HVAC systems will exclude most atmospheric aeroallergens such as pollen and fungi from interior spaces. By contrast, poorly designed HVAC systems may provide for amplification of fungi and actinomycetes in wet niches in the system. Pollen and fungi may enter indoor environments through the air conveyance system itself or through infiltration of the building envelope when the HVAC system is improperly operated or maintained. (EPA, 1991a; Morey, 1984, 1988; Robertson, 1988; Woods, 1988, 1989b). The following section discusses the various components of the HVAC system (e.g., outdoor air intakes, filters, heat exchanges, humidifiers) in the context of source control.
Some residential and most commercial HVAC systems are designed and installed to provide outdoor air for ventilation through "makeup air intakes" that are directly connected by duct work to the HVAC systems. Microbiological contaminants from sanitary vents, toilet or building exhaust air, cooling towers, evaporative condensers, swimming pools, and saunas may contaminate poorly located outdoor air intakes. Outdoor air intakes of some HVAC systems may be located at grade or below grade levels. These outdoor air intakes and the pathways (e.g., metal or concrete ducts) connecting them to the air-handling unit of the HVAC system can collect leaves and other debris that can plug bottom drains. In addition, water and debris that collect in poorly maintained and drained outdoor air pits and areaways provide amplifications sites for microorganisms.
The protection of outdoor air intakes is unfortunately given only minor and inadequate attention in ventilation codes and standards. Section 5.12 of ASHRAE Standard 62-1989 (ASHRAE, 1989b) states that special care should be taken to avoid entrainment of moisture drift from cooling towers into outdoor air intakes. Mechanical codes, such as the Southern Building Code Congress International (SBCCI), state that it is acceptable to locate outlets such as those of chimneys or sanitary sewers as close as 10 feet away horizontally and 2 feet above HVAC system outdoor air intakes (SBCCI, 1990, Section 513). Guidelines for construction and equipment of hospital and medical facilities state that outdoor air intakes shall be located at least 25 feet from exhaust outlets and vents (DHHS, 1987, Section 7.31). The protection of outdoor air intakes from cooling towers is not specifically mentioned in this guideline.
When outdoor air is provided to HVAC systems, it is usually mixed in a compartment or plenum with return air from the occupied spaces before it is filtered or thermally treated. This mixed-air plenum can collect debris such as leaves and feathers if bird or leaf screens on the upstream outdoor air