You can fix this
Likely cause: an old or dirty filter
A dirty filter can go with mold or mildew in the ductwork, which gives off a musty smell. Replace the filter first.
Why this happens
Lennox says old filters can let mold and mildew grow inside the ductwork and cause a musty odor. Trane says a musty smell from the registers can mean mold and mildew in the ductwork or the air filter, and that this can be a hazard, especially in humid climates.
How to fix it
- This page is only for a musty smell. If you smell gas or rotten eggs, or you see smoke or flames, leave the house now, touch no switch, and call 911 or your gas utility from outside. If the smell is sharp, like burning plastic or electrical wiring, set the thermostat to Off and call an HVAC technician.
- Keep the thermostat set to Off while you swap the filter. Lennox says to turn the unit off before you check it.
- Replace the filter with the size and type your furnace maker specifies. Carrier and Trane both say many standard filters need changing every 30 to 90 days.
- Turn the thermostat back to Heat and run the furnace. Give it some time and see if the smell fades.
- If the smell is still there with a clean filter, call an HVAC company. Trane says it can come back until the extra humidity behind it is dealt with.
- Don't open the furnace or the ductwork.
When to stop. If the musty smell is still there after a clean filter, turn the thermostat to Off and call an HVAC technician. If you smell gas or rotten eggs, leave the house and call 911 or your gas utility from outside. If a CO alarm sounds, or anyone has a headache, dizziness, weakness or nausea, get outside to fresh air and call 911.
If this does not fix it, tell the HVAC technician
- That the smell is musty, and whether it comes from the registers or the furnace.
- When the filter was last changed and how it looked.
- Whether the house feels humid or has damp spots.
Build the full list: what to tell the HVAC technician.
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