Furnace smells: burning, gas or musty

A furnace smell can be harmless, like dust burning off the first time you run it in the fall, or serious, like gas or burning plastic. Let's sort out which one you have, and what to do next.

Also searched as: furnace smells like burning, furnace burning smell first time, furnace smells like gas, furnace smells musty.

Before you start. If you smell gas or rotten eggs, or you see smoke or flames, leave the house now. Don't touch switches or use a phone inside. Call 911 or your gas utility from outside.

Every question and answer on this page, in order

Question 1: What does the smell most remind you of?

Pick the first one that fits. If you're not sure it isn't gas, choose the gas answer.

Gas or rotten eggs, or I'm not sure it isn't: Gas smell: leave the house now
Burning plastic, hot wiring or an electrical smell, or I see smoke: go to question 2
Burnt dust, the first time the furnace has run this season: go to question 3
Musty or damp: go to question 4
Something else, or I can't tell: Have it checked: a lasting or unidentified smell

Question 2: Do you see smoke or flames, in the furnace area or anywhere in the house?

Yes: Emergency: get out and call 911
No, only the smell: Stop: burning plastic or an electrical smell

Question 3: How would you describe the smell, and how long has it lasted?

A mild, dusty smell from the registers is the one to look for. Note when you first started the furnace.

Mild and dusty, and it has been less than a few hours since the furnace started: Likely normal: a first-start smell
It has lasted more than a few hours, is getting stronger, or keeps coming back: Have it checked: a lasting or unidentified smell
It's sharp, like plastic, electrical wiring or gunpowder: go to question 2

Question 4: Did the musty smell show up only when you first ran the furnace after a long time off, and then fade?

Yes, it faded as the furnace ran: Likely normal: a first-start smell
No, it's still there or it comes back: go to question 5

Question 5: Set the thermostat to Off, then look at the furnace filter. How does it look?

Lennox says to turn the unit off before you check the filter. Look only at the filter; don't open anything else.

Dirty, gray, damp, or I can't remember when it was changed: Likely cause: an old or dirty filter
Clean and recent, or I can't find it: Have it checked: a musty smell at the registers

Possible results

Need the words for the phone call? Build a what-to-tell-the-technician checklist, or read when to call an hvac pro.