A musty swamp cooler usually has stale water, scale or old pads. Drain and clean the pan, replace pads that are caked or sour, and the smell usually goes. Climate Pros Tucson is a referral service for licensed HVAC pros. Not every pro works on evaporative coolers, so ask before booking.

What causes the smell?

  • Stagnant water. Water left in the pan after a stretch off grows algae and bacteria.
  • Old or scaled pads. Pads hold moisture and mineral. Tucson Water says hard water leaves calcium deposits in evaporative coolers and advises regular pad changes and an in-line filter.
  • Dirt and debris in the pan. Leaves and dust break down into a damp, sour film.
  • Dead insects or a nest in the cabinet.
  • Not enough bleed or refresh. Water that never changes concentrates minerals. Some coolers use a bleed-off or a dump setting, so check your manual.

How do you fix it?

Turn the power off and shut the water supply first.

  1. Drain the pan, and scrub out scale and slime. Rinse well.
  2. Remove the pads. Replace any that are crumbling, caked or sour.
  3. Clear the pump screen and check the float valve.
  4. Refill and run the fan alone for a bit, then add the pump.
  5. In the dry season, don’t let the pan sit with standing water when the cooler is off.

Follow the manufacturer’s cleaning guidance and don’t mix chemicals. Use only products the manual allows.

Is it the cooler, or the AC?

If the house has both a cooler and refrigerated air, check which vents the smell comes from. A musty smell from the AC vents has different causes, like a wet coil or a clogged drain. Read why does my AC smell and AC drain line clogged.

Does the monsoon make it worse?

Yes. The official monsoon runs June 15 through September 30, and at Tucson’s airport average dew points jump into the high 50s in July and August. Humid air slows evaporation, so pads and the cabinet stay wet longer, and damp pads are where musty smells start. If the smell shows up in July, check the pads first. See monsoon humidity and AC.

When should you stop and switch?

If pads, scale and smell keep returning every season, compare keeping the cooler against refrigerated air in swamp cooler vs AC. A pro can quote AC installation or tell you what they can do for a cooler. See the Tucson HVAC hub.

When cleaning doesn’t clear it

If the smell stays after cleaning, or you want to ask what a pro can help with, call (520) 353-2520 and we’ll match you with a licensed local pro.