My AC has started to turn on with the warm weather and with that I've been smelling a gross musty sour smell coming from the vents (only when it cools and not when heater is on). I last changed the air filter in January and had gotten all my AC ducts cleaned last fall. However, I had an AC Duct Cleaning company come today to inspect and the AC didn't smell anymore! That's so weird because it had been smelling bad the last 3 times it ran!

He said that if there is no smell anymore, then it isn't mold and was likely the smell of the condensation dripping from the piping so I don't need to do anything. He didn't even inspect anything because he said there'd be no point with no smell. He told me to not run the heater tonight and run the AC again tomorrow to see if there's a smell. If no smell, it's fine and if there is a smell, then they'd go in to clean it.

I wanted to get a second opinion on whether it's fine to just leave it if the smell magically disappeared or if I NEED to get the coils cleaned? I've called several AC companies and I get such different answers from each of them it's hard to know what is the "right" thing to do. One said to install an iWay filter into my unit, another said that I'd need my whole AC unit replaced.

Has anyone else encountered the musty smell going away on its own?


It is a simple case of dirt in your evaporator getting wet.

You now know more than either of the idiots you had come out and check your system. This should have been done on the first call!!!

You can go to most home stores and get aerosol cans of coil cleaner to spray on the coils (the ones inside close to the air handler. The coil usually has a access panel just for this reason.

While you are at the store pick up some condensate pan sanitizer tablets or strips to put in the pan these also help to prevent mold in the pan.

The smell comes from dust and mold collecting on the coils and fins. When the ac is running these get cold and condense moisture out of the air , dirt, mold and water with air blowing over it. Very commonly said to smell like dirty socks. Search this site and you will find a few additional references.

Get new HVAC guys and if you decide to fix it and have success call the company and complain then the BBB as this is really simple.

  • Thank you so much! I'll look into coil cleaners and sanitizer tablets/strips and YouTube videos on how to do it. Was there a link you were going to provide with additional references? – Stephanie Mar 31 at 22:42
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    Check out u-tube “how to clean inside of ac evaporator coil”. There are dozens of them, the ones I use with trainees requires my password sorry I can’t give that out but the only difference is they push a brand of cleaner. The important thing is no acids. I use a lot of nu-Calgon cleaners but there are many brands you won’t need to purchase gallons of concentrate like I do. – Ed Beal Mar 31 at 23:56
  • @Stephanie Make sure there are no dirty socks in there.. :-) – JACK Apr 1 at 14:29

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