We are just finishing a major remodeling/home addition project.  While working on the AC system, the contractor found (you are not going to believe this!) several rags/towels soaking in the drain pan under the AC coil SEALED INSIDE the plenum.  The rags were saturated with water and covered in mold!  Of course, we removed them immediately.  We then had the whole system cleaned, including ductwork, coil, etc.  The cleaning company did use a biocide, though after reading some of your archived articles, I am not sure that was a good idea.  The AC engineer said, however, that air had been blowing across the moldy rags and circulating for who knows how long!  My concern is that my family is still suffering from allergy symptoms that seem to be related to our house.  (We just spent 10 days on vacation and were amazingly free of breathing and sinus problems which began again immediately upon our arrival at home…).  What further steps should be taken to clean the AC system and our house?  Should the ducts be treated again, or with something else?  We use the Filtrete “purple” filters and I change them every month or 6 weeks.  Should I leave the system fan running constantly?  (I always heard that could cause condensation problems…?)  I also have room air cleaners (HEPA) in each of my children’s rooms.  By the way, our health seems to be worse since the winter months have necessitated the use of the heating system.  Is that because the warm air “kicks up” the mold spores that are in the ductwork?  Thanks for your help!-K., Texas

            Well, rags/towels in the condensate drip pan of an HVAC system is quite novel.  Sounds like someone tried to fix a problem with a leaky condensate drip line by trying to soak up the water than the AC unit was producing.  Of course it wouldn’t have worked.

            Nevertheless, any organic materials sitting in water in the HVAC system is going to be an excellent mold medium.  It was likely to have been a significant source of mold spores particularly during the heating season; with renewal of the infestation during the air conditioning season.

            The use of biocides on HVAC systems components is definitely not recommended.  USEPA especially advises against such use.

            The pattern you describe of your family’s respiratory symptoms resolving/getting better when away from your house and returning after you return is one of those key observations that connects health problems to a building environment.  This is a classic pattern for mold and other indoor contaminants.

            If the HVAC system/ducts were cleaned, there is no need to do it again.  Indeed I doubt that it will provide any benefits as far as your family’s respiratory health is concerned.

            I recommend the use of multi-pleated filter (MPF) modules installed in the cold air return of houses with 12-24 hour per day furnace/Act fan operation.  Manufacturers of such products include Aprilaire, Honeywell, Trion, etc.  The Filtrete filters I have seen in the hardware stores though pleated are unlikely to do as good of a job in capturing dust and mold spores as the MPF units I recommend.

            The use of a whole house MPF filtration system has two benefits.  It of course decreases airborne mold and dust concentrations.  It also (over weeks) reduces residual surface contamination associated with remediated mold problems.  Deposited spores on disturbance become resuspended and then are drawn into the filter medium.

            The nature of the rag/towel problem in your house suggests that previous owners may have had other problems with the house.  As such, I would inspect other parts of the house for mold such as basements, crawlspaces, attics, brick veneer, walls, etc.

            If your health problems are worse in the winter months,  that may be due to the fact that you spend more time indoors, and have the house closed up more often than at other times of the year.  This indicates that during winter months you are actually exposed to more mold.

            During the heating season condensate drip pans dry out and any microbial growth can be aerosolized and become airborne.  During the air conditioning season, mold spore aerosolization is less likely due to the presence of liquid water. 

December 12, 2003

Indoor Environmental Quality (2000), Thad Godish Ph.D., C.I.H

Direct E-mail 00tjgodish@bsu.edu