AC Smells Musty? Here’s what to do.

wp header logo 4
AR blog inline Apr27 musty air conditioner

It won’t be long before you start up your air conditioner for the first time this spring (if you haven’t done so already). But what if your AC smells musty and unpleasant after sitting unused all winter? Air conditioner smells can happen for a number of reasons.

Let’s take a look at what can cause that musty smell coming from your AC vents (or the unit itself) and how to get rid of it. 

Why your AC smells musty

When an air conditioner smells musty, it’s a sign of moisture where it doesn’t belong. Moist conditions allow mildew to grow, leading to that musty odor. 

The question is, where are the moisture and the mildew coming from? It could be one (or more) of the following:

  • Your ducts.  Built-up dust and grime in your HVAC ducts, along with accumulated moisture, can cause a musty smell that spreads through your space along with heated or cooled air.
  • The unit. If you have a though-the-wall type air conditioning system, moisture may have accumulated in the unit. Mildew spores can get in and grow as the air warms, bringing nasty smells when you turn on your AC.
  • Water leaks somewhere else. It’s possible that the musty smell may be coming from somewhere else in your space. It could be a poorly ventilated bathroom, locker room, or storage area that has a return duct delivering smelly air back into your AC system. If you have plumbing issues and water leaks somewhere, poor HVAC design can create airflow conditions that spread odors everywhere.

You probably already know that trying to mask air conditioner smells with sprays or scented candles isn’t going to get rid of the problem. Here’s what to do instead.

3 steps to get rid of air conditioner smells 

Here are 3 steps to take if your AC smells musty:

1. Have your AC system inspected and thoroughly cleaned

Start by bringing in a professional HVAC technician to look at your system components. He will change clogged filters and spot signs of water leaks and mold growth that could be causing odors. You might have condensate fluid leaking, a drain pain that’s overflowing, or other issue that’s gone undetected for long enough to allow mildew to grow. 

This can easily happen if you haven’t had your system maintained for a while. Air conditioning maintenance prevents issues like this because the problems that cause them get fixed before any damage is done. (That’s why it’s called preventative maintenance.)

Residential systems should get maintenance at least once a year, ideally in the spring before you start it up for the first time. Commercial air conditioning systems may need maintenance more frequently, depending on the usage conditions. It pays to talk to the experts and get a recommendation.

If you’re in the NYC metro area, Arista can help. Here’s an easy way to get an estimate for a commercial or luxury residential maintenance agreement.


2. Clean your HVAC ducts 

If your equipment is working well and doesn’t have any water leaks, the musty smell may be coming from the ducts that distribute cooled air throughout your space. (That’s assuming you have a ducted system.)

Your HVAC technician can take a look at your ducts when they inspect your equipment, and they may be able to see moisture and even mold growth. If so, you’ll need to bring in duct cleaning experts to clean and sanitize your ducts.

Get our helpful guide to learn more about the benefits of duct cleaning for improving indoor air quality.

3. Repair water leaks and adjust airflow 

If your AC smells bad but your equipment and ducts are fine, it’s time to look elsewhere in your space for the source of the odor. If water leaks are found, obviously you’ll need to get that taken care of to get rid of the musty smell. 

But why does the smell seem to be coming from your AC? The answer is air flow. Your HVAC system is designed to move conditioned air throughout your space. But sometimes it can spread odors along with the air moving through the ducts. 

Air flow is something that can be managed with proper ventilation and HVAC design. Talk to your HVAC technician about this if you have ongoing odor problems. You might benefit from moving ductwork, adding fans, or adding ventilation to make your space smell fresh again.