Furnace has a musty smell when running

Symptoms

• Damp, mildewy odor when the furnace fan runs
• Smell may be strongest right when the system turns on
• No visible smoke or overheating

Likely causes

• CommonDirty or damp air filterA filter that's stayed wet or dusty for a while can develop mold.
• CommonMold or mildew growth in ductworkDucts running through damp basements or crawlspaces can grow mold over time.
• PossibleHumidifier attached to the furnace with standing water or moldA whole-house humidifier's pad or reservoir can grow mildew if not maintained.
• PossibleCondensate drain pan with standing waterA pan that doesn't drain properly can hold water long enough to smell musty.

What to do

1. Replace the air filter, checking it for visible mold or dampness.
2. If there's a furnace humidifier, check and clean its water panel or pad according to the manufacturer's instructions.
3. Check any visible condensate drain pan for standing water and clean it if present.
4. Have ducts visually inspected where accessible for mold, especially in a damp basement or crawlspace.
5. Run a dehumidifier in damp areas near the furnace to reduce moisture that feeds mold growth.
6. If the smell persists after filter and humidifier checks, consider a professional duct inspection or cleaning.

Tools and parts

When to call a pro

Call a duct cleaning or HVAC professional if the musty smell persists after filter and humidifier checks; ducts may need cleaning.

Safety