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Community and Q&A

Odor from Minisplit Heads

j_in_colorado | Posted in Mechanicals on


We had a “hyperheat” Mistubishi mini-split system installed with three heads connected to one large outdoor compressor (late July 2021) in the Colorado foothills near Denver (very dry climate). Ever since it was installed, we have had an intermittent “sour sock” smell coming from each of the heads, however it only happens when the units are in cooling mode, and even then, only immediately after the unit stops cooling (e.g. it has reached its set point or we turn the thermostat up a couple of degrees). It dissipates after a few minutes, but it really unpleasant when it happens. I noticed it on the very first day we had them installed, so I don’t think it’s a case of us not cleaning them properly or anything.

I had the installing contractor out and they confirmed the smell, but couldn’t explain why it would have an odor “fresh” from the factory (they said they sometimes see this after a few years if the filters are never cleaned, for example). After a lot of back and forth (verified there’s no errors in the system, the condensate is draining properly, etc.), they suggested doing a coil cleaning, but we ended up not having it done, since the smell had gone away once we entered the heating season and I was (naively) hoping having them blasting heat all winter might rid them of the odor.

We used the units all through the winter for heat, and had no smells at all during that time, but as soon as we turned them back to cool a couple weeks back, the smell started again. I’m about to reach out to the installing contractor again, but wanted to ask here if anyone had seen something like this before. All our friends and neighbors we know with these systems have said they never have any odors from theirs, but I was hoping the community here might have more experience to draw from and have some suggestions.

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  1. Expert Member
    PETER Engle | | #1

    "Old Sock" syndrome is common in HVAC units, but usually not with brand-new systems. It is generally caused by bacterial growth in stagnant & dirty water that pools somewhere in the system long enough for the growth to get going. it goes away when the systems dry out, but can recur pretty quickly when they start up again.

    Unfortunately, none of this works with your statement that the smell occurred right away when it was installed and first operated. I still think it's water related. Maybe some gunk got into the coils either during manufacture, transport or installation. The coil cleaning would be a good idea. Shouldn't be necessary with a brand-new system, but a brand-new system shouldn't stink, either. I'd also ask to HVAC contractor to take another look at the drain pans inside the system to make sure that all of the water is draining out and not puddling in the system.

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