Why Your AC Smells Musty, and What to Do About It

Short answer: A musty or mildew smell from the AC usually means moisture is sitting somewhere it shouldn't: a clogged condensate drain or pan, a dirty evaporator coil, a damp or dirty filter, or moisture in the ductwork. Replace the filter and check the drain line first. If the smell stays, a technician should clean the coil and drain and look for leaks or duct problems.

Safety note: a musty smell is a comfort and maintenance problem, not an emergency. If you smell gas (a rotten-egg smell), leave the house now and call 911 or Southwest Gas from outside. If you smell hot plastic or burning electrical, turn the system off at the thermostat and schedule a visit. If a carbon monoxide alarm sounds, get everyone out and call 911 from outside.

Why air conditioners can smell musty

An air conditioner cools by pulling heat and moisture out of the air. The moisture condenses on the indoor evaporator coil, drips into a drain pan, and runs out a condensate drain line. When that water doesn't drain away, or when dust and moisture sit together on the coil or filter, mold and mildew can start to grow. The blower then carries that smell straight to your vents.

Las Vegas air is dry most of the year, so many people are surprised to find this problem here. But the coil still makes water every time it cools, and the dust that collects on coils and filters in the desert gives mildew something to hold on to.

Common causes

1. Clogged condensate drain or drain pan

This is the most common cause. Algae, dust and debris can slow or block the drain line. Water backs up in the pan and sits. You might also see water around the indoor unit, a tripped float switch that shut the AC off, or rust in the pan. See our AC leaking water guide for more on drains.

2. A dirty evaporator coil

Dust that gets past the filter sticks to the wet coil. Over time it can grow a film that smells. A dirty coil also cools less well. Our dirty AC coil guide covers the signs.

3. The filter

A filter that's overdue, or one that got damp, can hold odors. It's the easiest thing to check. If you're not sure what size you need, see how to find your air filter size.

4. Ductwork

Gaps or leaks in ducts that run through a hot attic can pull in dusty attic air. Ducts that sweat, or a duct that has collected moisture, can smell musty too. Our ductwork problems guide explains what to look for.

5. A coil that iced and thawed

If the coil frosted over from low airflow or low refrigerant and then melted, extra water can overflow the pan and soak nearby insulation. Our frozen AC coil guide covers that problem.

6. A system that sat unused

After a long break, such as the first warm days of spring, standing water in the pan or drain can smell when the AC first starts. If it persists after the system has run for a while, look for one of the causes above.

What you can check safely

1. Replace the filter. Turn the system off at the thermostat first, then swap it. Make sure the airflow arrow points toward the unit.
2. Look at the drain. Find the condensate line where it exits the house, often near the outdoor unit or by the indoor unit. If it isn't dripping while the AC runs, or if there's standing water in the pan, the line may be clogged.
3. Check the float switch. Some systems have a small safety switch in the pan or line that shuts the AC off when water backs up. If your AC stopped cooling along with the smell, that may be why.
4. Sniff a few vents. If the smell comes from every vent, the source is likely at the indoor unit. If it's one room, it may be that duct run or something in that room.
5. Rule out the house. A smell can come from a sink drain, a damp carpet or a stored item near a return vent. The return pulls that air into the system and spreads it.

What not to do

• Don't spray household cleaners, bleach or air fresheners into the vents or onto the coil. Some products can damage coil metal, and fragrances only cover the smell.
• Don't remove panels on the indoor unit. The blower and controls carry line voltage.
• Don't keep running the AC if water is overflowing the pan. That can damage ceilings, drywall and flooring.

What a technician does

A proper visit usually includes clearing and flushing the condensate drain, cleaning or treating the drain pan, inspecting and cleaning the evaporator coil, checking the float switch, and checking airflow and refrigerant performance. If the source is in the ducts, they'll look for leaks, damp insulation or disconnected sections. If you want to talk about filtration or air cleaning beyond the basics, see our indoor air quality service.

When to call

Call if the smell is still there after a new filter, if water is backing up or leaking, if the AC shut off along with the smell, or if you can see mold on the unit or around vents. See our AC repair service, or our Las Vegas AC repair page.

FAQ

Why does my AC smell musty when it first turns on?

Usually moisture is sitting in the drain pan, drain line, coil or filter, and the blower carries the smell to the vents. Replace the filter and check the drain first. If it continues, have the coil and drain cleaned.

Is a musty AC smell dangerous?

It's usually a sign of mildew or standing water rather than an emergency. People with allergies or asthma may notice it more. A smell of gas, burning plastic or an electrical smell is different and needs action right away.

Can a clogged AC drain cause a smell?

Yes. A clogged drain lets water sit in the pan, where algae and mildew can grow. It can also overflow and cause water damage or shut the AC off on a float switch.

Can I spray cleaner on my AC coil?

We don't recommend it. Some cleaners damage coil metal or leave residue, and the coil sits next to electrical parts. A technician can clean the coil with the right products.

Why does only one room smell musty?

If the smell is in one room, the cause may be that duct run, a damp spot in the room, or something near the vent. If it's every vent, the source is more likely at the indoor unit.

What hours can I reach Colton?

Colton Air Conditioning is open Monday to Friday, 8am to 5pm. You can call or book online during those hours.

Colton Air Conditioning

Colton Air Conditioning, 6545 S. Fort Apache Rd Ste 135 #467, Las Vegas, NV 89148. Nevada contractor license 0092120. Hours: Monday to Friday, 8am to 5pm. Call (702) 738-6976 or book online during those hours.

Want routine maintenance on a schedule? See our Comfort Club maintenance plans: Gold, Platinum and Diamond tiers are all listed there.