Why do my vents smell musty when the heat or AC turns on?
Usually because the system is pulling in air from somewhere it shouldn’t. In many Bay Area homes, that somewhere is the crawl space or the attic, through a torn, crushed or disconnected duct.
When the heater or AC runs, air moves fast through the ducts. If a duct is damaged, it pulls in whatever air surrounds it. Under a house with damp soil, old insulation or rodent droppings, that air smells musty, and the vents blow it straight into your rooms.
Is the smell coming from the ducts or the crawl space?
The timing tells you a lot. A smell that only shows up when the system runs points to the ducts. A smell that is there all the time, strongest near the floor, points to the crawl space itself.
- Only when the heat or AC is on: likely a duct problem, often a damaged duct under the house or in the attic.
- All the time, near the floor: likely the crawl space, rising through gaps in the floor.
- Worse after rain: moisture under the house.
- Like urine or ammonia: rodents, in the crawl space, the attic or the ducts.
Often it is both: a damp, dirty crawl space and a damaged duct running through it. Our post on why a crawl space smells covers the crawl space side in detail.
How does crawl space air get into the ducts?
Through any opening in a duct that runs under the house. A torn outer jacket, a crushed section or a joint that has come apart all let crawl space air in.
Ducts under the house have a hard life. Rodents chew them and nest on them, they sag and end up lying on the dirt, and old connections work loose over the years. From inside the house you would never know, until the smell tells you.
What if the vents smell like urine or a dead animal?
That points to rodents. They chew into ducts, nest on them and sometimes inside them, and their droppings and urine end up in the airflow.
Chew holes, droppings on or inside the ducts, and dust blowing out of the vents when the system starts are all signs. A dead-animal smell that came on suddenly can mean a rodent died in or near a duct. Either way, the openings rodents use have to be sealed too, or the problem comes back. See rodent proofing.
Can the AC itself cause a musty smell?
It can. A dirty air filter, or moisture around the indoor coil and its drain pan, can also give off a musty smell when the AC runs.
Changing the filter is the easy first step, and an HVAC technician can check the coil and the drain. If the smell stays after that, look at the ducts and the space they run through.
Will cleaning the ducts fix the smell?
Not when the ducts are torn, crushed or soiled by rodents. Cleaning doesn’t close a tear or reconnect a loose joint, so the system keeps pulling in the same air.
Damaged ducts are replaced with new ones. We install new flexible insulated ducts, JPL brand and UL 181 listed, run and supported to fit your attic or crawl space. See air duct replacement.
How do you fix a musty smell from the vents for good?
Fix the source, then fix the path. Air fresheners and scented filters only cover it for a while.
- Clean the crawl space: debris, droppings and soiled insulation come out, and the space is sanitized. See crawl space cleaning.
- Seal the openings: vents and gaps closed with galvanized steel mesh, putty and concrete, so animals stay out.
- Cover the soil: a vapor barrier over the ground keeps ground moisture down.
- Replace damaged ducts: new flexible insulated ducts, connected and supported.
Why does the smell get worse after rain?
Rain wets the soil under the house, and damp soil raises the moisture in the crawl space air. If a damaged duct runs through that space, the system pulls in more of that damp, musty air.
Older crawl spaces with bare soil or torn plastic are hit hardest. A vapor barrier over the soil helps with this part; see our guide to crawl space vapor barriers.
How can I check the ducts myself?
Look from the crawl space opening or the attic hatch with a flashlight, without crawling in. Flexible ducts look like large round tubes with a silver or gray outer jacket.
Look for tears, flattened sections, runs lying on the soil and droppings. If you see droppings, don’t touch them; the CDC says not to sweep or vacuum them. Our post on signs you need new air ducts lists what else to look for.
What we see on real jobs
When homeowners in Palo Alto, Menlo Park or Milpitas call about smelly vents, we usually find a mix of these under the house:
- Ducts with torn jackets, chew holes or joints that have come apart.
- Ducts sagging onto the soil.
- Droppings on the ground and on the ducts.
- Bare soil or old, torn plastic sheeting.
Duct replacement and crawl space cleaning are each one-day jobs, and many homeowners do them together. Customers who book two or more services get 10% off (one discount per job). Book a free inspection or call us, and get a written quote the same day.