Q: Will installing a UV light in my client’s furnace ductwork eliminate bad odors in his HVAC system?

A: Jeff May of May Indoor Air Investigations (mayindoorair.com), in Tyngsborough, Mass., responds: Odors from air-conditioning systems are inevitable if the cooling coil condenses water from the air and if there is any biodegradable dust on the coil or in the condensate pan. Because condensed water cannot be eliminated from the system (unless the building is in a desert-dry environment), the only way to avoid odors effectively is to eliminate the presence of biodegradable dust, which can be done only with efficient filtration (at least MERV-8, but preferably MERV-11, filters). For builders, this also means keeping sawdust and other types of construction debris out of the system completely prior to occupancy.

UV irradiation can be a very effective deterrent to microbial growth in an HVAC system, but only if the entire coil and condensate pan are fully exposed to the UV light. An effective installation requires two or more long bulbs at the incoming side of the coil that cover an area at least as wide as the coil and the pan. Typical residential units with short or U-shaped bulbs cannot irradiate enough of the coil and pan to disinfect them completely.

And the UV bulb should not be installed as a means to kill spores or bacteria in the air. In order to create an effective-enough dose of UV to kill microbes in rapidly moving system air, the UV bulb would have to be in the supply duct—and would have to be well over 100 feet long.