Symptoms
- Musty odor noticeable in multiple or all rooms
- Smell stronger when the HVAC system is running
- No single obviously damp spot found in living areas
Likely causes
- CommonMoldy HVAC system or ductworkMold growing on a damp evaporator coil, drip pan, or inside ducts can spread odor through every vent.
- CommonDamp crawl space venting air into the houseAir pressure differences can pull crawl space odor up into living spaces, especially with an unsealed crawl space.
- PossibleBasement moisture issue circulating through open stairways or shared ductworkA musty basement can affect air quality upstairs even without visible signs there.
What to do
- Check if the smell gets stronger right when the HVAC system kicks on, pointing to the system itself.
- Replace the HVAC air filter if it's dirty or overdue, since a damp, dirty filter can harbor mold.
- Look at the indoor coil and drain pan (or have a technician do so) for slime or standing water, which are strong mold indicators.
- Check the crawl space for standing water, damp insulation, or exposed dirt floor, and address ventilation or a vapor barrier as needed.
- Have basement moisture addressed separately if that's also part of the source.
- Run an air purifier or increase ventilation temporarily while identifying the source.
- Schedule an HVAC technician to clean or service the system, and a mold specialist if a full-house source isn't obvious.
Tools and parts
- Flashlight
- HVAC filter
HVAC air filterSearch: “furnace air filter”
When to call a pro
Call an HVAC technician to inspect and clean the system, and a mold specialist if the source isn't found there.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.