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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

MonkeyZeus
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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 Apr 26 '22 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 Apr 26 '22 at 13:14
  • It could be, have you checked it for standing water, mold, musty smell or other unusual symptoms? – FreeMan Apr 26 '22 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 Apr 26 '22 at 13:23
  • Musty and _burnt dust_ are very different things. There must be some moisture where there shouldn't be moisture , and the heat causes it to evaporate and flow through the house. – isherwood Apr 27 '22 at 18:10
  • @isherwood It's certainly not burnt dust smell. The one thing I can think of when I first installed it is from page #20 of the [installation manual](https://www.acwholesalers.com/manuals/f96d05f2abe1793d3b99f4c91419d792.pdf) "The drain trap must be primed at time of installation." When I primed it, I overfilled it and flooded the impeller fan a bit. Some water sloshed around a bit but was evacuated and I assume that running it all winter would have dried things out. – MonkeyZeus Apr 27 '22 at 18:21
  • Yes, I was referring to FreeMan's reference to fall startup. – isherwood Apr 27 '22 at 18:21
  • @isherwood I appreciate that. The average person could easily mistake one for the other. Just wanted to assure you that I know the difference =) – MonkeyZeus Apr 27 '22 at 18:24
  • @isherwood Is it strange that the smell is never produced during a cooling cycle? – MonkeyZeus Apr 27 '22 at 18:34
  • Can you add some pictures (and perhaps diagrams) of the system maybe? I'm not always 100% familiar with the terms in english, and something visual might jog my memory. – MiG May 13 '22 at 12:20
  • @MiG I've added an image to my question. – MonkeyZeus May 13 '22 at 12:23
  • Can you share that installation manual pdf somewhere? I'm getting a (probably bad GDPR fix related) "access denied" here. Or can you post the model code? (example: "GMVC950453BX") – MiG May 13 '22 at 12:53
  • @MiG I've added model numbers to the equipment list in the post. Hopefully you can find the PDFs online in your country. Gotta love the GDPR nonsense... – MonkeyZeus May 13 '22 at 12:57
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    Thanks! Don't get me wrong, I'm quite happy with GDPR. It's just difficult when people do geo IP blocks rather than deploy something that works. – MiG May 13 '22 at 12:58
  • Did you ever get this resolved? – FreeMan Oct 13 '22 at 12:00
  • @FreeMan I have not yet. I cannot believe it's been 6 months since I posted this question :-O. Anyways, the heating season has started and there was a little bit of mustiness for the first few hours. I just haven't had time to open the evaporator coil to inspect it; even though that is my prime suspect, hah. – MonkeyZeus Oct 13 '22 at 12:51

4 Answers4

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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.

GOATNine
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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.

Andrew
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    The question states that the smell goes away after a few days. That makes the drain theory unlikely. – Chenmunka May 16 '22 at 06:23
  • 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 May 16 '22 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.

Gil
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  • Thanks, is mine UVC? See this bullet-point: "UV light above the evaporator coil: Honeywell UV2400U5000" – MonkeyZeus Oct 13 '22 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 Oct 15 '22 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.

blacksmith37
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