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Optimizing Ventilation Performance in Commercial BuildingsDavid W. Bearg, PE, CIH, Life Energy Associates, 20 Darton Street, Concord, MA 01742

978-369-5680 or sagefarm@aol.com

1.0 Introduction

The goal of Optimizing Ventilation Performance in Commercial Buildings is to achieve anddocument the delivery of adequate ventilation to the building occupants in such a way thatno more energy is required to accomplish this task then is absolutely necessary. The deliv-ery of adequate ventilation to building occupants is important because it is a basic require-ment for achieving good indoor air quality. As with any other building performance param-eter, it cannot be assessed and managed unless the amount of actual ventilation is knownwith some degree of certainty. Additional reasons for the understanding of ventilationperformance include the application of a Green Building philosophy, with the HVAC systemfunctioning as intended both initially and over the life of the building. The ability to assessventilation performance can also be considered a component of Commissioning for much thesame reason. This presentation will therefore include a discussion of how ventilation can beevaluated.

2.0 Assessing Ventilation Performance

The quantity of outdoor air entering the HVAC system does not necessarily indicate howmuch outdoor air actually gets delivered to the building occupants as ventilation. This isbecause of the critical role of the distribution components of the HVAC system in transfer-ring this outdoor air. In order to assess ventilation, feedback is needed on the quantity ofoutdoor air actually delivered to the occupants. One approach to this determination is tomeasure some component of the indoor air that is given off by the occupants in some pre-dictable manner. Since people exhale carbon dioxide (CO2) in concentrations that are about100 times greater than in clean outdoor air, its measurement can provide an excellent assess-ment of ventilation performance. That is, the exhaled concentration of CO2 is about 38,000to 40,000 parts per million (ppm), while clean outdoor air has a CO2 concentration of 380 to400 ppm. Recognizing that there is little hope of fine tuning ventilation performance unlessinformation on this performance is readily available, one needs to consider the best way tocollect this information.

3.0 Obtaining Information on Ventilation Performance

There are several ways of obtaining information on ventilation performance with the mea-surement of CO2 concentrations. Options vary from just a few grab samples, up throughcontinuous monitoring with automatic data logging. In all cases, some information onventilation performance is provided. The question remains, however, as to the minimum

There are several ways of obtaining information on ventilation performance with the mea-surement of CO2 concentrations. Options vary from just a few grab samples, up throughcontinuous monitoring with automatic data logging. In all cases, some information onventilation performance is provided. The question remains, however, as to the minimumlevel of CO2 monitoring data necessary for the optimization of ventilation performance. Thecriteria for assessing this include both the accuracy of the CO2 monitoring data and howrepresentative it is of building conditions. Accuracy is a function of the calibration of theCO2 detector and whether or not it is unduly influenced by the direct exhaled breath of eitherthe building occupants or the investigator. Unfortunately it has been my experience that CO2detectors have a definite tendency to drift. Therefore, if one is to have confidence in the CO2monitoring data obtained, there must be periodic calibration checks of the detector response.4.0 Representativeness of CO2 Monitoring Data

Since individual CO2 measurements reflect the dynamic interaction of the number and dura-tion of the people present and the ability of the HVAC system to dilute and remove aircontaminants, one should have a continuous polling of CO2 concentrations through the dayto determine both the maximum and minimum differences between indoor and outdoorvalues. The focus on the difference between indoor and outdoor CO2 concentrations comesfrom information in ASTM D6245, Standard Guide for Using Indoor Carbon Dioxide Con-centrations to Evaluate Indoor Air Quality and Ventilation. This resource not only points outthat it is this difference that can be used to calculate ventilation rates, it also states that whenusing CO2 concentration for the purposes described in this guide, the outdoor CO2 concentra-tion must be measured. This document also mentions a distance of 2 meters from anyoccupant as sufficient to avoid the effects of the air exhaled by individual people.

In terms of being representative of the building, the more locations included the better. Thelocations monitored should include both those spaces with the greatest potential for thehighest occupancies and those spaces served by the ends of the HVAC distribution system.5.0 Shared-Sensor, Vacuum-Draw Approach to CO2 Monitoring

One approach to obtaining the information necessary for optimizing ventilation performanceis with a monitoring system that uses a vacuum-draw, shared-sensor technology. With thisapproach, just one CO2 sensor is shared among up to 48 locations, with air from all locationsbeing analyzed by just this one individual sensor. The advantages of this approach overdistributed sensors are significant. This way anydifferences observed between differing locations will definitely be real, as opposed to possi-bly just being due to different responses of different sensors. In addition, the vacuum-drawapproach makes it easier to maintain the single sensor in calibration as compared with main-taining the calibration on numerous distributed sensors. A additional advantage of thevacuum-draw approach accrues from the greater flexibility of locating the end of the sam-pling tube approximately 2 meters from the nearest person as compared with distributedsensors that might be impacted by people much closer than 2 meters.

