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I have 2 Mitsubishi mini split units on the 2 additions on my house. One is a Mr. Slim Inverter and the other is an Inverter.

During the warmer months when I am running them in A/C mode, they frequently give off a pungent sour milk smell. It seems to be when it is particularly humid outside. Here's what I've tried to fix the smell.

  • Cleaning the removable/reusable blue plastic air filter
  • Pouring a cup of white vinegar over the evaporator coil
  • Pouring a cup of bleach over the evaporator coil
  • Spraying the evaporator coil with Nu Calgon Evap Cleaner
  • Removing an cleaning the blower wheel, drain pan and all surrounding plastic with the Nu Calgon cleaner.

These all seemed to help at first but the smell always comes back, sometimes within days. It is worth noting that the evaporator coil doesn't look particularly dirty. The drain pan was a bit funky and seemed to have a good bit of water but I checked and the drain is clearly allowing water to flow through to the outside.

Any ideas? The smell is very unpleasant and doing a number on our sinuses.

Here's the evaporator coil

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    hvac-talk.com/vbb/…. Good info here
    – Kris
    Commented Jul 24, 2018 at 22:53
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    I would not use vinegar on the evaporator/air handler because it could corrode aluminum and steel parts. Try spraying with 3 percent peroxide or pouring it over the evaporator coils. This should leave no residue or odor. Chlorine bleach is a lot more effective at killing mold, but would have an odor. Commented Jul 24, 2018 at 23:17
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    Where is the smell occurring? At the outdoor unit or from the units in your house? Commented Jul 24, 2018 at 23:23
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    Do you mini splits have PVC drains that go outside to get rid of the condensate? If do have you hooked up a wet vac shop vac to the pipes to try and clear out the algee growing in them? Commented Jul 25, 2018 at 14:40
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    its the water pan, treat the water in it, and better yet, keep the water out of it (improve drainage)
    – dandavis
    Commented Jul 25, 2018 at 18:54

6 Answers 6

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Posting my experience here as it may help others.

This happened for the first time to me last night -- same scenario: 2-year-old Mitsubishi units, very humid day (raining on and off), low cooling load. The whole room is damp and the air is smelly, after running for only ~1 hour.

What solved it for me what switching the unit to "dry" mode. 10 minutes later, the smell was gone and the room is no longer damp.

I suspect the mechanism at play -- as hinted in the thread @Kris posted in a comment on the question -- is that, at low cooling loads, modern mini-splits modulate their cooling output. Because of this, the coils don't get cool enough to condense a significant amount of water out of the air. This means that at high relative humidity, as the room cools, that moisture will be forced out of the air everywhere else. Moist everything + air movement = smells.

Traditional A/Cs don't have this issue, as they are either full on or full off, so have no trouble condensing water while they are cooling the air. Same goes for "dry" mode in a modern mini-split.

This is entirely an armchair theory; I am not an HVAC expert and would love to be corrected.

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    This did work for us as well... with the smell. However, the "dry/dehumidify" mode would make the room WAY too cold. Cleaning it with condenser cleaner would help temporarily but the smell would come back. The best thing we found was to run a separate dehumidifier in the rooms and that helped tremendously. Ultimately, I never really solved the issue and we have since moved so it is no longer my problem.
    – Andrew
    Commented Jun 9, 2021 at 19:08
  • Thanks for this answer, Chris - this worked for me! I've had similar problems to the OP for several years. I had an HVAC tech come and do an expensive cleaning of the coils, etc., but it only helped for a bit. I tried running dehumidify as you suggested and it seemed to work. Here's what I do in case anyone sees this: as soon as I detect a musty odor from the mini-split, I switch to dehumidify on full blast until the smell is gone. Usually this only takes about ten or fifteen minutes, but I did find that to initially fix the problem I had to run dehumidify overnight.
    – Bill
    Commented Jun 16, 2022 at 15:16
  • The other thing that helps, too (which was suggested to me by a Mitsubishi rep when I called about it), is to run the unit on COLD (not heat) but at the max temperature, which is 88 degrees for my unit. I imagine this dries out the unit, too, similar to a dehumidify. A combination of these two tricks (dehumidify and running on COLD at 88 degrees) fixes the problem for me and I can then switch back to normal A/C mode. It's not ideal that I have to do this, but it's still better than filling my prewar home with ductwork.
    – Bill
    Commented Jun 16, 2022 at 15:18
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Found this thread really helpful as I tackled this same issue, so thought I'd join and share my experience.

