A technician found mold in my HVAC system's fan blades. How do I keep mold from returning?

Asked by Charlene Smalls
Charlotte, NC

They suggested mold removal & sanitization of system and duct cleaning. The technician did say it is possible the mold could return because of where the system is located.


Michael Holcomb

Answered by Michael Holcomb

Byron Center, MI

Alliance for Environmental Sustainability (Headquarters)

September 20, 2012


In responding to your question I will assume, for the moment, that what the technician identified as mold was actually mold.

Mold is everywhere! (So don’t over react when you hear the word.) Contrary to popular belief mold is mostly a good thing. It is an important component in nature’s food chain. Ideally we would want to prevent mold propagation in our living spaces. Proper air flow, ventilation, even heating/cooling, and cleanliness are all important issues in reducing the potential for a mold problem in your home.

Follow the suggestions below if they apply and you should live in a mold responsible environment.

Mold grows when there is a water source, food source, air and temperatures above 40-degrees Fahrenheit. If you eliminate any of these factors from the equation the mold is history. 

Inhospitable environment for mold

Fan blades do not typically present a hospitable environment for mold growth. Air movement is likely to reduce mold growth.

However, if your fan assembly (ducts, AC coils, etc.) haven’t been cleaned in years you could build up dust and other food sources for mold on the interior of the HVAC system that permit mold growth as moisture becomes available.

  • In summer months conditioned air passing through an HVAC system located in a humid crawl space might be prone to condensation when warm moist air contacts the colder (conditioned) metal surfaces of the HVAC system.
  • If the HVAC components are dirty the added water may allow mold to grow.
  • We normally find the mold growth taking place in or on the ducts or in the drain pan of the evaporator coil. I’ve also found mold growing on dust buildup on the evaporator coil fins.

Clean your HVAC system every 3 years

Your technician is right to forewarn you that the problem may return in spite of cleaning and sanitizing, nothing stays clean forever.

So temporarily removing a potential food source is… well… a temporary solution. Reducing the water and food sources are the most effective long-term solutions.

You should plan on having your HVAC system professionally cleaned every three years. Consider it preventative maintenance.

  • The contractor hired to clean the system needs to clean the entire system: ducts, coils, fins, fans, and motors.
  • Sanitizing is not necessary unless you have lab results indicating that you actually had a mold problem.

Dirt/dust buildup on fan blade edges is often misdiagnosed as mold. If your technician took a sample and sent it to a registered testing laboratory you would know for sure. Otherwise you don’t know for sure that it was actually mold or that it was propagating (flourishing and growing). In my experience when you have an active mold problem in your home your nose knows.

If your crawl space has a dirt floor your source of moisture and mold (not to mention other soil gases) may come from the soil. Leaky, uninsulated ductwork may allow moisture, mold, dust and debris to be drawn into the ductwork resulting in potential unhealthy environment development in the HVAC system (and the interior living spaces of the house).

My recommendations

So, here is a list of recommendations that may or may not apply to your home:

  • Upgrade your furnace filter to a high efficient media filter with a MERV 10 rating (minimum as recommended by your HVAC contractor and the manufacturer of your equipment).
  • Regular replacement of your furnace filter (both heating and cooling seasons).
  • Professional duct cleaning every three years.
  • Have your HVAC ducts sealed with an approved duct sealing mastic.
  • Insulate ductwork in the crawlspace to eliminate surfaces from reaching dew point (when condensation forms).
  • Seal the floor of the crawl space to reduce potential moisture sources.


For more information:

Read "The AC supply grille in my daughter's room has dirty, crusty stuff on it. I think it's mold. What should I do?" a Q&A answered by Steve Saunders.

Tagged In: mold removal, heating cooling

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