Finding and Attacking Mold in the HVAC System

I turned on the AC in my home last spring and a musty smell gave me suspicion that there was mold growing somewhere in the ventilation system.  I determined that I would not stick my head in the sand, hoping it would go away after more use.  I didn’t want mold to be distributed through my house via the HVAC, so I planned a “search and destroy” mission!  

I have to say that I lack fear about a lot of things, but experience has made me a bit more cautious.   I used to reclaim wood and would pick up interesting pieces off the curb.  This resulted in making me violently ill one time when the scavenged wood had black mold on it. Mold is dangerous, so if you think your A/C system has mold, you will want to wear a protective mask (more than a thin cloth or paper mask; N95 is ideal) and wear old clothes: plan to shower, wash hair and launder your clothes afterwards.  If you are highly allergic to mold or just squeamish, this is best left to someone who isn’t.

Timing: You will need to shut down the AC for at least an hour, possibly several.  For this reason, and the fact that the air handler is often located in the attic, don’t plan this project in the heat of summer (at least in the daytime).


  • depending on the model of air handler, various screwdrivers and socket drivers
  • Small containers to hold the screws
  • A flashlight or headlamp
  • A spray bottle containing hydrogen peroxide (3% is the standard solution sold over the counter—works fine. ) OR
  • A spray bottle containing 50% white vinegar and 50% water.  However, DO NOT MIX VINEGAR WITH HYDROGEN PEROXIDE!  It creates poisonous fumes.
  • Paper towels or disposal rags
  • Plastic bag(s) to hold contaminated towels or rags
  • A cup of hydrogen peroxide in a small pouring cup OR A cup of vinegar . (DO NOT MIX VINEGAR WITH HYDROGEN PEROXIDE!)
  • A roll of foil HVAC tape.
  • A can of wasp spray on hand if your attic has any access to insects (ie. through a ridge or gable vent)

First, switch the AC off at the thermostat and place a piece of masking or blue tape over the switch.  Then identify the circuit breaker that controls the AC in the breaker box, switch it off and tape over that breaker.  Although we won’t be messing with electricity, you don’t want the fan to come on while you have the air handler open, blowing mold spores everywhere!

Set yourself up next to the air handler in the most comfortable way (usually these spaces are tight, I know!).  Identify the drain from the coil unit, which is usually white PVC pipe.  This pipe drains condensation that drips off the coil while it is cooling the air passing through.  The condensation is a major source of moisture for mold, so if the condensation is not draining properly, it can 1) harbor mold and 2) back up and flood the drip pan, causing water damage to insulation and sheetrock and more mold.  If you’re already experiencing problem #2, it’s probably due to algae growing in the pipe and blocking it.  You can go outside and use a wet-dry vacuum on the end of the pipe to clear the blockage, then keep going with the next step here.

The white plastic pipe coming from the air handler should have a small bend in it.  This acts like a P-trap in the drains of your house, keeping vapors in the unit with a liquid “trap” .  Ideally there is a capped “vent” on the pipe.  You’ll need to unscrew or pull the cap off, pour the liquid hydrogen peroxide OR vinegar in, and replace the cap.  This will prevent algae from growing in the pipe and blocking it.  HVAC professionals recommended to do this about once a month during peak cooling season.

Next, put on your mask and find the access door on the air handler that is closest to this pipe.  We’re going to check for mold in the drain pan.  Remove the door and shine the light inside.  If you find mold,  don’t touch it directly.  First, saturate the area with the hydrogen peroxide in the spray bottle.  Let it sit on the mold ten minutes.  Then, use the rags or paper towels to clean the surface thoroughly and dispose of them in plastic bags as you go.

While you are waiting for the hydrogen peroxide to take effect, you can remove other doors around the coil to see if the mold spread to other areas.  I found that the compressor motor body was covered in it (this is another reason you need to turn off the electricity before starting) and some other places inside the duct. 

Try to determine the source of the mold.  If the drain pan was draining correctly, but there is still mold, it may be that the unit needs more insulation internally or externally.  Insulation prevents condensation where there should be no condensation, which is everywhere except the evaporator coils!

If you can, apply a Mold Preventer spray to the problem areas.  Some are effective up to 3 months, so even if you are not successful at stopping the moisture problem right away, this will prevent the mold from recurring for a time (buys time). Here are some non-toxic options:

While you are in the attic, check for screws or joints that are loose, that can cause an air leak.  Warm air leaking into the system will cause condensation, so if you find one, try to seal it up with the appropriate fastener or at least, the HVAC tape.  Also check for missing insulation on ducts.  If your attic routinely gets into the 90 degF or hotter range, then the white condensation drain pipe (that drains cool water through a hot space) should also be insulated to avoid dripping into the attic.

Now that the mold is removed and prevented, you can seal everything back up and get clean!  Here are the follow-ups you need to look at to mitigate mold:

  • The other source of mold food is dust.  Less dust = less mold!  Use the best HVAC filters you can afford and be sure they fit well and you change them on schedule.  HEPA filters really do help.
  • Consider installing a dehumidifier to help with the efficiency and lower mold growth.
  • Consider installing a BP-2400 to continually prevent mold not only in the system, but throughout your home.
  • If your system is over ten years old, many HVAC professionals suggest replacement of the complete system for efficiency and cleanliness.  Get quotes and consider other types of heating/cooling systems, which may be less susceptible to mold issues.
  • Some HVAC companies apply a polyurethane coating coat to their heat exchangers, which keeps the coils cleaner (saving energy), discourages mold growth (better air), and extends coil life by 3-4x.  Madd Air in Texas is one of them. 

Mold in the HVAC system is worth looking into. as you and your family's health depend on it!