I live in a hot, humid climate and have a new higher-end Mitsubishi mini-split that emits really annoying "humid" smells.

It's new, so we can rule out dirt and mildew/mold. That's key, because all of the forums and HVAC sites always say that's the culprit, but it's not. That's probably why the poor OP (Andrew) cleaned and cleaned without improvement.

Let's also rule out the other commonly-cited culprit, namely the drain line or drain pan. I disassembled the interior unit just enough to to confirm that the drain pan actually drains properly. There was a very small amount of water left in at the bottom of the "V" shaped pan after I tested by slowly pouring half a gallon of water down it, which it drained liked a champ. Also, I confirmed that the unit was installed perfectly level. Essentially, the drainage system in these units appears to be made to handle potentially meaningful amounts of water during continuous operation, not to be perfectly dry, and that's fine.

The reason for the "humid" smell seems to Chris Pacejo's hypothesis, that mini splits don't cycle on/off like traditional AC Units but instead go into a lower-power mode and the fan keeps running.

The coils accumulate some condensation while the mini split is actively working to cooling.

When the mini split gets to around the target temperature and goes into low-power / maintenance mode, the air running over the coils now picks up the condensation remaining on the coils and gradually revaporizes the water back into the room.

You avoid this primarily by keeping the coils cold, and secondarily by making sure the fan speed isn't set really high and forcibly ejecting the condensation.

Dry Mode does both and hence works like a charm for the smell aspect, but you give up temperature and fan speed control to a meaningful extent.

Same explanation applies to posts on other sites that say they can eliminate the smell by setting the AC target temperature a few degrees lower. The lower temperature just means the mini-split has to work harder and won't go into low-power mode as often or even at all (depending on the specifics of your setup).

Traditional ACs are either fully on or fully off, which is why they won't have this smell issue as much as mini splits.

It also seems to depend on your specific unit's cooling capacity. My basic understanding is that cooling capacity consists of two parts: ability to lower air temperature and ability to condense water out of the air. My larger unit has the smell problem because it actually has the power to condense a lot (ironically, why I chose it). My smaller units are primarily for lowering air temperature and don't have much condensation capacity, likely explaining why they don't smell anywhere near as much.

I'm thinking that there's no way to get rid of the smell (other than masking it) without simply keeping the coils colder longer and on a moderate fan setting.

The good news is that it seems like it's not mold in this case (so smell isn't a health issue) and that we don't have to keep trying to reclean coils a million ways.

Just another hypothesis consistent with what others are seeing.

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    we found that running "dry mode" or the "dehumidify" mode would prevent the smell but would also freeze us out. For whatever reason, it still seemed to pump out cold air in that mode. The most helpful thing we did in the end was run a dehumidifier independently of the mini-split. This kept the moisture down enough to prevent the damp funk when it kicked into low-power mode. Really seems like a design flaw to me.
    – Andrew
    Commented Aug 19, 2022 at 14:16
  • Andrew's suggestion of adding a standalone dehumidifier is consistent with my experience. Humidity is often 70%+ in my area. Ran a test and on a 75% day. The large living room mini split running in Dry Mode lowered humidity to 67% after twelve hours. Still very humid and in 'mold growth' territory. Purchased a separate dehumidifier and it holds the humidity at 50%. In the summer, set the mini split on regular Cool Mode to offset the heat from the dehumidifier. In the winter, the dehumidifier heat is useful. Totally agree, mini split + dehumidifier is a good setup in humid climates.
    – Dave777
    Commented Aug 20, 2022 at 20:53
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I wonder if the mold is growing in a place in the drains where cleaning solution isn't reaching. I can think of different possibilities to get the cleaning solution on the entire inner surface of the drains:

