Bad Smell From Central Air Conditioner – What Causes Moldy Smell?

QUESTION: Bad smell from my AC. The central air conditioner in my house has a bad odor coming from the air vents. The air conditioner started blowing moldy musty air since the beginning of summer. The mold smell problem started when the hot weather temperatures brought high humidity. When I turn the AC thermostat to the cool setting the air smells moldy and musty. What is the best way to remove the moldy odor from the air conditioner?

Bad Smell From Central Air Conditioner - What Causes Moldy SmellAC Smells Bad: Questions And Answers

ANSWER: The AC unit could have mold growth near the evaporator coils or the condensation drain line. Dirt or dust on the evap coil can cause the condensation to turn into mold. If the drain line gets clogged, the water in the area sits for long periods. This can be a recipe for mold. Check the air handler in your home for signs of mildew or mold. Spray with germicidal cleaner. Clean the evap coils. Unclog the drain line. Change the air filter if you find signs of mold growth.


1. Remove and replace the AC air filter as it may contain mold.
2. Clean away any mold growth near the evaporator coil.
3. Ensure the condensate drain line is not blocked.
4. Inspect the home air ducts for mold or mildew growth.

When working on an Air Conditioning system, always turn the circuit breaker OFF before beginning work!


Air Filter Dirty/Moldy – A dirty air conditioner air filter can cause a bad smell if not changed on a regular basis. Change the AC air filter every 3 months to avoid any smells and AC problems. Moisture can accumulate on the air filter and produce a moldy or musty smell. Changing the air filter every 90 days will reduce any odors and mechanical problems.

Change AC Air FIlterAir Conditioner Air Filter
(may contain mold mildew and cause bad smell)

Evaporator Coil Mildew/Mold – A musty smell from your central AC can be caused from a dirty evap coil. The evaporator coil creates condensation when warm air flows over the coils. The condensation on the coil then combines with dirt or dust. This combination grows mold and mildew. This wet area is what causes the musty or moldy smell. The evaporator coil is located in the inside unit.

Clean Air Conditioner Indoor Air Handler/Condensation Drain/Evaporator Coils

Evaporator Coil Holding Odor – If the evaporator coil holds the odor, it can be cleaned. You can remove the moldy odor by accessing the evap coil and cleaning it. Find the air handler in your home. Cleaning the coil will neutralize the odor coming from the AC. Cleaning the rest of the inside of the unit to remove odor is also recommended. Always remember to check the condensation drain for clogs which can cause mold and mildew growth. SEE VIDEO ABOVE

Evaporator Coil Cleaner - Evap Foam No RinseAir Conditioning Coil Cleaner
(use to clean moldy dirty evap coil)

Condensate Drain Line Blocked/Clogged – This AC drain line drains the excess moisture from the evap coil. The drain line can get backed up. When this happens the water trapped in the line gets moldy. This backed up drain line causes the moldy smell. This drain line is located in the inside unit and feeds through your home to drain out the condensation.

AC Drain Line Clogged? – Unclog AC Drain Line Easy

Air Ducts Mildew/Mold – The moldy or musty smell can occur when the AC system leaks water into the ducts. If moisture accumulates in the duct work, mildew and mold can grow. The mildew and mold in the ducts will cause the musty smell. The mold spreads into your home whenever the AC is turned ON. This can be an unhealthy problem that needs attention. A leak in a duct occurs when there is an issue with the air conditioner draining system. We recommend having a professional clean the air ducts in your home.

AC Air Duct MoldAir Conditioner Air Duct Mold
(mold in air ducts cause musty moldy AC smell)


Lingering Odor? – If you cook fish, smoke, have pets, had a fire, etc, all these smells get “stuck” in the AC. The smelly air flows into the coil. The strong odors can permeate into the coil and the odors accumulate. When the AC is turned on to cool, all these heavy smells can still flow out of the AC and into your home. Cleaning the coil will remove any of these smells if done properly.

Air Conditioning Parts DiagramAC Parts Diagram
(image from Pinterest)

This job is not recommended by the user. Certain parts can easily be damaged. Have a professional clean your evap coils, unclog the drain line, and clean out your air ducts.

Still need help with a smelly AC? Please leave a comment below to describe your issue and we can help.

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