Your AC smells musty when it turns on: here is what is happening

You turn on the air and a few seconds later you get it: wet rag, basement, damp earth. It fades once the system has been running a while and it is back the next day. It is one of the most common complaints in Orlando and it nearly always has the same cause.

What is going on inside

Air conditioning does not only cool, it dries. As warm humid air passes over the cold evaporator coil, the vapor condenses into liquid water — gallons of it a day in summer. That water drops into a pan and leaves the house through one narrow pipe: the condensate line.

That line clogs easily, with dust and with the biological slime that forms on its own inside anything damp. When it clogs, the water backs up inside the air handler closet. And there you have everything mold needs: darkness, constant moisture and dust.

What you can check yourself

1. 1Look for water or water staining on the floor of the air handler closet.
2. 2Find the condensate outlet on the outside of the house: if the system is running in summer and nothing is dripping, be suspicious.
3. 3Check whether the emergency pan under the unit is holding standing water.
4. 4Change the filter if it has been in more than three months. A loaded filter makes everything worse.

When it is no longer just the drain

If the smell persists after the drain is cleared, if there is visible dark growth on the coil or in the flex duct, or if somebody in the house has respiratory symptoms that ease up outdoors, the growth is past the point where simple cleaning does it.

At that point the coil needs treatment, flex duct that can no longer be sanitized needs replacing, and the moisture source has to be corrected. Cleaning the ducts alone, without fixing the drain, guarantees the smell is back next summer.

Do the ducts need cleaning every year?

No. There is no fixed schedule, and be skeptical of anyone selling it as annual maintenance. It is justified when there is visible contamination, a persistent smell, a recent remodel, or if the house has had water or mold damage. If none of those apply, you probably do not need it yet.