Q: We moved into our 40 year old split level 6 months ago. I noticed a faint musty smell from some of the HVAC vents this summer. We had the ducts cleaned but we also had mold testing done. The mold test results were from a Petri dish growth from an open air sample coming from a supply vent register in our kitchen. The cladosporium raw count was 22 and the mucor count was tntc ( too numerous to count). I am confused about whether to be concerned and what to do. We also had micro-five air samples done prior in the upstairs hallway, outside a bathroom in the house, finding Cladosporium: 36, Alternaria: 6, Ascospores: 3, Epicoccum nigrum: 11, Ganoderma: 3, Pen/Asp group: 3, Pithomyces: 6, and Torula herbarum: 6. We had our ducts in the crawlspace cleaned but we are not sure what to do about the HVAC system based on the mold vent results. We have found that some of the duct work still has some drywall debris and dust in them even after being cleaned. Should we chemically clean the evaporator coils, sanitize the metal duct work, replace the panned ductwork and ductwork that was sitting on the ground before we dug it out and placed plastic under it? My husband sealed the ductwork with mastic. Should we get different filters and, if so, what type? We have electrostatic washable ones now. We have only been in the home since June and we are concerned for our boys who have allergies and asthma. We have had HVAC and air duct people in but no one seems to be able to give us direction. I would appreciate any direction on this problem.

A: The mold testing that was performed doesn't appear to be particularly useful. Tests performed by a professional mold inspector are generally more quantitative. They may include colony forming units per cubic meter of air or fungal structures per cubic meter of air. The open petri dish samples will always provide mold spores in the air as no building is sterile. The best mold testing would involve vacuuming dust from suspect sources and culturing them for mold and bacteria. Sampling air utilizing the best techniques does not identify the sources of contamination as well as collecting dust or tape samples from the sources. Also musty odors can be due to bacteria , mostly growing in fiberglass insulation.

Metal duct work can be effectively cleaned or flex duct if dirty can be easily replaced. Ducts also dry out fairly quickly when air is flowing. The most likely source of a musty odors is from contaminated acoustic insulation on the base or lining of an air handler. Insulation can remain wet and as a result can support mold/bacteria growth. The source of moisture in air handlers is often a result of poorly draining or backed up condensate pans. Plugged drains are a result of preventive maintenance. Poorly draining condensate pans can also be caused by the drain pan not be tilted or a trap missing on the drain line. Air handlers are often under negative air pressure and will pull air through drain lines rather than drain water out if the trap is missing. Traps are u shaped pieces of pipe that fill with water and prevent air from being pulled into the air handler.

If ducts are passing through a humid space or a crawl space with a dirt floor and have debris in them, then the duct cleaning was not done effectively. Ducts, that are not sealed effectively and have air leaking into them can be a problem. This occurs on return ducts. If ducts are supplying air then air is leaking out of, and not into, the ducts. This wastes energy and money but does not cause musty odors. Cold crawl spaces or attics can condense moisture in the interior of ductwork and cause microbial growth in debris or dust. Hopefully your husband has effectively sealed all joints in the ductwork.

Cleaning ductwork is reasonable if the filters in the past weren't effective in preventing deposited dust accumulation or debris remains from construction. If the ducts have remained dry and have only a small amount of dust, then duct cleaning may not solve your problem. Cleaning duct work that is internally lined with insulation that has gotten wet is not considered recommended. Lined ductwork with interior fiberglass lining that has become water-damaged should be replaced.

Filter selection and installation is critical in keeping the air clean as well as preventing dust from depositing on the coils, where energy transfer efficiency is effected, and duct surfaces. The most efficient filter that your air handler motor can tolerate is the one you want. Few residential HVAC systems have large fan motors and their air handlers can only tolerate filters with limited efficiency. Pleated filters allow more efficiency for dust retention because they provide more surface area for air to pass. You should consult your air handler maintenance technician to provide the best filter your system can handle. Washable metal filters are not usually very efficient. Many filters are designed to keep air handler coils clean, but do not remove the fine particles, such as mold and bacteria, that can impact breathing and health.

Mark Goldman, CMC
Senior Air Quality Scientist
EFI Global, Inc.

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