By Doug Hoffman

You might have experienced clients asking, “I smell mold inside my office; could it possibly be my heating system?” This is a great question because the HVAC system (heating, ventilation, and air conditioning) is usually a problem when considering indoor air quality (IAQ) issues but is not always the problem. Let me explain.

The HVAC system serves as the “heart” of the indoor environment. Whether a central air conditioning system or just a heating system, if you’re in conditioned space, much of the air goes through that system. I ask students to think about a building as a “living, breathing organism.”

In this very dynamic environment, the central HVAC or heating system carries the contaminated air to the return filter and into the air handler. Inside the unit, air is cooled or heated and then sent back as conditioned air through the supply ductwork and supply registers. Like the heart manages our blood flood and keeps our blood oxygenated, the HVAC system manages the air quality in our environment, keeps it clean, and cools or heats it.

The air conditioning system primarily is a dehumidification device and, when operating properly, pulls moisture out of the air. As it cools or heats, it also lowers the humidity and, thus, reduces the possibility of mold growth. However, it can also become a nest for mold to grow, so it’s a double-edged sword. Because an HVAC unit is moving most of the air in the conditioned space, it usually is a part of the problem. If it’s not operating properly or is dirty, it can be the problem.

When a NORMI-certified Mold Assessor does an IAQ/mold assessment, it is important to look at all aspects that could affect or influence the quality of air in the indoor environment. That certainly includes the central heating and/or cooling system. Filters that are not changed regularly or systems not serviced on a regular basis can cause “bio-nesting,” a place for mold to thrive and produce the mustiness you might be smelling. Because the system primarily dehumidifies, the moistest air in any environment goes into the system through the return plenum. This is also where the filter is located, and a filter that has trapped mold spores, dead skins cells, and other organic materials for mold to eat could be a source of the problem.

You can see how the filter media itself could become a nesting bed for mold to grow. That’s the “bio-nesting” concept in a nutshell.

As part of a holistic approach to solving indoor air quality problems, NORMI always recommends checking the heating and/or cooling system. When properly serviced, the system will run more efficiently, saving energy, and provide cleaner air throughout the building. It may not need service, but if it does, it should be addressed as part of the NORMI Sanitization Protocol. If you go to the trouble of resolving issues in the environment but ignore this part of the process, the building will quickly become re-contaminated, and that contamination will spread throughout the entire air-conditioned space.

Doug Hoffman is the CEO of NORMI, the National Organization of Remediators and Mold Inspectors. He can be reached via e-mail at