4 Reasons Your Air Conditioner Smells Musty

Written by: Paul Cathro

Updated on: February 1, 2023

A musty smelling air conditioner is a common problem that it is usually easy to diagnose. We explain how down below.

why your air conditioning unit smells bad

If you have recently switched on your air conditioning unit to be greeted with a horrid smell, your unit may be suffering from dirty sock syndrome.

Yes, this is a real term and is used to describe air conditioning units that give off an unpleasant odor when in use.

Most of the time these smells are musty or described as a type of mildew odor.

If your air conditioner smells bad, then it is not likely to be some hidden gym socks, rather one of the reasons we list below.

Why Does My Air Conditioner Smell?

Most of the time, dirty sock syndrome is not a major cause for concern if addressed early.

Sometimes, however, it can be a sign of a bigger issue and one that is harmful to you and your family’s health.

For example, if there is mold inside the air conditioning unit, you will need to get rid of it swiftly to avoid breathing in harmful mold spores being sprayed around your home.

You can learn more about mold and its health effects in this World Health Organization booklet.

If you're having problems with a musty smell coming from a portable air conditioner then we recommend our dedicated guide.

Otherwise, hopefully you'll find the answer you're looking for down below.

Mold Growth

The dark space of your unit is the perfect place for mold to grow. In some seasons the unit may go from being used to cool during the day and heat up the home on an evening.

It is these seasons that are most likely to cause mold growth because of the combination of moisture, dust, and heat.

If the growth of mold inside the unit is a problem, you need to get rid of it quickly. You can do this by taking the unit apart in a well-ventilated space to clean the mold away using bleach and vacuuming any dust particles or blockages from the filter.

Replacing the filter is also a good idea if possible. As if you don't manage to remove every particle of mold then there's a good chance it will reappear again soon.

We have a dedicated guide that explains how to remove mold from  an air conditioner safely.

If taking the unit apart is not something you feel comfortable with or in case it will void any insurance, it is best to seek out professional help in the form of an HVAC engineer.

why your air conditioning unit smells 1

Dirty or Damaged Evaporator Coils

A dirty, moldy or damaged evaporator coil can be one of the most common reasons that your AC smells musty.

There are ways you can tell if this is the main problem, including checking the airflow that comes out of the vents.

If the airflow feels weaker than usual, it is likely to be a problem with the evaporator coil. Another sign for concern is if the evaporator coil itself is iced over.

The evaporator coil is an open bar for mold as it offers three out of four necessities for mold to grow here, i.e., moisture, no UV rays to kill mold spores and heat. The fourth one is easy to come by, which is organic matter such as dust.

The solution? Again, it is best to consult with your AC provider to get a professional clean that will remove dust and stop the coil from becoming a breeding ground for mold.

Clogged Drain Line

Your air conditioner is programmed and designed to keep you safe if there is a problem. One way it does this is by an emergency shut down.

If your AC shuts itself off very soon after being turned on, it is likely caused by water overflowing the condensate pan.

Condensation that accumulates on the evaporator coil will drop into the drain pan which is then flushed away through the drain line.

However, the drain line can clog up due to dust also falling from the coil and the water overflows in the pan. The end result is more chance for mold to grow and give off a mildew smell.

The best solution in these circumstances is to clear the main cause of the problem, the drain line. You can unblock it by using a DIY-made drain cleaner. Alternatively, you can call in the professionals to do it for you.

Leaking Air Duct

If you have had your AC running for a long time and your home doesn’t seem to be cooling down, or worse, it is becoming dusty, there is a chance that your air duct is leaking.

A leaking air duct will draw in air from around the home and sometimes this can be dusty attic air. The end result is an ineffective system as well as an increased chance of mold throughout the unit, including the evaporator coil.

The only way to address this issue is to seal the leak. There are products on the market that will help you do this yourself. Again, you can always ask a professional for assistance if you are unsure or do not have the time.

If none of the above four issues seem to be the problem, it is best to ask the experts. They may know other signs to look for or be able to do a deeper investigation into your unit to find the root of the problem.


Many of the underlying issues mentioned above don’t just lead to a smelly air conditioning unit, but they also lead to mold. Breathing in these spores is a serious health concern if not addresses quickly.

If you start smelling musty smells, usually straight after turning the unit on, do not ignore it.

Ignoring the problem is not only a hazard to your family’s health, but it can also make the situation worse and expensive to fix.

Always consider the warranty of the unit before taking it apart to make DIY fixes or cleaning it yourself.

About the Author Paul Cathro

Paul is an ex-HVAC engineer with 5 years 'in the trade'.

He acquired tinyhousehugeideas.com in 2022 and aims to make it the internet's most comprehensive HVAC resource for small homes in the next few years.

You can learn more about Paul's story here.

Browse his published work on the website here.

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