6 Common Causes of Musty AC Odors (and How To Fix Them)

If your AC smells musty, then it’s normal to feel concerned. We recommend inspecting and fixing this issue as soon as possible. The easiest way to fix an AC that smells like mildew is to contact a professional HVAC repair company, like Total Comfort.

We understand that many homeowners like to make repairs themselves. If this sounds like you, we’d love to help. In this article, we answer all your questions about what to do when an air conditioner smells musty. If you attempt our recommended fixes but your AC still smells strange, contact our team today. We offer no-obligation estimates and upfront pricing on all services. 

Is It Normal for an AC To Smell Bad When It’s Turned On?

If you’re dusting off your air conditioner after not using it all winter, you can expect a strange odor at first. But if these odors persist, you should not ignore them. A healthy air conditioner shouldn’t put off any odors. If you smell burning, mildew, or other unpleasant odors, something is likely wrong with your system.  

6 Common Reasons & Solutions to Why Your AC Smells Bad 

If your air conditioner smells bad when turned on, it’s time to grab some gloves and inspect your unit. Here are 6 of the most common issues our HVAC technicians notice when repairing a musty AC unit: 

Dirty Filters

Dirty air filters are the culprit behind all sorts of air conditioner problems. If your air filters aren’t regularly swapped, then dust and other debris will collect on the filter. This can produce foul odors and make individual AC parts malfunction. 

Solution: Most air filters should be replaced every 1 to 3 months. Your air conditioner likely came with a maintenance guide—you can likely find the manufacturer’s recommendation inside. Some air conditioners require a HEPA filter, which filters smaller particles and offers greater protection from foul odors. 

Dirty Air Ducts

An air conditioner shouldn’t be dirty, especially in summer when it is most often used. Dirty air filters and air ducts can cause the whole system to shut down—this threat is signaled by musty odors. 

Solution: Make a quarterly reminder to inspect and clean your air ducts. Some air ducts cannot be cleaned without professional assistance. If cleaning your ducts has no effect, they may be damaged. Air conditioning repair professionals can inspect and fix damaged air ducts. 

Blocked Condensate Line

If your air conditioner’s condensate drain line is clogged, excess moisture will build up inside your unit. Mold and mildew thrive in dark, moist areas, so your air conditioner’s combination of darkness and moisture makes it a potential breeding ground. 

Solution: Remove blockages with a mix of warm water and vinegar. If your condensate line becomes blocked more than once, be sure to regularly inspect it in the future. 

Mold Growth

Your AC might smell musty because of mold growth caused by faulty evaporator coils, motors, and other components. If water sits inside your unit undisturbed, it will promote mold and mildew growth within 24 to 48 hours.

Solution: Clean your AC unit’s components with warm water and detergent. Be sure not to use harsh detergent or an excessive amount of bleach during the cleaning process. We recommend a maximum of 1 part bleach to 4 or 5 parts water. If mold and mildew are a persistent problem for your unit, ask an HVAC repair professional about installing UV lights in your unit. 

Improper Unit Size

Units that are too small or large will increase a home’s humidity, leading to mold and mildew growth. Air conditioners must be properly sized during installation to meet a home’s cooling needs. 

Solution: Consult an air conditioning replacement service, such as Total Comfort. Our expert technicians will determine whether your current unit was improperly sized or installed. We can replace your unit if necessary, taking extensive steps to improve indoor air quality. 

Full Drain Pan

A full drain pan is a common reason why your air conditioner is leaking water. Drain pans can become full of standing water. If this water mixes with dirt and debris, then your air conditioner will emit musty odors. A full drain pan can be a health hazard, so it’s important to check on it regularly. 

Solution: Inspect your drain pan weekly during high-demand cooling seasons. If you aren’t using your air conditioning as frequently, you can inspect it once a month. Contact a professional AC repair service if you find water pouring out of your AC unit. 

How To Prevent Musty AC Odors in the Future

Air conditioners must be regularly cleaned and maintained, just like any other machine. If your AC is neglected, it will run poorly, accumulate dust, and give off foul odors. These issues can be prevented by taking the following steps:

• Clean and replace your air filters as needed (usually every 1–3 months).
• Make sure the insulation and sealing around your air conditioner are in good condition.
• Inspect and clean AC parts regularly—this includes air ducts.
• Schedule regular air conditioning maintenance with Total Comfort.

When To Call a Professional HVAC Technician

If you have tried the solutions listed above, but your AC still smells musty, it may be time to contact a professional HVAC service. Total Comfort has been Minneapolis-St. Paul’s go-to heating and cooling team since 1956. 

Our NATE-certified technicians service all makes and models of air conditioners, tailoring their skills to your home comfort needs. If your HVAC smells musty, our cooling experts will quickly diagnose the problem and quote you upfront for repairs. We don’t believe in charging hidden fees—when we give you a number, we stick to it.

If your air conditioner needs to be cleaned rather than repaired, we will make sure it is spotless inside and out. This involves changing air filters, removing water from the drain pan, cleaning coils, and more. Our team is happy to give you cleaning tips you can use in the future. 

Frequently Asked Questions