  1. Add cleaning solution as directed, then use a shop vac for a short time on the end to vigorously pull cleaning solution through the drain.
  2. Add solution as directed. then plug the drain to let the solution entirely fill the drain.
  3. Make up a quart of cleaning solution in a jug and introduce this into the unit from the discharge end outside so that the solution completely fills the drain. This risks the possibility of an overflow of solution into the evaporator unit inside and so one would have to be careful. I'd use a length of clear neoprene tubing with swollen sections so you could cut at a swelling to get an end of the right diameter to seal the discharge end of the drain. By alternately raising and lowering the jug one could flush the drain repeatedly.
  4. Attach a 6-ft or 8-ft length of neoprene tubing to the end of the outflow tube outside and raise the end of the neoprene tubing to the height of the drain pan inside the evaporator. Introduce the cleaning solution into the drain inside as instructed. The solution should completely fill the drain system and overflow out the end of the neoprene tubing.
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I wanted to share my experience here, since I've been having this problem for the last few years. We've tried a lot of things to fix this. For the first two years we had the system, there were no problems, but once this started happening, it was very challenging to resolve. I spoke to our HVAC company and even called the manufacturer directly. Some suggestions I got:

  1. Apply a diluted cleaning solution to the coils.
  2. Run the unit on COLD at max temperature (88 degrees).
  3. Do a professional cleaning of the unit (we did both the bag-and-clean and the kind where the technician takes the unit apart and cleans it)

All of these helped, at least for a bit, but the problem eventually came back.

I finally found a solution after thinking about Chris's answer above and discussing with my HVAC technician. As he says, conventional HVAC systems are binary: they're either on full-blast, or they're off. Mini-split units, on the other hand, are variable-speed. They will slow the fans down once they're near their target temperature.

But this made me realize that I often run my AC at MAX fan speed, where I'm not giving the mini-split the ability to slow down the fans when necessary. I decided to try setting the fan on "AUTO" mode and never using any other mode. In the past, I'd often set the fan on max to cool the house quickly. But based on what my HVAC tech told me (and Chris's answer above), this isn't a good idea - when the air reaches the desired temperature, the cooling coil turns itself off, but your fans are still blowing on max, letting humidity and other nasty stuff build up on the now-warm coil and blowing it out into your home. In auto mode, the fans adjust themselves to be slower, or barely on, when the coil is off.

Another thing that worked (in parallel with the above) was a product from Nu-Calgon called ClenAir MiniFresh. My HVAC tech mentioned it to me, and I ordered it online. You rest it on the vent above the mini-split and turn it on. As air flows through it, it "reacts with odor-causing compounds and eliminates them." I tried this on both of my mini-splits. Right away, the smell improved.

I can now run them on any temperature (using AUTO fan speed only) and it seems to work. I hope this helps someone!

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I have one and it's been great, so I was interested. @dandavis said something about a "water pan", and I was curious where that was. I think he is probably right, but I stumbled upon this detailed vid about cleaning these things. In here somewhere (I am confident) is the solution to your problem: https://www.youtube.com/watch?v=2e6by0tV8h4

The main points are (1) that there are a lot more nooks and crannies that may be cleaned than just filters; and (2) that -- as an owner/operator of a similar product -- I believe @dandavis is right and the water pan is the culprit. (The video includes a nice disassembly of a unit, including how to access and clean said water pan.)

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I think there are 3 things that are happening:

  • During cooling the coils are below the dew point so water condenses on it, rather than evaporate.

  • Air also contains dust particles and other aromatic compounds from outgassing of furniture, clothes, sweat, etc, these get mixed into the water on the fins.

  • The metal fins of the coil act as a catalyst to cause chemical reactions that would not happen otherwise. That's also the reason why metals have "metal smell", it's not the metal that smells (it does not evaporate), but the fat, sweat, etc. that quickly oxidizes when you touch it due to the catalytic effect of the metal.

When the cooling cycle is over the coil warms up so the liquids on them evaporate, and the smelly reaction products get blown into the room.

The solution would be to reprogram AC like this:

  • At the end of the cooling cycle, the blower fan turns off entirely so the smelly stuff is not blown into the room.

  • A coil drying cycle that briefly switches the unit into heating mode for a few minutes to dry the coils to prevent actual mold growth, again without running blower while doing this. It should run 1-2 times a day while the AC is in use. So the smelly stuff go up to ceiling rather than get blown into the air.

My AUX AC has the latter "Anti-F" mode, but there is no way to turn off the blower fans at the end of the cooling cycle.

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