Have it checked: a lasting or unidentified smell

Why this happens

Carrier says a persistent, worsening or unidentified furnace odor deserves professional inspection, and that a burnt-dust smell lasting beyond a few hours or growing stronger is not normal dust burn-off. Lennox says that if a smell persists or gets worse, you should call a qualified professional rather than attempt a fix yourself. Carrier also says an oily smell from an oil furnace can point to a leak or burner problem.

What to do right now

1. If you smell gas or rotten eggs, or you see smoke or flames, leave the house now and call 911 or your gas utility from outside. Don't touch switches or use a phone inside.
2. Write down what the smell is like and where it is strongest. Carrier says the type and location help you decide what to do.
3. Set the thermostat to Off if the smell is strong, has lasted more than a few hours, or is like burning plastic, electrical wiring or gunpowder. Carrier says to have the system inspected rather than keep running it.
4. You can check the filter, with the thermostat set to Off, and replace it if it is dirty. Carrier says not to try to diagnose or repair electrical or combustion problems yourself.
5. Don't open the furnace.
6. Call an HVAC technician. If you have an oil furnace and the smell is oily, say so.
7. Lennox says carbon monoxide has no smell, so keep working CO detectors near sleeping areas. CDC says the same.

When to stop. If you smell gas or rotten eggs, or see smoke or flames, 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.

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