Got the blower wheel and coil cleaned last week and the smell is gone

We had almost the same thing in our 2006 Rheem down here in Florida. The smell only showed up when the unit first started, then faded after a few minutes, wich had me convinced it was the ducts too. Turns out the drain pan had a bunch of sludge in it and the drain line was barely draining, so there was always a little standing water under the coil. The musty smell was basicly coming off that water every time the blower started pushing air across it.

The tech did the coil cleaning and the blower at the same visit, but he also treated the pan and flushed the line with some kind of tablet thing he drops in that slowly dissolves. He told me to pour a cup of white vinegar down the drain pipe outside every couple months from now on. Cheap and easy, and so far the smell has stayed gone through a full summer. One thing he said stuck with me: a dirty coil holds moisture longer, so it stays damp between cycles, and damp is where the smell lives. So if anyone gets the cleaning done and the smell comes back in a few weeks, check the drain side before assuming it did not work. My wife also started leaving the fan set to on instead of auto for a while, and she swears that helped dry things out between runs.

I went the DIY route on this instead of calling anyone. My air handler is a 2013 Goodman up in the attic, and I just could not stomach the quotes I was getting. Bought a spray can of no-rinse coil cleaner and a cheap rat tail brush, pulled the blower panel, and vacuumed the wheel with a shop vac rather than pulling the whole motor out. Cramped, sweaty, took me a solid three hours to do what a pro does in two, but the wheel came clean and the airflow did pick back up. Here is my diffrent take though. Cleaning only fixed about half my smell problem. The other half was the fan setting. I had always left the thermostat on auto, so the coil just sat wet between cycles and that damp film kept regrowing. Once I started running the fan on during the heavy summer months, the coil had a chance to dry out and the musty smell basically stopped coming back.

The other thing nobody tells you is how fast it gets dirty again if your return ducts leak or your filters fit sloppy. Mine had gaps around the edges, so dust was bypassing the filter entirely. Some foil tape around the filter slot and pleated filters instead of the cheap fiberglass ones, and two years later it still looks decent when I peek in there.

Did this on my own system a couple years back, a 14 year old Trane air handler, and the numbers convinced me more than anything. Before the cleaning my temp split across the coil was around 12 degrees, after it was up near 19. That is a big chunk of capacity you get back for the cost of one service call. One thing I would add, swap the filter right after the job even if it looks new. All that dust you knock loose ends up sitting on the filter face and it will choke airflow again in a week if you leave the old one in.

Adding my own bit to this. Had my 15 year old air handler cleaned two springs ago and the startup smell got way better, but there was still a faint whiff for the first minute or two after the blower kicked in. What finally killed the rest of it was a small UV light the tech offered to mount above the coil. I figured it was an upsell, said yes anyway, and that turned out to be the missing piece. He explained the light keeps stuff from growing back on the coil surface between cleanings. Might not be needed for everyone, but if you get the cleaning done and a little smell still hangs around, that could be why. One heads up, the bulb dims before it actually dies, so swap it every year or two even if it still glows.