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Why My Air Conditioner Is Emitting A Foul Smell?

If your air conditioner is emitting a foul smell while running, it’s important that you get it identified and removed right away. More so if the unit is running in tropical climate, because sultry weather conditions are not exactly favorable circumstances for air-conditioning units to run if they are not used and serviced properly.

For AC users experiencing bad odor emissions from their units, the following can be the causes.

Is your AC too large for your room?

ACs need to fit in perfectly to the room size – not more, not less. For example if your room is not too large, a 1-1.5 ton AC would be big enough to cut back on the humidity levels and cool it down. But if your ACs cooling capacity is larger than what is required, it can make way for a musty smell to come across. This is because larger than required ACs are not able to perform the ‘latent cooling’ function properly (which accounts for 70% of an AC unit’s work), i.e. dehumidifying the air to the right extent, in spite of chilling down the air much quicker. Growth of mold spores and stale smell are normal thereafter.

So if you have a reasonably bigger unit in your home and experiencing a foul smell, it can well be because of its size. Call for a professional to check that. 600 SF/ton-800SF/ton is the standard requirement. For older set-ups 400SF/ton can be needed also. Anything larger than this must not however be installed without consulting a contractor.

Is the water drain clogged?

ACs which run for extended hours are often seen to emit foul smell because of clogged water drainage systems in them. In case of the drain being blocked with dirt and grime particles, the water stays back in the ‘condensate’ tray and with time this starts emitting a foul smell. In a tropical climate it is not unusual for bacteria and/or mold to grow within the condenser coil also, which on running the AC, spreads a rotten smell in the house.

Removing the AC cover and freshening up the tray and the drainage holes can solve this problem.

Is your AC being used properly?

Leading AC manufacturers are of the opinion that foul smell emissions can also occur if your air-conditioning unit is not being used properly. Running the AC on ‘Low’ constantly can be damaging for the indoor air quality for instance. This is because dehumidification will not be carried out to the right levels here also, resulting in mold growths and obnoxious odors thereafter.

As a remedial measure, put your AC on the ‘Fan’ mode for sometime so that any accumulated moisture in the vent system of the unit gets released and/or dried up.

In some cases water seepage into the pipes of a central AC, dirty air filters, engine fluid leakage and rotten dead animals in the system’s duct can also induce it to emit a stink. So if you find that your air conditioner is emitting a foul smell its time perhaps to call for the maintenance team to identify and amend it. Your house will start smelling fresh again.

29 Responses so far.

  1. Imelda says:

    Thanks for sharing this. Lately and for a week now, our AC had been emitting a foul, rotting smell. I’ve told the hubby I can smell it really bad all day long as long as I am inside the house but he said he can’t and so does his buddy said he don’t smell it either. So until now nothing has been done to it! What could happen if our AC has been left unchecked for long? HELP!

    • Alex Poh says:

      Hi Imelda, sorry for the late reply as I’m very busy running my day business the last couple of months. If your husband and his buddy can’t smell it, it does not meant that everything is alright. If you are technical enough, you can try to resolve it. Otherwise, it is best to seek a help of a qualified professional to diagnosed where the smell is coming from before things get worse. Hopefully that helps!

  2. R B says:

    Finally! This is a recent problem and I have a very sensitive sniffer AND I had eliminated all other potential issues. My AC has been running for extended periods of time. I just figured I was running it too hard even though we have had a mild summer. I never thought to think of my room air conditioner (RAC). RAC’s, of course, bring in air directly from the outside. I can smell grass if my or my neighbor cut our grass, etc. I was actually thinking there might be a dead animal outside the unit (it’s elevated) but it’s not quite that, well, overpowering. It is, however, continuous and sickening.

    I now have an avenue of attack and I thank you, more than you can know. Many, many thanks.

  3. Mike Thompson says:

    Hi, I seemed to be cursed with my A/C system. Around 5 years back, my A/C company convinced me to upgrade my system to a high-end Goodman 3-Ton unit. It has been nothing but a headache. Our A/C emits terrible foul odors, and our A/C company can only recommend to dump more and more money on fancy air purification and/or filters, etc.

