Does this match your problem?
- Musty smell strongest when AC first turns on
- Dirty-sock smell during heating cycles
- Odor is worst in one specific room
- You can smell mildew near return vents
- Bathrooms or closets smell 'old' even when clean
Quick check first
Sniff a supply vent within 30 seconds of the AC starting. If the smell is coming from the vents, the source is in your HVAC system. If not, it's a room-specific moisture issue.
Step-by-step troubleshooting
1. Inspect the air filter
Pull the filter and look at it closely, and smell it. A damp, dust-caked filter grows biofilm, and every cycle pushes that odor through the house. If it's gray, matted, or smells sour, you've found at least part of the source. Which filter grade to move to (MERV 8 vs MERV 11 for odor-carrying particles) and the replacement routine are covered in the DIY Quick-Fix Playbook, or a tech can sort it on a visit.
2. Inspect the drain pan and condensate line
Standing water in the drain pan under the indoor unit grows mold within days. Look for water, sludge, or a slimy film in the pan, and check the access tee near the unit for a backed-up trap. On a humid day with the AC running, the exterior drain stub should drip; a dry stub means the line is blocked and water is sitting where it shouldn't. Any of those confirms the source. Clearing and treating the line properly is covered step by step in the Playbook, or we can do it during a visit.
3. Inspect the evaporator coil
The coil sits in constant condensation during summer. Over years, it builds up biofilm, the 'dirty sock' smell. Coil cleaning is a pro job (accessing it usually requires cutting into the plenum). If your system is over 5 years old and has never had coil service, this is likely.
4. Look for wet insulation or drywall
Walk the house and sniff at floor level near exterior walls, under sinks, around water heaters, and in closets on shared plumbing walls. A slow leak inside a wall smells musty long before you see damage.
5. Check the crawlspace or basement
Return-air leaks in crawlspaces pull musty crawlspace air directly into your HVAC and distribute it through the house. If your crawlspace smells musty, your house eventually will too.
6. Check the ductwork and returns
Pull a couple of return grilles and look inside with a flashlight: heavy dust mats, visible mold speckling on the liner, or a distinctly musty draft at the return all point to the duct system itself as the reservoir. Duct interiors and coil surfaces are not DIY territory, remediation there is a professional NADCA-certified duct cleaning, usually paired with a UV-C coil light to stop biofilm from regrowing. Once you've identified it here, book a visit and we'll scope it.
Stop and call a licensed pro if:
- •Odor returns within a week of filter/drain cleaning
- •You see visible mold on the coil, drain pan, or vent grilles
- •Anyone in the home is experiencing new respiratory symptoms
- •Smell is paired with rising humidity readings
- •You suspect a hidden water leak inside a wall