I have been trying to track down a wet dog / dirty sock smell coming from the HVAC. This lead to opening up the evaporator coil box and looking inside.

I see a lot of rust and some liquid. I do not see any ice, and we have not been running the AC very much, though it has been running a bit. The unit was manufactured in 2002, and I do see things that look like mold. It is probably time to replace the whole thing.

Can you determine from these photos where the liquid might be coming from? If I replace the evaporator coil, do I also need to replace the outdoor condenser? (it's from 2003, and the label mentions HCFC 22).

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    Refrigerant is inside the coil tubes. If it affected the outside, that would mean it leaked out and your AC would not work at all. So if it is cooling then the problem is outside the coil. The catch is that if the coil corrodes through from the outside then your refrigerant will all leak out and no cooling. So definitely worth figuring out. On a 20-year-old unit the odds are that its time for a new system, but I can't say for sure (which is why I am only commenting). Commented May 29, 2023 at 15:54
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    No common refrigerants promote rust. Nor remain liquified at atmospheric pressure. Liquid is likely to be condensed water from humidity in the air. Replacement will almost certainly be complete replacement. As such, be sure to contemplate alternative complete replacements, such as a good (highly efficient) mini-split rather than "like for like" replacement.
    – Ecnerwal
    Commented May 29, 2023 at 15:54
  • These look like copper tubes and they dont rust. If they were oxidising because of moisture, they would go green. And refrigerants dont cause rust. Commented May 29, 2023 at 21:22

2 Answers 2


If it were me I would not worry about replacing it, it appears you have a good one. Rust is to be expected and at this point the discoloring on the tubes is probably copper oxide,again not to worry that is normal. As for the smell you did not state if it is on the outside of the coil or the inside of the home.

The smell is probably from mold which needs three things to grow, Food (dirt on the coil and fins), Air (ambient) and Moisture (collected on the fins and tubes). The mold would be caused by "not running it to often", this gives the mold time to warm up and grow. This is typical of most AC units when there first turned on in the spring, the smell lasts only for a short time. You can have or do it yourself by cleaning the coils. I do not use bleach, I use Hydrogen peroxide, no strong smell. A little will go a long way. I just spray it on from a small bottle. Note the peroxide will not last long in the bottle as light and air will destroy it but that is normal.

Have a great summer and enjoy the energy savings.

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    After opening things up properly and looking, one problem is obvious: the evaporator coil unit itself is not level, so the condensate is not draining very well. The drain hole is at the front left of the unit, but the whole thing is tilted slightly to the back and right. so there's a stagnant pool of gross water there.
    – negacao
    Commented May 30, 2023 at 12:36
  • The smell FWIW - it's coming out the ducts. It seems to be present in both the evaporator coil AND the furnace blower. :(
    – negacao
    Commented May 30, 2023 at 12:37
  • Level the coil and clean first. The smell will travel into the ducts with the air.
    – Gil
    Commented May 30, 2023 at 14:58

I see no rust ; very likely red copper oxide. I would not mess with it unless you know there is a leak.

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