Shedding Light on Moldy Ductwork

Ed Utterback, President

Ductbusters Indoor Pollution Control
Terre Haute, Indiana

Ductmaster W.J. Kowalski responds:

U tube lamps are easy to install in ductwork and can last for more than a year before replacement is needed.

A: Dozens of different species of fungus and bacteria can colonize air handling unit equipment. Inspections and testing are normally required to identify a problem, but the first indication of equipment or building contamination may occur when the occupants become chronically ill.

Mold grows in HVAC units when spores enter from the environment and germinate in the presence of moisture or nutrients. Drain pans, cooling coils, duct walls, duct insulation, and dust are typically present in HVAC systems and can foster the growth of a variety of fungal spores. Mold growth inside an HVAC unit should first be mitigated by controlling the moisture source. Drain pans and condensate drains should be designed and installed correctly so that water does not stand inside the unit.

In particularly bad cases, regular cleanings with bleach may also help. A low-maintenance alternative is to install an ultraviolet (UV) lamp. Some recent studies (Shaughnessey et al, 1999; Scheir and Fencl, 1996) have shown that UV lamps can effectively control microbial growth in HVAC units, on filters, and in cooling coils. The exact size and number of lamps you should use depends on the particular situation, but for a typical residential system, a UV lamp of 4 to 9 watts might do the job without incurring high energy costs.

Many types of UV lamp are available (check with any manufacturer or distributor), but U tube lamps are probably the easiest to install. Just drill out a hole in the side of the duct and bolt the lamp fixture to the duct wall. The lamp must illuminate the surface where the mold is growing. Adding reflective surfaces to focus the light can better than double effectiveness. UV light is harmful to the eyes and skin, so take the necessary precautions, such as wearing goggles when testing, and make sure that no UV light can escape from the system (for example, through gaps or grilles).

UV lamps typically require a dust filter placed just upstream of the lamp to keep dust from impinging on the lamp surface. Otherwise, without regular maintenance, performance will suffer. Better than a dust filter, however, is an ASHRAE 25% (or higher) filter that can also remove most mold spores. For residential applications, the combination of this type of filter with a low-wattage UVGI lamp will be effective in controlling microbial growth if the surface receives adequate exposure to UV. Depending on the HVAC system, the use of higher-wattage lamps may even help to reduce airborne viruses and bacteria inside residences. This can be especially beneficial when children and infants are present.

For more information, see my article in Heating, Piping, and Air Conditioning (January 2000), p. 100.

--W.J. Kowalski

W. J. Kowalski is a doctoral candidate in Architectural Engineering at the Pennsylvania State University.

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