    What I have concluded (I think) is the problem surfaces in the cool weather in Florida when the A/C runs only a small amount of time during the day. We do not have a problem in summer when the A/C is running many hours in the day.

    I have thought that the air ducts may be dirty, and we had those cleaned to the tune of $900. Or that the air was just getting stale in the ducts, and when the A/C kicks on it pushes out the stale air. But I tried leaving the fan on low to keep constant circulation, but the foul odor is still here.

    The problem came into existence with our new A/C system. I am about to give up on the high-end Goodman air handler and replace it. I am convinced that the Goodman air handler is a crappy product. If you have some suggestions I would appreciate it.

    • Ed Zweiger says:

      Wow. They just installed a Goodman and stinks. Smell from day one and keeps getting worse. Very dry AZ climate. Stupjd air handler gets confused in falling temp at night and fan runs all night wssting energy. Terrible unit. They did put in half ton bigger which is no doubt part of problem. Super cold. But dumb air handler.

    • Jessica says:

      We have a Carrier unit and have the same problem when the AC isn’t running constantly. We actually had all of the ducts and the handler replaced. Still smells! It’s awful.

  4. Alex Poh says:

    Him Mike, I’m sorry to hear about your problems with your A/C system. You can try to check on the piping to see whether there are any leaks. However, i think it is best to get another a/c company to check out your A/C system before dumping it. I hope that helps.

  5. Nelly says:

    April 5,2014 Package Unit – Rheem 3.5 ton 16 Seer – New A/C unit with a heat pump – installed on 03/18/14 and tested on 03/22/14. The very day of testing the unit a terrible metallic burning smell that stink the entire house came in. At the Company who installed it and also did a complete new Duct work recommendation we run it on fan with windows open for few hours – did not help. We run it on cool w/ windows cracked – did not help. We run it in cool with windows closed it got worst. The house is complete empty – has ben clean several times so there is no other odors to blame. The unit is a package unit that seats outside on a slat that was built. The house has only tile floors – so there is no rugs. This smell is so horrific that makes me sick the minute I enter the home. However, my husband said he doesn’t smell anything. Three of my friends also smell it. one got sick in the first 10 minutes she was in the house – started with a terrible headache and had to leave.Please if you have any information where to get help, pls help us.

    • Alex Poh says:

      Hi Nelly, it is highly possible that your odors may have come from parts overheating causing parts like the insulation on the wiring to melt. It is best to get another company to check it for you. You may have got yourself a faulty unit however since yours is a new A/C, check whether you can do a one for one exchange if the odor still persists.

  6. Alejandra says:

    Hi I wanted to ask we bought a new ac ton of 4 about 2 months ago and know it started to get a really bad smell when it stops runing the smell comes from main vent please help need to know what it could be.

  7. Susan Reisenauer says:

    Do not sell my email!,,
    The capacitor burning out will emit a kerosene like smell. Get it replaced, as it is hard on the unit. That’s what happened to my new one, and did it ever stink. It went all through the vents. They go out frequently.

  8. Denise says:

    Help! My a/c in my apartment keeps letting out a musty smell when the air turns on. The apt. Had my all my air ducts cleaned out and the drip pan tray ive cleaned several times. They also come by to drop in condensation pan tablets to mask the problem and they also sprayed some cleaner on the a/c coils to clean the years of dirt off. Like i said they cleaned the air ducts and used the chemical inside there to kill whatever was inside? Its been about a month and the musty odor went away and now its back! Im so frustrated! I dont know what to do next to get rid of the odor? I live in south texas where it is humid. Any suggestions would be greatly appreciated.

