If I leave the furnace fan always running for an extended period of time (to circulate and filter the air), when the central AC turns on there is a not-so-fresh smell that is emitted through the vents. It almost smells wet or stale.

This smell eventually disappears if the fan is not run in between AC cycles.

There is a MERV-16 filter, which is regularly replaced, located within the system, between the air intake and the furnace.

What is this smell? What is causing it? Besides not running the fan, how do you prevent it?

2 Answers 2


Same thing my the system in my house. Uses a MERV 16 5". Smell was the filter itself. Dont know what it picked up, but it was generating its own odor. Whew ! I think it started after I baked a beef roast. which smoked a bit when in the oven. Maybe something in rhe meat ?

  • Thanks. I changed the filter and I feel like the smell reduced a little, but is still present when I turn on the AC after running the fan. If I don't run the fan in between AC cycles, the smell disappears. Commented Feb 27, 2015 at 3:48

AKA: Dirty Sock Syndrome (paraphrased), caused by the growth of mold and bacteria on the coil. Heat pumps (central HVAC) are particularly susceptible because, unlike conventional heat exchangers, their heating cycles are not hot enough to kill the microbes that thrive on their wet coils during the cooling season. Instead, the temperature is just warm enough to slowly “cook off” their organic odors, producing that gym sock odor.

Clean the coil attached to your furnace.

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  • Thank you for your excellent answer. I truly hope it is not mold, as I have mold allergies. I had the installer come out and I described the smell to him. I turned on the AC so he could smell it. It is like a wet or stale smell, similar to wet wood. He said that Dirty Sock Syndrome is different and is an unmistakable horrible smell. I hope he is correct. Commented Feb 27, 2015 at 3:46

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