Hello all, I recently noticed a musty/stale smell from all my supply registers when a/c is on. It had run fine during summer this year and I have been using gas heating for a while. Last weekend the weather got quite a bit warmer and it was rainy, so I turned cooling on to bring the humidity down. That was when all of a sudden I noticed a strong smell from all of the supply registers. Not really stinky but just stale, like an old house. I turned it back to heating, it was not as noticeable, but it could be the hot air is masking it.

I had 2 HVAC techs come out to take a look, evaporator coils looks ok, drain pan is dry (no standing water, just some rust in there). Also took the bottom panel off to look at the blower, no obvious thing in there. Filter was changed back in September, looks clean.

The 2nd tech went ahead and wiped clean the pan, foam cleaned the evaporator coil......it still smells, although not as bad as before (could be the cleaner smell masking it).

Where else can the smell come from??? The ducts were just cleaned last July.