Q: I was recently called back to a home because of a bad odor in the HVAC system. What caused it and how can I avoid the problem in the future?

A: Jeffrey May of May Indoor Air Investigations in Tyngsborough, Mass., responds.

The odor you describe is due to the presence of bacteria, yeast, or mold. Here’s what happens: In many geographical areas the coils and condensate pans in central A/C systems stay wet during operation. In hot and humid conditions, builders often operate the A/C during construction to cool the site. Sawdust gets sucked into the return system and some of it ends up on the coils and in the condensate pan. The cellulose serves as food for the microorganisms, which then grow and produce the odors.

The simplest solution is to completely avoid operating the central A/C during construction. If cooling is necessary, use portable or window-mounted A/C units. Another option is to keep dust out of the ducts with a pre-filter on all returns (MERV 2 or 3 roll filter material is available from building supply stores), and to install at least a 4-inch-thick MERV 11 media filter at the A/C unit itself to further prevent dust from getting on the coils and pan. Electronic filters are ineffective because they lose efficiency when dirty, and typical 1-inch fiberglass furnace filters (about MERV 3) won’t stop heavy dust from construction getting in.

Also keep in mind that operating the hot-air system during construction in the winter will contaminate the ducts and A/C unit with dust as well. And don’t forget to keep jobsite dogs outdoors to avoid contaminating the entire HVAC system with dog dander for clients who have allergies.