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Why does my AC smell musty?

A musty smell when your AC runs usually points to moisture and mold issues inside the system. Here are the most common causes:

  • Dirty or clogged air filter – Dust and moisture trapped in a filter can create a mildew smell when air blows through.
  • Mold or mildew in the evaporator coils or drip pan – If water isn’t draining properly, it can stagnate and encourage mold growth.
  • Clogged condensate drain line – When the drain line gets blocked, water backs up and creates a musty odor.
  • Dirty ductwork – Dust, humidity, and organic matter in ducts can cause mold and mildew to develop.
  • High indoor humidity – In Florida especially, excess moisture in the air can linger in your HVAC system and contribute to odors.

👉 What to do:

  1. Replace your AC filter if it’s dirty.
  2. Check that the condensate drain line is clear and flowing.
  3. Schedule a professional AC cleaning if coils or ducts are contaminated.
  4. Consider using a dehumidifier or setting your thermostat to “auto” instead of “on,” so moisture doesn’t sit in the system.

Here’s a step-by-step checklist you can follow if your AC smells musty when it runs:

✅ 1. Check and Replace the Air Filter

  • Turn off your AC.
  • Remove the filter (usually near the return air vent or in the air handler).
  • If it looks dirty or damp, replace it with a new one (choose a filter rated for Florida humidity if possible).

✅ 2. Inspect the Condensate Drain Line

  • Find the drain line (a small PVC pipe near your indoor unit).
  • Check for clogs or standing water.
  • Pour a mixture of 1 cup white vinegar + 1 cup warm water down the drain to kill mildew and clear buildup.

✅ 3. Clean the Drip Pan

  • Locate the pan under your indoor unit.
  • Empty any standing water.
  • Wipe it clean with vinegar or a mild bleach solution.

✅ 4. Check the Evaporator Coils

  • Open the access panel (turn off power first).
  • Look for dust or mold on the coils.
  • If dirty, use a no-rinse coil cleaner (available at hardware stores).

✅ 5. Run the Fan Setting

  • After cleaning, set your thermostat to “fan only” for 20–30 minutes.
  • This helps dry out moisture inside the system.

✅ 6. Control Humidity

  • Keep indoor humidity below 50% if possible.
  • Use a standalone dehumidifier or whole-home dehumidifier if your home feels damp.

✅ 7. Call a Klein Cooling Professional If…

  • The musty smell continues after cleaning.
  • You see visible mold in ducts or coils.
  • The drain line keeps clogging.