A musty smell from the vents means moisture is feeding mold or mildew somewhere along the air path. The usual places are the indoor coil of an air conditioner or heat pump, a drain pan that holds water, a clogged condensate line, a damp filter, ductwork that runs through a damp crawlspace or garage, or the crawlspace itself when leaky return ducts draw air from it. Find the water and you have found the cause; covering the smell changes nothing.
Is it really musty?
Different smells mean different things, and two of them are urgent:
- Musty, damp, earthy, like a wet basement: moisture and growth. That is this page.
- Dusty or slightly burnt the first time the heat runs in the fall, fading after the first few cycles: dust burning off the furnace, usually normal. See our guide to the burning smell when the heater first comes on.
- Rotten eggs or sulfur: a possible gas leak. Leave the house, then call the gas utility or 911 from outside.
- Hot plastic or electrical burning: turn the system off at the thermostat and the furnace switch, and call.
Where the smell usually comes from
| When or where you notice it | Likely source | What it takes |
|---|---|---|
| Strongest in the first minutes after the system starts | Mildew on the indoor coil or in its drain pan | Coil and pan cleaning |
| Mostly when the AC or heat pump is cooling | Wet coil, a pan that doesn’t drain, a clogged condensate line | Drain clearing; pan check |
| A “dirty sock” odor from a heat pump at startup | Growth on the indoor coil | Coil cleaning; a UV lamp to help prevent regrowth |
| Musty with the heat on, worse in certain rooms | Moisture or mold in one duct run, often through a damp crawlspace or garage | Duct inspection; repair or cleaning |
| Damp filter, rust or water stains near the furnace | Water from a drain line or a leaking fitting | Drain or fitting repair; new filter |
| Musty even with the system off | The house: crawlspace moisture, a leak, a bathroom without ventilation | Find the moisture; humidity control |
If your home has a furnace only and no air conditioning, there is no indoor coil. Look first at the ducts and the spaces they pass through, and at the condensate drain if you have a high-efficiency furnace with white plastic vent pipes.
What you can check
- Replace the air filter, and look at the old one: damp or spotted means moisture is reaching it.
- Look at the registers and grilles for visible mold.
- Find where the condensate drain line ends. When cooling runs, it should drip; a dry line with water around the indoor unit means a clog.
- If the drain pan is visible without removing panels, look for standing water.
- Check the crawlspace or the garage under the house for standing water, wet insulation or a damp smell.
Don’t spray bleach or household cleaners into the registers or ducts: the fumes travel through the house and the source stays wet. Don’t run an ozone generator to cover the smell either.
When it needs a technician
Call when you see mold, when the smell is still there after a new filter, when the coil or drain needs cleaning, when you suspect mold inside the ducts, or when you can’t find the source. A technician opens the coil cabinet, inspects the coil and pan, flushes the drain, looks inside the ducts with a camera, and checks whether leaky return ducts are pulling air from the crawlspace or garage.
From there the work is coil cleaning, duct cleaning when the ducts are actually contaminated, sealing or repairing the leaky runs, or UV light installation. A UV lamp aimed at the coil helps keep its surface clean, but it won’t fix a drain that holds water. Mold spread over a wide area of the house itself is a job for a mold remediation specialist, and we will tell you so.
Local note: fog-belt damp
Daly City’s marine layer keeps the air cool and damp for much of the year, and the ground-floor garages and crawlspaces under many mid-century homes rarely dry out. Ducts that run through those spaces, and return leaks that draw from them, bring that damp air into the house. Sealing the returns and controlling moisture, with a dehumidifier where needed, fixes the cause. Our fog-belt humidity guide and the page on high humidity go further.