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$93 Main Line Drain Clearing Get Offer

Why Does My AC Vent Have a Musty Smell?

Is a musty smell coming from your AC vents?

You might notice a musty smell coming from your AC vents. It’s not exactly a pleasant odor and seems impossible to get rid of as it circulates throughout your home.

Mold is most likely the cause of the musty smell. When an AC problem occurs, the system cannot dehumidify the home as well. Lack of dehumidification combined with Florida’s humidity means that your home can become particularly susceptible to mold growth.

Six main AC problems can cause mold growth and, in turn, the musty smell coming out of your vents. In this article, we’ll go over the potential causes and both DIY and professional solutions for each one:

  • Dirty Evaporator Coil
  • Clogged Condensate Drain Line
  • Leaky Ductwork
  • Filthy Air Filters
  • Oversized AC Unit

When you realize mold causes a musty smell, you’ll probably want to get it addressed and fast. If you suspect one of the above problems is the culprit, contact an AC technician and mold remediation as soon as possible.

Want your AC inspected by a professional? Call Red Cap. You can also schedule an appointment online. We provide same-day AC inspections and repairs.

Dirty Evaporator Coil

A dirty evaporator coil can block airflow into your AC system.

Your AC’s evaporator coil is critical to the system being able to cool the air. It uses refrigerant, a cold chemical agent that cools the warm air passing over the evaporator coil.

Unfortunately, the coil can get caked with dirt over time, creating a breeding ground for mold. If there’s mold, you’ll smell an unpleasant musty odor.

The reason a dirty evaporator coil can cause mold is due to:

  • Darkness: Sunlight can’t reach and kill mold since the evaporator coil is in a dark space.
  • Moisture: Condensation forms when warm air flows over a cold evaporator coil.
  • Warm temperature (80°+): Warm air constantly flows over the evaporator coil.

Because mold grows when spores land on wet surfaces, the combination of moisture and warm temperatures (plus the lack of sunlight) encourages fungal growth.

A dirty evaporator coil can also impact your AC’s airflow, causing the system to work harder to cool your home and leading to higher utility bills (or even a system breakdown).


If your AC’s evaporator coil is dirty, contact a professional technician to clean it carefully during a maintenance inspection.

Want to avoid this issue? Get regular AC system checks to identify issues early on and prevent future breakdowns. Since Florida is a naturally humid area, you might also want to consider hiring an HVAC professional to install a UV lamp in your ductwork to kill future mold growth.

Clogged Condensate Drain Line

A clogged condensate drain line can facilitate mold growth.

An efficient AC removes moisture from your home. That moisture normally collects in your AC’s drain pan and exits through the condensate drain line. However, a clogged condensate drain allows water to just sit in the drain line with nowhere to go, facilitating mold growth and a musty odor.

You can determine if you have a clogged drain line by checking your AC’s indoor unit. Beyond smelling a musty odor, do you see pools of water around it?


You have two options for clearing out a clogged condensate drain line:

  1. Use a wet/dry vacuum to clear the line yourself
  2. Contact a professional to clear the condensate drain line for you

If you go for the DIY option, locate the end of the condensate drain line, which you can find coming out of the wall outside near your outdoor AC unit. Next, connect the vacuum to the end of the line, turn it on, and see if it manages to suck up any debris.

If the DIY option doesn’t work, your clog might be further up the drain line and out of reach. You will need a professional to clear the clog.

Leaky Ductwork

Leaky ductwork allows 20-30% of your conditioned air to escape.

Central air conditioning moves air through a ductwork system. When ducts get leaks or holes, your AC can lose up to 20-30% of its conditioned air. Even worse, ducts are typically located in spaces like attics or crawl spaces where it’s easy for dust, dirt, and/or mold spores to sneak in. When this occurs, you might notice a musty smell coming out of your AC vent.

You likely have leaky ductwork if you notice a musty smell and any of the below red flags:

  • Your AC Doesn’t Cool Your Home Like It Used to
  • There Is Dust On or Around Your AC Vents
  • Your Home Feels More Humid Than Normal
  • You See an Increase in Energy Bills


Contact an HVAC professional to inspect your ducts for leaks. If you have leaky ducts, they will clean and seal them up to fix holes or loose connections.

Want to avoid this issue? Annual ductwork inspections and cleanings can identify issues early on and help you avoid mold building up in your ducts.

Filthy Air Filters

When your air filter looks like the left one, it’s time to change it. / Source: Department of Energy

Your AC’s air filter catches airborne contaminants like dirt, dust, and pollen. If you go too long without changing it, your air filter can become quite filthy. A dirty air filter reduces warm airflow into your system, blocking it from getting cooled and increasing humidity in your home. The higher the humidity in your home, the higher the chance for mold growth and musty odors.

A filthy air filter can also allow:

  • Moisture and Dust to Settle in Your Ductwork
  • Dust or Dirt to Attach to Your Evaporator Coil


If you suspect a filthy air filter is behind the musty smell in your home, you can fix the problem by changing your air filter. Because of Florida’s high humidity, it’s important to change your air filter at least every three months.

Want to avoid this issue? Consider upgrading to a higher MERV filter to stop mold spores from entering your home’s ducts and air. MERV is the efficiency rating for air filters. The higher the MERV rating, the more efficient the filter is at trapping mold spores. However, MERV filters get clogged faster than regular filters due to having a tighter weave, so you need to replace them more frequently.

Oversized AC Unit

If you have an oversized AC for your home, it will cool your space too quickly and cause your AC to “short-cycle” or turn on and off frequently. Because ACs remove moisture from the air on top of cooling the air, they can’t do their job correctly if they need to shut off constantly. They won't be able to run long enough to dehumidify the space.

When the increased humidity in your home meets dust in your evaporator coil or ducts, mold can form, causing a musty smell.


Contact a professional HVAC technician to check if your AC is oversized. If so, a system replacement might be prudent for better energy efficiency and cost savings. Not to mention a long-term solution for avoiding mold growth and musty smells.

Want to avoid this issue? Always get a professional technician who performs a Manual J Load Calculation while sizing your new AC.

Dealing With a Musty AC Smell in Tampa or Pinellas? Contact Red Cap Today

Red Cap's highly-trained AC technicians can handle any of your AC problems.

Here at Red Cap Plumbing & Air, we’ve seen it all. So if you need an AC repairAC system check, or AC installation in the Tampa or Pinellas areas, you can count on our trusted technicians to get your AC back in tip-top shape and eliminate that musty smell in no time.

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