AC drain pan mold, the puddle under the coil

The smell has a waterline

You changed the filter. You wiped the register. The first blast of cooling still smells like a wet basement. Before you price a duct-cleaning package, look at the drain pan. Air conditioners make water on the coil on purpose. That water is supposed to leave through a condensate line and drip outside or into a pump. When the line algae-clogs, the pan fills, the blower moves air across a pond, and the house gets the smell. That is AC drain pan mold territory: not always a carpet of growth, always a moisture fault in the air path.

A clear line, then a judgment call

Find the air handler with the power off at the switch or breaker you understand. The pan is the shallow tray under the coil, and the line is usually PVC leaving the pan. If you see standing water, the drain is not doing its job. Many clogs clear from the outdoor end of the line with a wet/dry vac for a few seconds. Some pans are obvious in a closet air handler. Some are buried in a furnace you should not disassemble if you cannot put it back combustion-safe.

Do not seal the flue. Do not block combustion air. Do not spray bleach into a pan and then run the blower into the bedrooms. If the equipment burns fuel, test the carbon monoxide alarm before you button anything up. Dizzy, headache, or flu-like symptoms with the equipment running means leave the house and call for help.

A treatment tablet in the pan is maintenance after the line flows. It is not a repair for a disconnected hose filling the closet. If the closet floor is wet, you are also looking at ceiling damage below. Dry that before it becomes the bathroom ceiling story from the other side of the floor.

Tape does not fix a pond

Seal return leaks on the same weekend if the smoke test fails, with UL 181 foil tape or mastic. A tight return and a full pan can both be true. The pan is why the supply smells wet. The return is why the bedrooms smell like the crawlspace. Do both, in that order if water is actively sitting there.

Insulation inside the cabinet that is already fuzzy is a professional job. Shut the door and call. About 10 square feet is the homeowner line for ordinary surface growth, and HVAC insulation is called out separately because it is in the airstream.

Then let the rest of the protocol catch up

The drain is a satisfying Saturday because the drip outside is proof. The assessment will tell you whether the pan was the lead item or the washer and the nursery are still waiting. Unlock ClearNest for $99 for the seven steps and Ava on the member side, so the next smell does not restart the argument from zero. Run the cooling, walk to the outdoor line, and watch it drip before you call the job finished.