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

  • 2
    hvac-talk.com/vbb/…. Good info here
    – Kris
    Jul 24 '18 at 22:53
  • 1
    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. Jul 24 '18 at 23:17
  • 1
    Where is the smell occurring? At the outdoor unit or from the units in your house? Jul 24 '18 at 23:23
  • 1
    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? Jul 25 '18 at 14:40
  • 1
    @LimoDRIVER yes, they do have the PVC drains. I debated trying this idea. Seems to be worth a shot. I'll report back.
    – Andrew
    Jul 25 '18 at 18:25

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.

  • 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
    Jun 9 at 19:08

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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