    • Trudy says:

      What ever happened? Did you get it resolved? We live in Lake Charles, LA and have the same thing happening. I have had so much work done on the darn AC I am about out of my mind. I am running AC on fan right now, It smells better but when I turn it off and turn it back on again, it smells terrible. Like smelly shoes! We had a black mildew attack this past summer. Please let me know. We had the coil cleaned, put fans in the attic to put air on the AC, had the ducts cleaned and some replaced (almost all!), unit outside cleaned, a UV light installed and the vent ridge replaced. Not to mention Ozoned house, threw away furniture, etc. (yes, it was growing in the furniture!!!!). Anyway, please let me now. Blessings

  9. Tim says:

    Last year I ran my portable 12,000 Btu unit in an apartment and it worked great. I recently bought a house and I stored the unit in a room in my basement till I recently needed it. Now I have it running in the living room and down the hall in my bathroom and master bedroom I get a musty smell that is almost unbearable. The longer it runs the worse it gets. I opened the unit and cause I read it could be mold on cooling lines. I cleaned all that with hot water and vinegar and smell is still there.

  10. Gopal says:


  11. Angie says:

    I have 5 window units and my daughters is the only one that stinks and is the only one that is beside the front porch light which attracts a ton of bugs which are getting into the vents of her window unit. Her room will smell like a dirty wet dog and the air coming off of it from the outside smells like dead animal which will work it’s way to the inside as well. What can we do and how can we prevent bugs from getting into the back of the unit? I think that may be why hers is smelling so bad.

  12. Merve fetter says:

    Hi! Thank you for all the helpful info. This year we turned our ac on and i have noticed a bad smell.. It s not like mold but it smells like a dusty room thats been closed for months. I have called a company and they checked the evaporator coils, they said theres no mold or no repairs needed. He speayed some lysol on it but the smell still exist.. What else di you think can cause it ? Any info will be greatly appreciated i have too kids and my anxiety rises everytime i turn the ac on. Thanks in advance.

  13. Shanzid says:

    Actually I realized my room size is small to fit 2 ton A/C. That’s why it is emitting a foul smell? Is there any way to resolve it?

  14. babu says:

    My new 1.5 ton Samsung Ac gives bad foul smell after 10 mins .service engg unable to solve this at work shop level.help me.

  15. Augustina says:

    In new house 4 mos. live in the south, very humid. I have been experiencing a subtle, somewhat acrid odor that irritates my nose. Trouble is I am the only one affected. What to do?

  16. Brenda says:

    Could dirty/clogged coils in my central a/c unit cause a burnt or chemical smell in my home? I look forward to hearing from you.

  17. Amy says:

    We had a new indoor unit and several parts of the duct work installed. We live in FL. When the a/c turns on it smells awful. (like the dirty sock thing) Once it runs a minutes it either goes away or we are getting used to it. What can we do? The people that installed the new unit said they would charge us a service call to come out and look unless it was directly cause from something they did!

  18. Lola says:

    How can only one room smell when the air comes on. This is a central air unit. Old house….registers on the wall. Can animals get into the duct work?

  19. Lola says:

    Foul smell only in one room when central air comes on.

  20. Sanuel says:

    Hi Iam suffering a lot with my ac.its Ogeneral 1.5ton. It’s emitting too much foul smell whenever I run it ..Like the smell of a wet cloth kept without drying for a long time.. Initially after starting up the smell arrives and after 5-7mins it just goes away..I’m like tortured everyday by this Smell.Called for service ..No response ..they checked both units..They say it’s normal and it’s not an issue
    . Pls help me out ..For which I would be so grateful..Thank you

  21. Mansi says:

    Actually we recently got our ac serviced totally. But after four to five days we noticed that the the water was not dripping and after that there was stinky smell. I read your article it was helpfull but just wanted to ask that how should we solve this problem of clogged water drainage system. Its a split ac of Lg company.

  22. Ryan says:

    We’ve been using our ac for about 8 years now, i regularly call a ac cleaner to clean our ac. But this past few days we’ve been smelling a burnt like smell. And I think it’s the capacitor. But the ac still runs fine. Is it bad to our health if we continue using it?.. it’s 3rd day today to be exact.. need answer pls

  23. […] actually, this problem has nothing to do with bad smelling socks. Dirty sock syndrome is when your air conditioner produces a foul smell when it is turned on. This odor is often described as moldy and musty which is an unpleasant smell […]