By Andy Kahn, co-owner, 32 Degrees AC & Furnace Repair
You switch on the air, and a damp-basement smell rolls out of the vents. Most people assume the ducts are dirty and start pricing duct cleaning. In our experience across the Las Vegas Valley, that smell almost always starts closer to the equipment: in the condensate drain pan, the drain line, or on the indoor coil. 32 Degrees AC & Furnace Repair doesn’t offer duct cleaning. We fix the part of the system that’s actually making the smell.
Why a desert AC can smell like a swamp
Even in dry air, your evaporator coil pulls water out of the house every hour it runs. That water drips into a pan and leaves through a drain line, usually PVC running to a sink, a floor drain, or outside. When that path slows down, the water sits. Standing water in a dark cabinet grows a slime that smells sour or musty, and the blower pushes that smell into every room.
Las Vegas adds two problems. Fine dust gets past loose filters and settles in the wet pan, building sludge faster than you’d expect. And in attic installs, summer heat bakes whatever is sitting there.
Common sources, from most to least likely
- Partly clogged drain line. Water drains slowly, so the pan never fully dries out. Sometimes you’ll see a float switch shut the system off, or water near the air handler.
- Rusted or cracked drain pan. Older steel pans in homes built in the 1990s and early 2000s rust through. Water sits in the low spots or drips where it shouldn’t.
- Dirty evaporator coil. Dust packs into the fins, stays damp and holds odor. It also cuts airflow and cooling.
- Wet or collapsed insulation in the cabinet. It soaks up water and odor.
- Missing trap or a dry trap. Without a proper trap, the system can pull sewer or attic air back through the drain line.
Dirty ducts are rarely the root cause. If the source is fixed and the smell is gone, there’s nothing left to clean.
What you can check safely
- Look at the filter. If it’s gray and bowed, replace it.
- Find the drain line where it ends outside or at a sink. On a hot afternoon with the AC running, it should drip steadily.
- Look around the indoor unit for water, rust stains or a tripped float switch.
- Note when the smell happens: only at startup, all the time, or only after the system sits off.
Don’t pour bleach straight into the cabinet or spray fragrance into the return. Bleach can damage the coil and pan, and fragrance only hides the problem for a day or two.
What we do on a service call
A technician opens the air handler, checks the pan, the trap, the drain line and the float switch, and inspects the coil. We clear and flush the drain, check the slope, and tell you whether the pan or coil needs attention. You get the findings before any repair. A routine AC maintenance visit includes checking the drain, which is the easiest way to keep the smell from coming back.
If the smell is burning, electrical or like rotten eggs, turn the system off and call right away. That’s a different problem from a musty drain.
When to call
If the smell lasts more than a day or two after a filter change, or you see water near the unit, call 32 Degrees AC & Furnace Repair at 702-635-9505 or book online. We answer 24/7 and serve Las Vegas, North Las Vegas and Henderson.
NV License #0088234
Related reading: furnace smells at first startup and is damaged ductwork hurting your air quality. Need a fix now? See our AC repair service.