The main supply air duct work immediately downstream of the fan and the heat exchanger is usually constructed of galvanized sheet metal and can be either externally or internally insulated with fiberglass. However, in some residences and commercial installations, duct work itself is constructed of a rigid fiberboard that is intrinsically insulated. Internal fiberglass or fiberboard liner either in the fan or main air supply plenum should have sealed surfaces to prevent the erosion of fibers into the airstream. The internal insulation in these plenums should be protected from emissions from humidifiers or from water droplet carryover from heat exchangers to reduce the likelihood of microbiological amplification.
In some commercial buildings, a separate system may be installed to heat and cool perimeter zones that are more affected by outdoor climatic conditions than interior zones that are not in contact with the building envelope. Fan coil and induction units often mounted in sheet metal enclosures along exterior walls, are two common types of peripheral units that are used to condition air in perimeter zones.
Fan coil units contain small fans, low-efficiency filters, and small heat exchangers with small drain pans. Fan coil units condition and recirculate room air (often without any outdoor air) in peripheral zones. Sheet metal enclosures are usually lined along interior surfaces with porous insulation. Large buildings may contain hundreds of these units, and consequently, maintenance is often neglected. Fan coil units often accummulate dirt and debris, becoming amplifiers and disseminators of microorganisms during the air-conditioning season, when the heat exchanger actively contributes moisture to each unit's enclosure (Morey et al., 1986).
Induction units are provided with primary air from a central AHU. This air, containing a percentage of outdoor air, exits each induction unit through a series of nozzles that induce a flow of room air, and with which it is mixed and supplied to the room. Induction units usually contain low-efficiency filters and a heat exchanger that removes some sensible heat but little latent heat or moisture. Under design operating conditions, condensate pans in induction units are expected to contain little if any water. Thus, they are less likely to be sites for microbiological amplification, but they are likely to be reservoirs for pollen and phylloplane fungi, especially if the primary air has been poorly filtered.
In many commercial buildings, the cavities or plenums above the finished ceilings are used as unducted passageways for air returned from the