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The HVAC System and Mold Prevention

by Amanda Demsky
mold HVAC

Mold is a prevalent problem in homes and buildings and it can cause serious health issues and damage. HVAC (Heating, Ventilation, and Air Conditioning) systems are a vital component to preventing mold growth by controlling the moisture and humidity levels. This blog post will address how HVAC systems play a role in mold prevention, and how you can utilize that advantage in your own home.


The HVAC System: A Life-force of the Home

Within the walls, flooring, and ceilings of your domain, the HVAC (heating, ventilating, and air-conditioning) system is busily catering to your specific comfort needs. Much like the circulatory or cardiovascular system of our bodies, the HVAC system regulates and monitors the ebb and flow of a home’s life sources: air and temperature.

A complete HVAC system is usually comprised of a “split system,” which consists of two separate devices: the “central cooling system” and the “central heating system.” In many cases, a home is equipped with only a central heating system, which could be due to the financial limitations of the original home owner, or because the climate in that specific area is generally cooler all year-round. In the case of a split system, however, both the cooling and the heating systems are “matched,” or paired. This means that they share the same ductwork and ventilation throughout the house and—depending on the type and configuration—both systems can actually work in sync with one another by combining specific processes, despite their seemingly contradictory functions.

How the Forced-Air Central Cooling System Works

The primary components of the central cooling system (CCS) are contained within a cabinet just outside the home. The CCS is comprised of various cooling and heating exchangers (i.e. copper tubing), as well as valves and a mutual sharing of the central heating system’s ductwork (and occasionally, the CHS’s blower fan); but the most significant mechanisms of the CCS are the evaporator coil, condenser coil, expansion valve, and the compressor.

The compressor pumps a chemical coolant, commonly referred to as “refrigerant” through an expansion valve, rapidly cooling it in the process before it enters the evaporator coil. The blower fan passes the home’s warm air across the evaporator coil, which causes the air to exchange its heat with the refrigerant, essentially “cooling the air.”

After that, the heated refrigerant is passed from the evaporator coil to the condenser coil, where it is carried back outside to the compressor and the cycle repeats itself.

How the Forced-Air Central Heating System Works

The main feature of the central heating system (CHS) is the heating appliance, which requires a fuel of either gas or oil. In some cases, a homeowner can select and install a heating appliance that can incorporate both fuel types, otherwise known as a “hybrid packaged system.” For the purpose of this article, we will focus on the most common heating appliance: the forced-air furnace.

While the furnace has many parts, it mainly consists of burners (delivery and burning of fuel), heat-exchanging units, a blower fan, and a flue or exhaust for hazardous gaseous by-products. (Due to safety precautions, furnaces are typically located in basements, garages, or utility closets.)

The burners generate combustion gases, which are passed through a heat-exchanging unit. The blower fan then pushes air across the heat exchanger, warming it in the process. From there, the warm air is blown through a series of ducts, distributing warm air throughout your home.

The HVAC System and Mold Prevention

If appropriately used and maintained, the HVAC can help prevent a mold infestation, but it should be noted that it does not act as a central dehumidifier, despite the fact that it can help provide some stability in the moisture level of your home. If there is a pre-existing mold or moisture issue, the HVAC cannot fight mold on its own.

The type of air and moisture level that is permitted to go through the ventilation system, is the same air and moisture level that will be circulated throughout your home (depending on temperatures and water issues present within and without the home).

What does this mean exactly? It means, that while your HVAC is an excellent source of mold prevention and all-around personal comfort, it still depends on the maintenance and preventative actions of the homeowner in order for it to perform at its best and provide protection for the home and its inhabitants.


Just like our own bodies, we get out of them what we put into them. Your HVAC system should be treated the same way. Don’t expect circulating air to solve a moisture or mold issue in one room of your house, for example. That’s an invitation for disaster. The HVAC will do what it does best: carry and push that moist, mold-spore-infested air from one room to the next, spreading the mold like a plague upon the house.

Mold and moisture issues must be remedied immediately upon discovery, or the HVAC system will work against the home. Mold spores will travel from room to room. The ventilation shafts, air ducts, and filters will harbor mold infestations, corrupting the entire HVAC system permanently. Unless a homeowner is willing to replace the whole HVAC skeletal structure and start fresh, it is imperative that any source of moisture concern be addressed and repaired as quickly as possible.

The first step to protecting and enabling the HVAC to perform well against the possibility of mold is to understand mold—its growth process and relationship with moisture and air currents. MoldBlogger.com is a resource for information regarding molds, their prevention, and what actions to take if your home or your body has been infiltrated by molds or yeasts. But, in short, what you need to know about mold in regards to your HVAC system is the following:

  • Mold is an opportunistic fungal microorganism that needs moisture, darkness, warmth, and decomposing organic material (nutrients) to survive. The HVAC system can only regulate the home’s temperature, which will not entirely solve a humidity issue. This means that the moisture content, lighting, and cleaning of a space—removal and upkeep of organic material, such as dust, wood-based furniture—is the responsibility of the homeowner.
  • The spores (reproductive agents) are most often carried on air currents, floating in and out of living spaces by the thousands (sometimes millions). This is a very troubling scenario for the HVAC system, as it was designed to move air, along with whatever is floating in the air. While there are filters involved in the HVAC system processes, they are mainly designed to catch dust, lint, and similar debris, which just so happens to be mold’s usual snack. This means that even if the filter catches the mold spores, they could take root on the filter and feed off of the organic material that is trapped there as well. It is very important that the homeowner replace the filters frequently to avoid possible infestation and spreading.
  • Unfortunately, the range of temperatures that provide optimal growth success for mold are the very same temperatures that provide optimal comfort to the human body. Mold is capable of thriving and progressing its rate of growth between 60 degrees and 80 degrees Fahrenheit, but the 70 to 80-degree range provides molds with the most beneficial heat and moisture levels to really conquer a home. This is where teamwork between the HVAC system and the homeowner really comes into play. Not only should the HVAC be well-maintained and specific mold and moisture issues cleared up, the homeowner should also consider keeping their thermostat set at 68 degrees Fahrenheit, as this falls just under the most beneficial growth temperature of mold and stays just above the more uncomfortable temperatures for the human body.
  • Both mold and the human body thrive in a habitat of 50% humidity or higher. Even with the thermostat leveled around 68 degrees Fahrenheit, the relative humidity of your home still depends on the temperature, pressure, and moisture conditions outside your home. However, with the HVAC system running efficiently, the relative humidity of your home could range anywhere from 25% to 35%, generally speaking, regardless of what’s occurring outside.

It should be noted that, while it’s vital to maintain mold-preventative moisture levels, it is also important to be mindful of your own personal health and comfort. A relative humidity below 30% can not only dehydrate your body, it can also leave you susceptible to frequent illness, due to the drying out of the nasal passages. Use great care and consideration when determining what is best for you and your home. However, if the relative humidity is above 50%—even with the aid of an efficient HVAC system—you may want to consider investing in a dehumidifier.

Remember, moisture concerns for the body can be easily rectified by drinking more water and avoiding foods and beverages that dehydrate the body, such as caffeine. If the possibility of mold is far greater than the risk of personal dehydration, then a relative humidity of 30% is ideal for your home.

For more information regarding HVAC maintenance, please contact your local HVAC supplier/installer or check out Proper Maintenance for the Forced-Air Central Cooling System in the Fight Against Mold, which gives more detailed examples of how to maintain the efficiency of your CCS. With a little team effort between you—the homeowner—and the HVAC system, your home and your loved ones will be well-guarded in the fight against mold.

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