Molds and their odors can be found in many places. Before any contractor can start remediation, he or she must first determine if the source is from the HVAC system - so it’s best to walk around the building to see where the odor is coming from. Even testing can’t be relied on here, because both odors and spores can usually be found distributed throughout the whole house as well as the HVAC system. So, some sleuthing abilities are required.

Realize that molds are really just plants, and as plants they need a few things to grow, such as soil and water. So, since few molds can grow without water, you must look for damp places where they propagate, such as in bathrooms, basements, inside walls and ceilings, around plumbing fixtures and pipes, etc. However, too often that wet place where mold is growing is inside the air handler and ductwork.

One word of caution here: Some molds are extremely toxic, and remediation may require the use of medical-quality masks.

Air Handlers

I don’t know if I’ve ever seen an old air handler that didn’t show some signs of mold when it wasn’t regularly serviced. After they operate for many years and the coil, pan and insulation get dirty, the condensate water provides all that is needed to water the “soil,” which gives mold spores that are naturally being sucked through the system a perfect medium to grow during the cooling season.

Mold can be killed by bactericides, such as chlorine bleach, but remember that many of these are strong corrosives that can destroy the cabinet, coil and pan. And, unless the growing medium (which may also be inside the insulation) is removed, the cure is just a stop-gap measure, for it will grow back.

While regular servicing and cleaning (with a non-corrosive bactericide) will provide a short-term solution to the problem, two longer lasting remedies are UV lights (which have to be replaced each year) and ozone generators. Also, contaminated insulation should be cleaned and covered with a plastic or foil lining, or replaced. I have also seen insulation sprayed with an antimicrobial paint, but that diminishes its insulating properties.


Of course, in a perfect world, there should be no soil or water to provide a growing medium for mold or bacteria inside ducts, so duct cleaning and other remediation should be unnecessary. After all, any dirt would have to make its way through the air filter and then through the wet air conditioner evaporator coil. And where would water come from?

However, especially in older buildings where an air conditioner was retrofitted into an existing heating system, there was once no evaporator coil to act as a filter, so huge buildups of dust and dirt are possible. And many people just don’t change their air filters - or they throw them away entirely - so some buildup is possible, especially where the ducts are insulated on the interior, and the rough fiberglass lining becomes a collecting spot.

As for moisture, when condensate pans overflow, or where air velocity across the evaporator exceeds 450 fpm, some blow off into the ducts becomes possible and this situation would have to be corrected before any remediation is started! Then add the droppings (and dead bodies) of rats, mice, cockroaches, and buildups of cooking grease, and we have a ripe medium for all sorts of nasty growth. This might in fact require a duct cleaning.

However, the problem with duct cleaning is that it loosens the insulation fibers, creating another pollutant, and once the growth of mold and bacteria is there, it will be hard to kill. An ozone generator or coating all surfaces with an antimicrobial paint (do you carry this?) may be required.