Why your AC smells musty when it turns on, and how to fix it

The AC clicks on, the air moves, and for the first minute the house smells like a wet basement. Then it fades until the next cycle. In Central Texas this is one of the most common calls we get from May to October, and the ducts are usually not the cause.

The three usual suspects

• The evaporator coil. It sits inside the air handler, runs cold and wet all summer, and collects dust on its fins. Dust plus moisture plus dark is the recipe for the smell. This is the number one cause.
• The return plenum. The box behind your return grille is often the original one from when the house was built, lined with insulation that has been damp for years. Even a new AC unit is usually connected to the old plenum.
• The condensate drain. A partly clogged drain line leaves standing water in the pan under the coil. That water is what you smell.

How to tell which one it is

If the smell is strongest in the first minute of a cooling cycle and fades, it is almost always the coil or the pan. If it lingers and is present even when the fan runs without cooling, look at the plenum. If there is water near the indoor unit or the secondary drain pan is wet, the drain is involved. An inspection settles it in under an hour, and we photograph what we find so you are not taking anyone's word for it.

What actually fixes it

• Coil cleaning, done with the access panel off and the coil cleaned front and back, not sprayed through the filter slot.
• Blower wheel cleaning, because the wheel collects the same film as the coil and throws it into every room.
• Drain line clearing and a check of the trap.
• Replacing the plenum liner or the plenum itself when the insulation inside is stained or growing something. A duct cleaning on top of a plenum with microbial growth just moves the smell around.
• A UV light or air scrubber at the coil if the house has a history of the problem. Useful after the cleaning, not instead of it.

What does not fix it

Air fresheners in the returns, a new filter alone, or a $99 vent vacuuming. The smell comes from the wet parts of the system and those are behind the panels.

A recent example

A homeowner in the Georgetown area called us before moving into a house because his son had allergies. The AC was three years old. The plenum behind it was the original twenty-year-old box, and the insulation inside was black. We replaced the plenum and added a scrubber. In his words on Google, no allergy problems in the new home so far.