Mold growth is an all-too-common issue that many homeowners have with their HVAC systems, especially during the summer months, when homeowners need their air conditioning most. A combination of humid weather, high moisture and lax maintenance can cause mold to spread rapidly throughout the HVAC system. Not only can it leave behind unpleasant odors, but it can also trigger allergies and asthma symptoms as well as contribute to poor overall indoor air quality.

Getting to the Heart of Your HVAC Mold Problem

In order to tackle your HVAC system's mold problem, you'll first need to find out where mold is likely to grow within the unit. Dark, damp areas with lots of excess moisture are typically ideal places for mold and mildew to thrive, so it's crucial to look in areas within your HVAC system where these conditions are likely to exist:

  • Evaporator coil - The evaporator coil is tasked with removing latent heat from indoor air and transferring it outdoors. Mold spores can accumulate on the coil, and the high-moisture environment of the plenum encasing the coil can spur mold growth on the coil itself, leading to blockages that reduce cooling performance.
  • Condensate drip tray - As the evaporator coil removes latent heat, it also causes excess moisture to condense into liquid form and fall into the condensate drip tray below. Standing water within the drip tray can also spur mold and algae growth.
  • Drainage lines - Condensate drainage lines that are blocked by debris can also play host to mold and algae growth.
  • Air ducts - As mold spores migrate throughout the HVAC system, some spores can embed themselves within the ductwork. Humid conditions inside your home or excess moisture from the HVAC system itself can cause these spores to flourish.

You should also take a look at your HVAC system's air filter. Once your air filter becomes full of dust and debris, it could allow unfiltered air to pass between the microscopic cracks and gaps between the air filter and the return air duct. This unfiltered air could carry mold spores that would have normally been trapped by a cleaner air filter.

Taking Care of Business

Start by cleaning the evaporator coil. Remove the A/C plenum access cover and spray the entire evaporator coil with a foaming no-rinse cleaner. As the cleaning solution sits on the coil, the heavy-duty detergents will liquefy any residue found on the coil while the foaming action creates its own rinsing action. You could also use a soft-bristle brush and a mild detergent to manually clean the coil, but physical contact could create a risk of damaging the delicate fins lining the coil.

Next, focus on removing mold and algae growth from the condensate drip tray. Use a wet/dry shop vacuum to remove any standing water from the condensate drip tray. Fill a spray bottle with white distilled vinegar and spray the vinegar on the drip tray surface. Allow the vinegar to sit for an hour and then rinse the tray with water.

If there's a blockage in the drainage line, use a small drain auger to break up the clog. You can also use the suction from the wet/dry shop vacuum to dislodge some clogs. Afterwards, pour a 1/4-cup of vinegar down the drain to disinfect the line.

For mold and mildew within the ducts, you can use a mold removal solution and a fogger to push the solution throughout the duct system. However, it's usually best to have a professional perform this task or use a similar method of mold remediation for the best results.

Don't Forget to Change Your Air Filter

Last but not least, you'll want to change your air filter before using your HVAC system again. By changing your air filter, you won't have to worry about new mold spores infiltrating your recently-cleaned HVAC system. In addition, a fresh air filter will also boost your HVAC system's overall performance and energy-efficiency. 

If you feel you cannot do this on your own, you can also contact HVAC contractors or air conditioner repair companies or visit websites like for more information and options.