According to Manual D, Residential Duct Systems, Air Conditioning Contractors of America, "Researchers speculate that microorganisms - which are commonly found in the soil, water and air - are deposited on the indoor coil during the summer, when the coil is cold and wet, and that these colonies thrive and grow during the heating season, when the indoor coil is dry and warm. However, this coating, by itself, does not produce the odor problem, even when spores are entrained in the supply air. Evidently, the odor is produced when the airborne contaminants are burned. This condition can only occur during the heat pump defrost cycle when the electric resistance heater is energized, but even then, it might not occur unless the indoor coil is wet."
Three conditions are normally associated with an odor problem:
1. Biological contaminants must be deposited on the coil.
2. The indoor coil might have to be wet.
3. The electric resistance heat must be on.
"Note that two of these three conditions cause the occurrence of the odor problem to be decidedly arbitrary, according to Manual D. "Evidently there are many homes that do not have a contaminated coil. And, even if the coil is coated, the indoor humidity (during the heating season) may be too low to cause wetting of the indoor coil during the defrost cycle. In any case, if the first condition is eliminated (no contamination), the problem cannot occur. Therefore, duct leakage could be a contributing factor if the leakage introduces contaminants into the return air or causes contaminated air to infiltrate into the conditioned space."
The Air Conditioning and Refrigeration Institute in its "General Maintenance Guidelines for Improving Indoor Air Environment" states: "In the cooling season, humidity control will be largely dependent on the equipment selected, thermostat setting(s), and the amount of air moving across the evaporator coil. Keeping the relative humidity below 60% in the cooling season will help prevent the growth of mold, mildew, bacteria, etc. in ducts, in hvac equipment, and on walls and ceilings."
In addition, "All hvacr systems must be kept clean so that the build-up of dirt, dust and microbes and other possible contaminants can be minimized. This will reduce the probability of mold and bacteria growth which causes odors, respiratory problems or other potential health problems. This includes the ductwork, coils, air filters, condensate drain pans, and equipment cabinet, etc."
There are a number of ways to treat or prevent this problem. Foster Products Corp. and Virginia KMP carry chemical treatments designed for use on cooling coils.
Honeywell offers a different approach, one using ultraviolet lights to kill germs and mold at the cooling coil, available in single or dual lamp models. Lennox Inds. offers a similar product, shown here, the Healthy Climate germicidal light said to be effective in killing viruses, mildew and microbes in ducts and coil. Gerson Gavin, marketing manager, said this is the highest intensity UV bulb currently in the market, with power consumption "about the same as a normal fluorescent bulb." The light comes complete with gaskets and a template for easy installation.