6.0 Calculating Ventilation Rates from CO2 Concentrations

People generate CO2 as a function of their metabolism. This generation rate is therefore afunction of their activity level. Published values of this generation rate place this CO2 gen-eration rate at 0.0106 cubic feet per minute (0.3 liters per minute). Multiplying this value bya million, to make it consistent with CO2 readings expressed in terms of parts per million,yields an equation where at equilibrium the ventilation rate is equal to 10,600 divided by thedifference between indoor and outdoor CO2 concentrations in ppm. For example, if the goalis to achieve a minimum ventilation rate of 20 cfm of outdoor air per person (for adultsdoing light work), then the indoor to outdoor CO2 concentration differential should notexceed 530 ppm. Thus, if the outdoor CO2 concentration is 370 ppm, then the indoor valuesshould not exceed 900 ppm.

7.0 Additional Aspects of Ventilation Performance

In addition to the delivery of outdoor air to the building occupants in adequate amounts, theintended function of ventilation is to maintain the occupied spaces at a positive pressure withrespect to the outdoors so as to prevent the infiltration of unconditioned outdoor air. Theproblem with relying on just the measurement of CO2 concentrations to determine this how-ever will be frustrating because it will be impossible to know if the resulting low values aredue to infiltration or just generous ventilation. To better know if infiltration is occurringthen, the monitoring of CO2 concentrations can be supplemented with the monitoring ofabsolute humidity levels. Then as the data is viewed over time, if infiltration is occurring itwill be reflected on those days when there is an observable difference in absolute humiditybetween the outdoors and core indoor locations. The extent of infiltration will be reflectedby the amount that humidity values approach those of the outdoors as compared with the restof the indoor values.Monitoring of absolute humidity levels will provide information on not only the existence ofinfiltration but will also permit assessment of humidity control for both dehumidification andhumidification specifications. This monitoring will also provide identification of any indoorsources of moisture. This information is also importbecause the availability of indoor sources of moisture can be a precursor to the indoorgrowth of microorganisms.

A vacuum-draw, shared sensor monitoring system has the ability to include not only themeasurement of humidity levels but also the measurement of carbon monoxide (CO) concen-trations. This monitoring will be very important in buildings with basement garages andloading docks as well as nearby roadways. Since buildings during the heating season havethe potential for air contaminants at the lower levels to be distributed upward in the buildingdue to thermal stack effects, these lower level sources of air contaminants can adverselyimpact the occupied areas of the building. The monitoring of CO concentrations at theselocations can therefore assess whether the combination of physical isolation and pressuredifferentials created by the HVAC system are adequately preventing vehicle related aircontaminants from being distributed to the building occupants. Monitoring of CO levels canalso identify and assess the impact of nearby vehicle exhaust on the air quality of the outdoorair being drawn into the building.

8.0 Options for Optimizing Ventilation Performance

In addition to monitoring to provide feedback information on ventilation performance thereare several options in the operation and design of buildings that have the potential to im-prove ventilation performance. In terms of operation, for instance, increasing the supply air(SA) temperature in VAV systems will cause more air to be provided to achieve the samedegree of cooling as compared with a cooler SA temperature. This resulting increased airvolume may therefore correct the operation of VAV systems where the VAV boxes fre-quently go to minimums thus causing ventilation deficiencies.

From a design perspective, more options become available. The delivery of outdoor air canbe separated from that of maintaining thermal comfort so that even when thermal conditionsare achieved, there is still adequate ventilation being provided. The geometry between thelocation of supply air registers and exhausts can also be modified to provide displacementventilation rather than merely dilution ventilation. Displacement ventilation offers benefitsin terms of higher ventilation effectiveness for the air moving through a space to dilute andremove air contaminants. In addition, better control of pressures in the building can beachieved with the use of relief fans as opposed to the use of return fans. Using a relief fanmay allow a ducted return air system on a VAV air distribution arrangement without induc-ing depressurization of the ceiling plenum or building envelope.

Optimizing ventilation can therefore be achieved by a combination of having feedback onventilation performance and design details that favor good ventilation performance.