I am having trouble getting rid of a smell in my AC (installed 2016), 2 visits by an HVAC professional have resulted in no luck.


There is a musty smell, from all vents, as the AC is turning on and just after it turns off. While the air is at its peak cold, there is no smell.

  • We have a variable speed fan, which we were told could/should run all the time (on low) to keep air circulating. We like to leave this on in the summer (off in the winter).
  • If we turn the fan off, smell isn't noticeable, as smell is worst when fan runs after AC.
  • First visit by HVAC professional, we were told everything was fine, probably dirty air ducts.

After this first visit, I did some investigating, I carefully removed the outer panel over the AC evaporator coils. When I did this I could instantly smell the smell coming off of the coils as soon as the compressor came on, again no smell while its running, but smell comes back as the compressor turns off and remains for a few minutes.

HVAC guy came back, decided evaporator coils needed to be cleaned, sprayed them, and left. Smell got better, especially while you could smell the cleaner, but came back after a few days.

After this, I purchased a can of cleaner and repeated the process, and again the smell came back, but doesn't seem to be as bad.

  • Do I need to just keep re-applying the cleaner until its gone? The coils look clean (the HVAC guy removed the inner panel to see the inside, and its pretty clean.
  • In the last 4 years, we have run the fan on low 24/7 during the summer and haven't had this issue.
  • I poured water/clorox down the pan/drain, but it looked pretty clean.

I don't think its dirty ducts, but am willing to pay to get them cleaned, but want some more advice before spending money on something that has questionable effect.


  • 2
    Have you always changed the filters? You might need to have the coils removed from the air handler and pressure cleaned or at least water sprayed.
    – JACK
    Commented Jul 16, 2020 at 23:36
  • 2
    Never use pressure cleaners on hvac coils it takes very little pressure to bend the fins and ruin the coils , removal is not feasible as the system would have to be recovered down to a vacuum then replaced and pumped to a high vacuum. Simple cleaning in place is needed.
    – Ed Beal
    Commented Jul 18, 2020 at 15:52
  • Do you have a humidifier?
    – rogerdpack
    Commented May 23, 2021 at 4:13
  • We use a humidifier occasionally when we have sick kids, croup, etc. But not regularly, we are in a dry desert climate too. Commented May 24, 2021 at 19:57
  • Agree that dirty coils is the problem here, but that's a symptom of dirt and grease getting through your filter (esp since it's not coming clean easily). Make sure you're using quality filters, not the really cheap ones, and changing them pretty often. Running the system on recirculate, you'll need a new one every 1-3 months. Good ones cost $5-10 each. Obvs that adds up; buy in bulk. And if you're cooking a lot with oil, putting grease in the air, this is all the more important as it's hard to clean off coils.
    – pbristow
    Commented May 4, 2022 at 2:09

2 Answers 2


Musty smells in the evaporator coils are common. You really need a foaming cleaner and the coils need to be cleaned 2-3 times to get everything out. I use a product called Nu-brite from My-Calgon it is a non acidic foaming coil cleaner, make sure your drain is working prior to use or it will make a mess. The foam pushes the dirt out.
What you are smelling is dust and grime, and when the system runs it collects moisture. Now that dust and grime are the “smelly socks” breeding ground - you don’t smell it mid cycle because it is frozen at the start and end. When the coils are thawing you get the full odor.

1 cleaning usually helps. 2 may do it, but 3 has fixed even the worst (smokers) coils I have encountered, with a clear water rinse after each cleaning.
Coil cleaning is suggested at least 1 time per year and with heavy use 2x.

Always have a filter in your system or this can start in just a few hours with dusty conditions.

Whatever you use don’t use an acid based cleaner. Acids damage the aluminum fins and shorten the coil life.

  • Thanks for such a great explanation, especially why it smells at start and end! The smell is going away, I may give it another spray. The HVAC technician didn't spray much, I used almost a whole can and its almost gone now, hopefully another spraying will wipe it out.. Commented Jul 19, 2020 at 18:56
  • 1
    I haven’t used that spray but it sounds similar to nu-brite the foaming action pushes the dirt out. Possibly the difference is cost a gallon of nubrite will outlast more than a dozen cans of aerosol but for a home owner that might be a lifetime supply and you don’t have to mix, if you like it I will mention it to customers that like to do some of their own cleaning in between a yearly check up.
    – Ed Beal
    Commented Jul 19, 2020 at 23:36
  • Quick follow up, after spraying is it best to leave the AC off for a while, or can I use it right after. Can instructions don't provide a clear answer. Commented Jul 22, 2020 at 18:24
  • You want it to quit foaming and a quick rinse is even better but once it is done you can go back to using it. I know one guy that puts it on with the fan running but that spatters stuff everywhere I would not recommend that.
    – Ed Beal
    Commented Jul 22, 2020 at 19:37
  • 1
    @slattery, nope I wasn't able to get rid of it. My wife can't smell it for some reason, and I have just learned to live with it for now. I have tried so many different things and HVAC people, etc. I figure its that way till I replace it. Commented May 25, 2023 at 16:20

It's possible that you have a clogged condensate drain line and the condensate is pooling in the pan under your condenser and growing mildew / mold that then gets sucked into the air ducts. Did the HVAC service techs check that? It's not hard to clean them out yourself if you have a wet-vac, and a yearly flush with vinegar will keep them from smelling.

Here's an example: https://www.myfrugalhome.com/how-to-clean-a-clogged-ac-condensate-drain-line/

  • Vinegar is a bad thing on evaporator coils and galvanized metal it actually eats both never use acids even mild acids unless you want the sheet metal pan to rust and the fins and coils to fail prematurely. The tech would have noticed a plugged drain when he cleaned them.
    – Ed Beal
    Commented Jul 17, 2020 at 19:35
  • 1
    Roger that, I have been using this: lowes.com/pd/… Commented Jul 19, 2020 at 18:54
  • I checked the condensate drain, its clean. Each time the smell has gotten better, I will give it another spray with AC cleaner and see. Commented Jul 19, 2020 at 18:54

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