Why Do My Air Ducts Smell Musty? Find the Moisture First

That smell is a moisture report. CDC notes that mold can smell musty, and EPA lists the inside of ductwork among the places where hidden mold can grow. EPA also says mold spores will not grow if moisture is not present, so the useful question is not "how do I make the smell go away" but "where is the system getting wet." Answer that, and the smell usually solves itself. Skip it, and every cleaning or spray becomes a temporary fix.

Where the dampness usually comes from

Each source below is named in federal guidance on ducts and mold.

The cooling coil and drain pan. EPA's guide to air duct cleaning calls cooling coils, which are designed to remove water from the air, a major source of moisture contamination that can lead to mold. The water they collect drips into a pan and leaves through a drain line. EPA says substantial standing water or debris in that pan indicates a problem requiring immediate attention. ENERGY STAR adds that a plugged condensate drain can cause water damage and affect indoor humidity. If you see water around the indoor unit in summer, our guide to water dripping from a furnace in summer walks through the drain.

Wet insulation near the coil. EPA tells homeowners to check any insulation near the cooling coils for wet spots. Wet duct insulation is a bigger deal than it sounds, because EPA says it cannot be effectively cleaned (more on that below).

Ducts sweating in attics, crawl spaces and garages. A cold supply duct running through a hot, humid attic or crawl space can collect condensation. EPA says ducts in non-air-conditioned spaces such as attics and crawl spaces should be properly sealed and insulated to keep moisture from condensation out of the system. ENERGY STAR's duct sealing page estimates that a typical house loses about 20 to 30 percent of the air moving through its ducts to leaks, holes and poor connections, and lists ducts located in an attic, crawlspace or garage as a warning sign of poorly performing ducts.

An oversized air conditioner. EPA says a unit that is too big will cycle on and off frequently, resulting in poor moisture removal, particularly in areas with high humidity. Short cycles remove less water, so the house and the ducts stay damper. Sizing is covered in our guide to what size central air unit you need.

High indoor humidity in general. EPA's mold, moisture and your home guide says to keep indoor humidity below 60 percent if possible, ideally between 30 and 50 percent. A home that sits above that range gives mold a foothold everywhere, ducts included. The thermostat's fan setting also affects how much moisture the system removes, which we cover in AC fan on or auto.

A 20-minute check you can do yourself

You do not need to open any ductwork for these. Turn the system off at the thermostat before you remove any panel.

1. Read the humidity. EPA says a humidity meter is a small, inexpensive instrument, about $10 to $50 at many hardware stores. Leave it in a main room for a day. A reading that stays above 60 percent points you toward a humidity problem first.
2. Look at the filter. ENERGY STAR says to inspect, clean or change air filters once a month. Swap a dirty or damp one before you judge the smell. Other odors, from burning dust to rotten eggs, are sorted out in our guide to HVAC smells.
3. Find the drain pan and drain line. With a flashlight, look for standing water, slime or debris in the pan under the coil and for a line that is not flowing. EPA's mold guide says to keep drip pans clean and drain lines unobstructed and flowing properly.
4. Check the ducts you can reach. In the attic, crawl space or garage, look for wet or stained insulation, beads of water on duct jackets, and torn or disconnected flex duct.
5. Sniff the source. If the smell is much stronger at one register than the others, that branch of duct is worth a closer look before you blame the coil.

When to stop running the system

EPA's mold cleanup guidance is direct: do not run the HVAC system if you know or suspect it is contaminated with mold, because it could spread mold throughout the building. EPA's examples are a system that is part of an identified moisture problem, or mold near the system's intake.

For context on scale, EPA says a moldy area smaller than about 10 square feet can usually be cleaned up by the homeowner, while larger growth or a lot of water damage calls for its commercial remediation guidance. Ducts are harder to judge, because EPA notes that many sections of a heating and cooling system cannot be inspected visually. EPA also says to dry wet or damp materials within 24 to 48 hours after a leak, since in most cases mold will not grow if you do.

CDC's mold overview says some people react to damp, moldy air with a stuffy nose, sore throat, coughing or wheezing, burning eyes or skin rash, and that people with asthma or mold allergies may have severe reactions. If someone in the house fits that description, move faster.

What actually fixes it, and what does not

Fix the water first

EPA says that if the conditions causing mold growth are not corrected, mold growth will recur. That makes the repair list fairly short:

• Clear and flush the condensate drain and make sure the pan drains properly.
• Have the coil cleaned if it is dirty or growing anything. Our page on how long evaporator coils last covers when a coil is worth cleaning versus replacing.
• Seal duct leaks with mastic or metal tape and insulate the ducts you can reach. ENERGY STAR says never use duct tape, because it does not last.
• Bring indoor humidity into EPA's range, with a dehumidifier if the air conditioner cannot do it alone. If yours has quit, see dehumidifier stopped working.

Replace wet insulation instead of cleaning it

This is the point most pages skip. EPA says that if insulated air ducts get wet or moldy, the insulation cannot be effectively cleaned and should be removed and replaced. It goes further: fiberglass or other insulation that is wet, visibly moldy, or has an unacceptable odor should be removed and replaced by a qualified heating and cooling contractor. A persistent musty smell from a lined duct section is, by EPA's own wording, a reason to replace that section.

EPA also says steam cleaning and other methods involving moisture should not be used on any kind of ductwork. Adding water to a moisture problem is not a fix.

Be skeptical of sprays, foggers and coatings

EPA says research has not demonstrated the effectiveness of chemical biocides or sealants in duct cleaning, or their potential adverse health effects. It notes that no products are currently registered by EPA as biocides for use on fiberglass duct board or fiberglass-lined ducts, and only a small number are registered for the inside of bare sheet metal ducts. Before a provider applies any biocide, EPA says they should show you visible evidence of microbial growth.

Duct cleaning has a narrow role

Cleaning makes sense when there is substantial visible mold on hard surfaces such as sheet metal ducts, when ducts are infested with rodents or insects, or when they are clogged with dust and debris. EPA says that when you see no sign of large dust or mold deposits, meaning no musty odor or visible mold growth, cleaning is probably unnecessary. A musty odor therefore is one of the cases where an inspection is justified. It is not proof that cleaning alone will help. Our guide to whether ductwork needs to be cleaned covers how to vet a duct cleaning company.

If you hire one, EPA recommends asking the provider to show you any mold they say exists, since a substance can look like mold without being mold. Only an expert can make that call, sometimes with a lab test, and EPA notes that some labs will check a sample on clear household tape for about $50. EPA also says to choose a provider that follows the National Air Duct Cleaners Association (NADCA) standards and, for fiberglass duct board or lined duct, the insulation manufacturers' recommended practice.

If the smell keeps coming back

A smell that returns a few weeks after a coil cleaning or duct cleaning means the moisture source is still there. Go back to the list: drain pan, wet insulation, sweating ducts, oversizing and indoor humidity. Our page on how long ductwork lasts explains how to judge whether a damp, sagging section is worth repairing or replacing. ENERGY STAR's maintenance checklist includes inspecting the condensate drain at each tune-up, so the drain gets looked at regularly.