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Why does my AC Smell Musty?

Updated: 2 days ago

During the summer months, the only thing we seem to long for is the cold embrace of the AC when we get home. But this time, you open your door and are hit with a wave of musty air. Why is your AC musty, and what can you do to fix it?

Don’t fret–a musty AC is not a sign that your machine has hit a dead end. For the most part, a good cleaning will get your AC back into tip-top shape and will alleviate your home of the wretched odor.

Let’s get down to brass tacks: what exactly causes the smell? Musty smells always equal moisture, which in turn could mean that your AC might be harboring mold or mildew. There are a few common reasons why your AC might be expelling this smell. This can include…

Mold or mildew in the air ducts or vents

If you smell must, the odor very well could be origination straight from the ducts or vents. If your AC is not running regularly or is functioning improperly and not dehumidifying the air, air ducts and vents can host mold or mildew and end up spitting the smell out into your space every time you turn on your AC.

dirty air duct
A dirty air duct

Clogged drain lines

Your drain lines are what carry condensate (water runoff) away from the unit and away from your home. Drain lines can become easily clogged and are generally a very easy fix, but they can also cause a lot of grief in a variety of ways–one of which is providing a nice home for mildew to form, which can cause the musty smell to emanate from there.

Clogged filters

Your AC’s filters catch anything and everything (ideally!) that passes through. Depending on the nasties that get caught in the filter, moisture itself can be harbored in the filters. This moisture can in turn develop into mold or mildew, thus causing the stench.

dirty air filters
A pair of dirty air filters

Full drain pan

Connected to the drain lines, a full drain pan with stagnant condensate can easily grow mold, mildew, or even algae. This can cause the smell and is one of the more exposed parts of the machine, making it easy for the smell to leak out into the home.

Incorrect AC unit size

Sometimes, an AC unit can be too small for the space that it’s supposed to be working in. When this happens, the unit works overtime to cool and dehumidify. If the unit is too small, it may not be able to dehumidify at a good rate for your space, thus leaving it to smell musty. Conversely, if the unit is too large, it may be pulling in the air too quickly and not allowing for proper dehumidification.

Although there can be other reasons why your AC smells musty, the above are the most common, and often easiest to resolve as well. You may have already guessed, but the best way to solve the issue of a smelly AC is to give it a proper cleaning. We recommend having one of our specialized technicians come out to perform a deep, thorough cleaning of your unit, as it can be easy for a novice to injure themselves or their unit when trying to conduct a deep cleaning.

If you are not sure where the smell is coming from, it’s always in your best interest to call a professional to get to the root of the problem. If the problem seems to be something that’s not readily apparent, it could be an issue of dirty ductwork or evaporator coils–which is definitely something you’d like a technician to look into!

The best thing to do for your HVAC system, of course, is to consult a professional. Start here by giving us a call here at Tri-Point Refrigeration and we would be ecstatic to take a look at your unit and give it a good scrubbing! This will ensure its longevity, as well as protect your wallet from costly repairs or replacements.

We’re excited to help you along the way, and with regular servicing, we can keep your equipment running smoothly and healthily, while also extending the reach of your dollar.

For a direct line to any of our employees ready on the line to help you, please call…(512) 651-4565 for our Austin, Texas community (806) 686-0050 for our Lubbock, Texas community. Or check out our website at


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