Engineering Manual o.. - HVAC.Amickracing

INDOOR AIR QUALITY FUNDAMENTALSRecirculation RequirementsRecirculation of air in HVAC systems is regulated bybuildings codes and other rules as well as ASHRAE 62-1989.The BOCA mechanical building code, for example, requiresfiltration to remove particulates above a certain level forrecirculation.Figure 3 shows locations where filtration might be appropriatein different system configurations. The filter shown in the spaceis located in a local makeup air unit if one is used. The filtershown in the outdoor airstream is utilized when outdoor aircannot meet the requirements of the building code, ASHRAErecommendations, or flow sensors or coils are located in theoutside air.Each of the filter locations shown might represent multipleparticulate, gas phase, and/or ozone generators.Ventilation OptimizationThe Ventilation Rate and Air Quality Procedures bothincrease outdoor air consumption and increase operating costunless techniques are used to minimize outdoor air when thespaces are not fully occupied. The two procedures requiredifferent approaches.One approach utilized for the Ventilation Rate Proceduremonitors the load, or airflow, of all the VAV terminal boxesand the outdoor air flow of the supply fan, calculates the requiredventilation fraction, and resets the outdoor airflow to theminimum outdoor air that will satisfy the critical space orspaces. See the latest version of ASHRAE Standard 62 foradditional information on critical zone reset application.An alternate method maintains a constant supply fan mixingbox static pressure as the supply fan load varies. Since the entireVentilation Rate Procedure is based on a series of assumptions,this lower cost, static pressure approach may be as effectiveand is certainly more cost effective.When the Air Quality Procedure is used, CO 2 monitoringand control has been used to reduce outdoor air to conserveenergy. However, CO 2 is only one of the possible contaminantin indoor air. It was intended as an index of odor quality on theassumption that if odor was low all other contaminants wouldalso be low. This is not necessarily so. Some designers use VOCSensors in lieu of or in addition to CO 2 sensing and control.The standard lists CO 2 , Chlordane, Ozone, and Radon levels.Table 3 references other contaminants which may be of concern.The engineer is left to decide which ones are to be monitored.CODES AND STANDARDSA host of regulatory agencies, code bodies, and advisoryagencies are involved in the maintenance of Air Quality. Someare concerned with Outdoor Air Quality while others areconcerned only with IAQ. These agencies and their jurisdictionsare listed in Table 5.GENERALEXHAUSTRETURNAIREXFILTRATIONOCCUPIEDSPACEENERGYRECOVERY UNITAIR CLEANERLOCATION VENTILATION AIROUTDOOR AIRMAKEUP AIRSUPPLYAIRLOCALALTERNATE AIR CLEANER LOCATIONSINFILTRATIONM10482Fig. 3. Possible Location for Filtration in an HVAC System.162ENGINEERING MANUAL OF AUTOMATIC CONTROL