As seen in: Issue 46

Mold And Filters: The Importance Of Cleaning Your HVAC Equipment

Your HVAC equipment is the heart of your grow room. All the air and environmental conditions come from this mechanical equipment. However, most people do not adequately sanitise or service it.

Be conscious of sanitising your HVAC equipment between each group.

Most air handlers, mini splits or other HVAC equipment come with factory-equipped filters. Sometimes, these filters are robust, and sometimes just a plastic mesh. Regardless of the filter provided, these items need constant maintenance. At a minimum, you should inspect these filters every 30 days. Most of the air in your room runs through these units, so any bugs, molds, or particulates you have in the air will be in those filters. The longer they plug up, the harder it is for your fan to push air, and the less effective your HVAC unit works. This also leads to lower efficiency and more power draw on your unit.

Cleaning Filters

Knowing that your filters are this important, you should learn what type of filter you have and how to replace or clean it. Plastic and metal mesh filters are less effective than their fabric counterparts, so you must consider cleaning them more often. Some fabric filters will only last 30 days, while HEPA filters can last over a year if properly maintained. Understanding these maintenance intervals is essential.

Preventing Mold

We typically like to describe the ability of mold to grow based on the “mold square”. This square is based on the four components needed for mold growth: food, moisture, temperature, and spores.

In a grow environment, you, unfortunately, can’t manipulate the temperature and humidity much, as they are a fixed requirement for the growing process. The only way to control mold in a grow facility is to contain food and spores. We can prevent spores with UV or similar technology. They also tend to be drastically reduced in buildings with indirect gas-fired heaters as they reach 400°F + surface temperatures and help kill them. In a cannabis facility, your best bet is to minimize the mold’s food with repeated and regular sanitization to stop mold growth. Direct filters are an excellent place for high amounts of “mold food” to build up.

Filter media and any area where moisture resides tend to have a higher potential for mold. Unfortunately, you cannot control all the aspects of mold, so the best option is to clean regularly and change filters often. Be conscious of sanitizing your HVAC equipment between each crop. I recommend keeping your HVAC fan running, even if the room isn’t operating. Any stagnant air in spaces with moisture will cause mold growth. I have been through buildings that didn’t appear to have any mold smell or problems, so the owner turned the HVAC unit off for a few weeks to save on power. The mold growth and odor were horrible when they turned it back on. You will always have spores in the air, so don’t allow them to grow.

Healthy Environments

For cannabis growing, we need to keep tight temperature and humidity conditions to maintain a healthy environment. You can see from the Sterling chart below what a healthy environment is not to have mold growth or viruses. You’ll note that both high and low humidity can cause different problems. While the Sterling chart was created to advise on human health in hospitals and health care, it can also show the necessity of an adequately controlled cannabis environment. It is essential to keep humidity between 40-60% to avoid pathogen growth. Knowing that many people try to keep at least 60% RH in a grow room makes it very difficult to control mold as it is on the edge of proliferation. You can see from this that we need to ensure we properly clean our filters so as not to concentrate mold in one spot.

Filter maintenance and regular sanitization of your HVAC will prevent years of problems and stop repeated crop issues. Filters collect thousands of aphids, grow all sorts of stuff, prevent proper airflow, and lead to many other problematic and beneficial things. So make sure to clean them often and pay attention to maintenance.

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  • Clover Rayner says:

    This article succinctly emphasizes the critical importance of regular HVAC equipment maintenance to prevent mold growth and maintain indoor air quality. It serves as a vital reminder for homeowners to prioritize cleaning their HVAC systems to ensure optimal performance and a healthy living environment.


Adam has provided planning and design services for cannabis and hemp cultivation and processing facilities over the last seven years with Stratus. His projects involve outdoor cultivation, indoor cultivation, drying, processing, extraction, storage, bottling and packaging, and more. Living on a hobby farm, Adam loves all plants, including flowers, vegetables, and microgreens, but is most passionate about hemp and is in awe of the fast-growing plant and all of the benefits it offers to humans and the environment alike.