was measured every tenminutes throughout a 24-hour period while the economizerwas used to directly control ventilation. Figure 11 is a graphfrom this data collection. Because space pressure varied errat-ically, a running average was used to show the general trend ofspace pressure. The space pressure trend was similar to the pat-tern for CO2 concentration.

It should be emphasized that the HVAC control system containsno provision for direct control of building pressure. What building

pressure control there is results from the control of the return airfan speed as directly proportional to the supply fan speed.

ConclusionsWith relatively simple and inexpensive hardware modifica-

tions, outside air ventilation control is achievable on present VAVbuildings that currently do not have direct ventilation controlschemes. In general, the economizer damper can be used to main-tain a constant supply of outside air at all times. Furthermore, thiscontrol scheme can be used to save significant amounts of energywhile conforming to the requirements of Standard 62-1989.

Castleman Hall displays regular predictable trends for IAQcomponents. While the VOM sensor appears to detect contam-inant on a semi-regular basis, the output is not directly trace-able to any human irritant. Although the results of this studyare not conclusive in this regard, the use of this device to con-trol ventilation is highly suspect at this time.

AcknowledgmentsThe authors wish to acknowledge with thanks the support

provided to the first author by an ASHRAE Graduate Grant-In-Aid. The authors are also grateful to the university’s phys-ical facilities department for their permission and assistance inmodifying, instrumenting and monitoring the HVAC system inCastleman Hall.

References

1. ASHRAE, “Ventilation for Acceptable Indoor Air Quality,”ASHRAE Standard 62-1989.

2. Janu, G., Wenger, J., Nesler, C., “Outdoor Air Flow Control forVAV Systems,”ASHRAE Journal, April 1995, p. 62

3. Graves, Larry R., “VAV Mixed Air Plenum Pressure Control,”Heating, Piping, Air Conditioning, August 1995, p. 53.

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Fig. 10: Outside air intakes for the Music Wing (above) and the Alumni Center (left).

Fig. 11: Space pressure when the economizer controlsventilation.

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