This is normal
Likely normal: a first-start smell
A mild, dusty smell the first time you run the furnace after months off is common and should fade. A musty smell can also just mean the system has not run in a while.
Why this happens
Carrier says dust settles on parts like the burner and heat exchanger over the summer, and burns off when the furnace first heats up, so you may smell it at the registers. Carrier says this typically lasts anywhere from about 30 minutes to a few hours. Trane says to wait it out, since the smell should fade fairly quickly. Lennox says a musty smell can simply mean the system hasn't run in a while.
What to do
- This page is only for a mild, dusty smell that fades. If you smell gas or rotten eggs, leave the house now, touch no switch, and call 911 or your gas utility from outside. If you see smoke or flames, leave the house now and call 911 from outside. If the smell is sharp, like plastic or electrical wiring, don't wait it out: set the thermostat to Off and call an HVAC technician.
- Let the furnace run, and stay aware of the smell. Carrier says you can ventilate the home if needed while you monitor it.
- Check the filter while you're at it. Trane suggests starting the heating season with a clean filter, and Carrier says to replace it if it is dirty. Set the thermostat to Off before you pull it, then set it back to Heat once the new filter is in.
- Lennox says to clean the area around the unit before the heating season starts, since dust and debris collect there. Do that before the first run next year, and clean only the outside area. Don't open the furnace.
- A smell can't tell you whether carbon monoxide is present, because CDC says it has no smell. CDC says to have CO detectors near every sleeping area.
When to stop. Carrier says to turn the furnace off and call a qualified HVAC professional if the smell lasts longer than a few hours, gets stronger or keeps returning, or if it smells like burning plastic or electrical wiring. If it smells like 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 it still worries you, tell the HVAC technician
- If it does not fade: how long the furnace has run and how the smell changed.
- Whether the smell is dusty, plastic, electrical, musty or gassy.
- When the furnace and its filter were last serviced or changed.
Build the full list: what to tell the HVAC technician.
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