Airborne particulates, including mold spores, that get circulated through your furnace and air conditioning system contaminate the living spaces of your home. Because the entire air volume of your home circulates through your heating and cooling ducts multiple times per day, an ongoing exchange of microorganisms such as mold spores may occur between the system and the house. Effective source control to combat mold means eliminating it in all locations. Because a mold colony inside an HVAC system can disperse millions of invisible spores into your breathing air, it’s important to take steps to keep the system mold-free.
The Mold Environment
Mold needs only a few ingredients to flourish: a dark space, moisture and food. This pretty well describes the interior of many HVAC ducts. For several reasons, these systems provide a perfect environment for mold growth. The sheltered environment inside ductwork and coils admits little light and is seldom disturbed or examined by homeowners. Moisture that promotes mold growth infiltrates ducts through a constant flow of humid air, and by condensation forming on the air conditioner evaporator coil as part of the humidity extraction process. Food for mold growth is provided by microscopic airborne particles of organic matter that originate in living spaces and are drawn into the ductwork.
Standard HVAC system air filters with Minimum Efficiency Reporting Value, or MERV, ratings of 4 and below are effective only for removing large inorganic airborne particulates that make up the constituents of household dust. These are the particles you can actually see floating around. Mold spores are generally 10 to 30 microns in size and not visible to the naked eye. To capture microorganisms this small, higher efficiency pleated filters rating from 8 to 12 on the MERV scale can be installed in the HVAC system. These filters remove particulates down to a size of 3 microns, including most mold spores. Filters with MERV ratings higher than 12 generally aren’t suitable for residential systems due to excessive airflow restriction.
For over 100 years, the germicidal properties of ultraviolet light have been used to sterilize surfaces and air in hospitals and to disinfect municipal water supplies. Exposure to ultraviolet wavelengths, also known as UV, kills microorganisms such as mold spores, bacteria and viruses by disrupting their DNA and reproductive capability. Ultraviolet lamps are used to neutralize mold in HVAC systems in two configurations. Lamps installed inside heating and cooling ductwork expose the moving airflow to UV light, killing airborne spores as they circulate through the ducts. A UV lamp array placed at the air conditioner’s evaporator coil continuously kills mold that accumulates on wet coil surfaces, preventing build-up that reduces coil efficiency and can obstruct airflow.
The Environmental Protection Agency recommends keeping household humidity below 50 percent. Indoor humidity is linked to the relative humidity in outside air that infiltrates the home, but normal human activities such as cooking, bathing and simply breathing also raise humidity within the confines of a house. High indoor humidity fosters mold growth in interior sites, including heating and cooling ducts, but your HVAC system can be part of the solution, too. A whole-house dehumidifier connects to the system through a bypass duct that pulls air out of the ductwork, diverts it through the dehumidifier unit, then channels it back into the return duct. The entire air volume in the home is continuously dehumidified per the setting on a humidistat, normally located on the wall next to the heating/cooling thermostat. Water removed from the air is conveyed through a drain line plumbed to the household sewer.
Gus Stephens has written about aviation, automotive and home technology for 15 years. His articles have appeared in major print outlets such as “Popular Mechanics” and “Invention & Technology.” Along the way, Gus earned a Bachelor of Arts in communications. If it flies, drives or just sits on your desk and blinks, he’s probably fixed it.