There are several pieces of mechanical equipment that are designed to improve indoor air quality but that don’t always live up to the hype.
Filtration: In my opinion, the primary function of a filter in an HVAC system is not to clean the air but to keep the blower cabinet, blower, and coil free of dust and biodegradable debris. The typical fiberglass furnace filter that you can see through is next to useless. An electronic filter is efficient in theory but loses its filtering capacity when it gets dusty, and such filters are rarely cleaned frequently enough. By-pass HEPA filters may seem like a good idea, but approximately 90% of the air by-passes the by-pass. And what about a turbulent flow precipitator? This consists of several media filters stacked in a holder with air passing over rather than through the filters. Many particles survive this journey and are then distributed by the mechanical system. I always recommend (as does ASHRAE) a pleated-media filter with a MERV rating of at least an 8; a rating of 11 is preferable for people with allergies and/or asthma.
A filter should never be positioned directly beneath the cooling coil, because when the blower is off, moisture may drip onto the filter, fueling mold growth that subsists on the dust captured by the filter itself.
Black mold grew on this filter, which was placed under the cooling coil (May Indoor Air Investigations LLC)
HVAC and ventilation systems are major sources of IAQ problems, so an indoor air quality professional should understand how such systems operate and should open up such equipment for inspection.
Ultra-violet lights: In a residential HVAC system, air passes too quickly over UV lights for the air to be disinfected. (To disinfect air in a duct, the lamp would have to be longer than 200 feet.) In addition, UV lights mayproduce ozone: an irritating gas and a major component in smog. Pleated-media filtration, the deeper the better, is always the way to go.
Air to air heat exchangers: Most HRVs and ERVs are not installed with pre-filters to keep the intake ducts clean by preventing the build-up of outdoor pollen, mold, other plant materials, and insects from the exterior, and the build-up of skin scales and lint from the interior. And the built-in filtration that I’ve seen in most units is completely inadequate. Some units don’t have drainage, so any fibrous lining material that is present may get wet. Moisture, dust, voila! Mold growth. I always recommend that such units have solid-surfaced interiors andbe cleaned as frequently as needed (so the units must be accessible); and that the filters supplied by the manufacturer be removed and in-line filter boxes (such as the FB6) with MERV-8pleated media filters be installed on both the exterior and interior air intakes.
Basement fan “dehumidifiers”: In theory, these exhaust air out of a basement, drawing drier air from upstairs down into the basement. But if the upstairs air is being cooled, the A/C system will have to run longer, negating any savings that an exhaust-fan-only unit can offer as opposed to a refrigerant-based unit such as a Santa Fe. If the upstairs air isn’t being cooled, the fan-only unit will draw humid air down into the basement, and moisture can condense on cool foundation walls, fueling mold growth. (I have even seen mold growth on such fan “dehumidifiers.”)
Mold spores remain potentially allergenic even when dead, so unless the relative humidity in below-grade spaces have been kept at no more than 60% in finished spaces and 50% in unfinished spaces, mold growth will most likely be present. Many times, this mold growth will not be visible to the naked eye but will still pose an exposure threat.
Air purifiers: These can be most helpful if they direct airflow into a person’s breathing zone (possibly directly or with the aid of a gooseneck), but the devices are no substitute for removing the sources of contaminants, allergens, and irritants from an indoor environment. And because air purifiers produce airflow, they can actually aerosolize allergenicparticles.
This article is contributed by Jeffrey C. May, founder and Principal Scientist at May Indoor Air Investigations in Tyngsborough, MA. A recognized national speaker on IAQ topics including at Maine Indoor Air Quality Council’s conferences, Jeff is author or co-author of 5 books in indoor air quality published by Johns Hopkins University Press, including Edition 2 of My House is Killing Me!, subtitled A Complete Guide to a Healthier Indoor Environment. He is also author of a monthly column in the Healthy Indoors Magazine titled “May’s Ways.” Jeff is a Certified Indoor Air Quality Professional (AEE) and a Council-Certified Microbial Consultant (ACAC), and is licensed as a mold assessor in NH and FL. He holds a B.A. from Columbia College (chemistry) and an M.A. from Harvard (organic chemistry). You can reach him firstname.lastname@example.org or 978-649-1055.