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When switching from cooling to heating in the fall we get about 2-3 days of musty air and then it's fine all winter. It's like wet sock/sneaker smell.

Yesterday, the A/C kicked on because our house hit 80°F. At night the furnace was switched to heating mode and the musty smell lasted a few hours.

One important thing to note is that the musty air is humid.

I checked my ductwork (trunk and branches) and see no mold and they're not smelly. Cooling mode never produces a smell. The furnace is in an unfinished basement and we have a dehumidifier that keeps things at 50% or less. The filter is not musty and is replaced 3-4 times per year.

We've even had a few consecutive warm winter days where the furnace didn't turn on and once it did there was no smell.

The unit is not short-cycling. Heating/cooling cycles always last 15 minutes or more because the ECM starts slow and ramps up as needed.

What could be the source?


Equipment list:

  • 97% Goodman ECM variable-speed 60k btu furnace: GMVM970603BN
  • 2-ton 2-stage Goodman outdoor condenser: GSXC180241
  • 3-ton Goodman evaporator coil: CAPF3636B6 with TX2N4
  • AHRI #201506361
  • UV light above the evaporator coil: Honeywell UV2400U5000
  • Brand new ductwork
  • 4" MERV 11 filter installed on return duct
  • All installed January 2021

I don't have a picture of my personal setup but this image is almost identical to what I have:

enter image description here

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    We usually get a musty smell from our furnace when we first kick it on in the fall, so that doesn't strike me as too odd. Though we didn't this year because the HVAC was replaced last summer. However, having the musty smell after having the AC on during the day then heat at night is unusual.
    – FreeMan
    Commented Apr 26, 2022 at 13:02
  • @FreeMan Yeah I'm familiar with average furnace mustiness but this is like wet sock smell. I guess the main difference between a heating and cooling cycle is that the evaporator coil is producing moisture so maybe my drain pan is to blame?
    – MonkeyZeus
    Commented Apr 26, 2022 at 13:14
  • It could be, have you checked it for standing water, mold, musty smell or other unusual symptoms?
    – FreeMan
    Commented Apr 26, 2022 at 13:16
  • I did last fall and it all looked normal. The water drains into the condensate pump with zero issues. If the drain pan was to blame then wouldn't I get a smell when the A/C runs as well? The A/C air smells perfectly crisp 100% of the time. Recently, I stayed in a Florida home with musty A/C air because of short-cycling so I definitely know what bad A/C air is like.
    – MonkeyZeus
    Commented Apr 26, 2022 at 13:23
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    Commented May 13, 2022 at 12:58

4 Answers 4

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The source of the musty smell probably exists between where the furnace is and where the ductwork for the furnace and A/C join together. Since the smell only results when you switch from A/C to heat, I would guess that the cold air form the A/C is causing condensation to form in the ductwork. when the heat kicks on, it dries (and potentially dislodges) any damp dust in the ducting. That evaporated moisture will almost certainly carry some dust with it as well. It's worth checking the moisture content of the air coming from the A/C, which may identify an issue with the A/C moisture system.

If you have a whole home humidifier, that may be dumping moisture into the system unexpectedly (I know my Honeywell unit runs whenever the blower is on, and the blower will run with the A/C as well as the heat), which can result in the above conditions as well.

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floor drain

I believe the source of this smell is this. If you have any drains like this in your basement, pour some hot bleach water down them.

I believe we don't smell it as much here during the winter because the ground is frozen.

Maybe have a plumber come out with a camera and snake the plumbing to see if anything is wrong.

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    The question states that the smell goes away after a few days. That makes the drain theory unlikely.
    – Chenmunka
    Commented May 16, 2022 at 6:23
  • Sorry, I forgot to mention that I have a condensate pump that pumps the water 20 feet away from the furnace. Good theory though!
    – MonkeyZeus
    Commented May 16, 2022 at 10:36
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The musty smell is mold, this is normal. This typically occurs on the AC evaporator coil. If you look it is no longer a shiny metal evaporator like it was when new. When the heat is turned on it kills the mold and only some spores are left for the next cooling season. In your picture the combustion chamber is completely separate from the evaporator however the air flows from the heating chamber (bonnet) across the condenser coils.

Per the CDC, “molds can cause nasal stuffiness, throat irritation, coughing or wheezing, eye irritation, or, in some cases, skin irritation. People with mold allergies may have more severe reactions."

In my home I added a UV-C germicidal lamps. I installed one near the evaporator coil and other in the return vent. I have been told that Long-term usage of UV germicidal lamps not only tackles existing mold growth, but it also plays a role in preventing future mold, mildew, and bacteria growth. The result of this we are experiencing a lot less resportial problems and the smell is gone. I installed them in the middle of a heating season so it took a year to realize the results.

You can kill the existing mold by using bleach or hydrogen peroxide to clean the evaporator and other surfaces. My recommendation is to install the UV-C (note it must be the -C) and then replace the bulb every year or two at the most depending on how they are installed (on all the time or cycled). My systems blower runs 24/7 year around. That keeps the air clean and the temperatures even.

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  • Thanks, is mine UVC? See this bullet-point: "UV light above the evaporator coil: Honeywell UV2400U5000"
    – MonkeyZeus
    Commented Oct 13, 2022 at 16:54
  • I will take a SWAG and say it is not enough UV light or the wrong color (not UVC). I looked at the data I found from Honeywell and it did not state either. Also these need a minimum size depending on your location and size of your home. I just turned mine and no smell and I am heating several thousand sq ft. The lights and blower were on 24/7 all summer. There should be some information on how to size it from Honeywell but I did not find it.
    – Gil
    Commented Oct 15, 2022 at 18:41
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Some smell comes from heating household dust deposited on furnace during the cooling season. We use more expensive corrugated filters and change them every two months. We get no trace of odor during winter start or other times because the filters remove nearly all dust from the system. It is a 4 year old AC /gas furnace and 25 year old ducts, no humidifier.

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