Not an ideal environment for mold
HVAC Fan blades don’t typically present a hospitable environment for mold growth as air flow typically prevents it. The cooling coil is more suited for mold, bacteria and yeast growth due to the presence of condensation. 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 2-3 years
You should plan on having your HVAC system professionally cleaned every two – three years depending on its location and the dustiness of your home. Consider it a type of preventative maintenance. We recommend using a NADCA certified firm where the ducts, coils, fans and motors are all cleaned and sanitized. But nothing stays clean forever so think of it as a routine household cleaning item. The best way to keep mold under control is to eliminate the moisture source. 
Common moisture culprits
If your crawl space or Basement has a dirt floor, lacks insulation, is poorly ventilated, has a french drain, etc. then your source of moisture intrusion (and other soil gases) might be located there. Natural stack effect may draw that unhealthy moisture-laden air up into the home. Leaky, un-insulated ductwork may allow moisture, mold, and dust to be drawn into the duct work resulting in an unhealthy environment in the HVAC system.
So what should I do?
Here’s a list of some common recommendations that may apply to your home:
- Upgrade furnace filter to a high efficient media filter (min. MERV 10 rating as recommended by your HVAC contractor / HVAC manufacturer).
- Replace your furnace filter before both heating and cooling seasons.
- Have ducts cleaned every 2-3 years by a NADCA certified duct cleaner.
- Have HVAC ducts sealed with an approved duct sealing mastic.
- Insulate duct work in the attic and basement / crawlspace to prevent ducts from sweating
- Reduce potential moisture sources with use of vapor retarders, dehumidifiers, improved ventilation, spray foam insulation, etc.
