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Dirty Sock Syndrome

Dirty Sock Syndrome



Every once and a while, we get a call from a concerned customer who asks: “Why does it smell like a dirty locker room when I run my A/C?” If that sounds familiar, then believe it or not, your A/C system may have Dirty Sock Syndrome.

How would you know if you have Dirty Sock Syndrome and not some other issue causing a foul odor? Well, with Dirty Sock Syndrome, the smell only occurs when the HVAC system is turned on to ‘cool.’

Contrary to what many folks may think, Dirty Sock Syndrome is not caused by a faulty HVAC system. It is actually caused by a buildup of bacteria on the evaporator coil of the HVAC system. The evaporator coil is made of aluminum which is extremely porous. The coil is very cool and very damp, creating the perfect condition to attract and accumulate bacteria. In some cases, this buildup of bacteria is caused by high VOC counts in paint or glue, pet dander, dust, or mold that exists in the home.

Dirty sock syndrome can be triggered by frequently switching your system between heat and cool. The warming and then cooling of the bacteria which has collected on the coil can jumpstart the growth of the bacteria and spread the stench.

Lately, we have received what seems to be a high volume of customers calling about their smelly A/C systems. This is most likely due to the odd winter we had this year, where some people were turning on their heater one day and then their A/C the next. The bacteria builds up while the system is off, creating an unpleasant odor when the A/C is turned back on. Fortunately, it only affects about 1% of systems that we installed last year, it’s not serious, and it can be fixed.


  1. Have a UV Light filter installed at the time of your new HVAC system installation. This will kill the bacteria and prevent Dirty Sock Syndrome from occurring. Additionally, you will have extremely clean and purified air in your home.
  2. If you already have Dirty Sock Syndrome in your home, you need to have the coil cleaned. After the coil is cleaned, it would be wise to have a UV light installed into your HVAC system. This will prevent bacteria from growing there again and stop the odors before they start.
  3. Another solution to this problem is to have the coil coated with an epoxy, which will prevent the buildup of bacteria.

Again, Dirty Sock Syndrome is not the result of a faulty or deficient HVAC system. It is caused by bacteria that is formed from something pre-existing in the home. But fear not, this syndrome is both preventable and easily fixed – you don’t have to live with a smelly HVAC system for long.

Do you have any questions about Dirty Sock Syndrome?


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