A dusty, warm smell when you first run the furnace each season is normal and clears within a day; a rotten-egg smell means a possible gas leak and requires leaving the house; burning plastic, rubber or hot metal means shut the furnace off; and musty or stale smells usually mean moisture, mold or a dirty filter. The smell is the clue, so match yours to the table below.
Rotten eggs or sulfur: possible gas leak
Natural gas is odorless, so utilities add a rotten-egg smell. If you smell it, do not turn lights or the furnace on or off, do not use a phone inside, and do not light anything. Open a door on the way out, leave, and call your gas utility’s emergency line or emergency services from outside. Do not go back in until it is cleared.
Smell guide
| Smell | Usual cause | What to do |
|---|---|---|
| Dusty, warm smell at first use of the season | Dust burning off the heat exchanger and vents | Run it with windows open; should clear in a few hours |
| Burning dust that persists | Clogged filter or dirty burners | Replace the filter; see when to change it |
| Burning plastic or rubber | Foreign object, melted part or blower belt | Turn the furnace off and call a technician |
| Hot electrical or fishy smell | Overheating wiring or a failing motor or board | Turn it off at the breaker and call a technician |
| Musty, damp or sweaty-sock smell | Mold or moisture in the ducts, filter or coil | Replace the filter, check for moisture; see below |
| Oil smell (oil-fired furnace) | Leak, soot or a dirty burner | Turn off and call an oil heating technician |
| Rotten eggs or sulfur | Possible gas leak | Leave and call the gas utility |
Dust and burning dust
A furnace that has sat idle all summer collects dust that burns off on the first few runs. Open some windows, and the smell should fade within a few hours to a day. If it lingers, a clogged filter is restricting airflow and cooking the dust; replace it. A dirty filter can also lead to shutdowns, covered in furnace keeps shutting off.
Burning plastic, rubber or electrical
These smells are not normal. A child’s toy that fell into a floor vent, a belt slipping on an older blower, a melted wire insulation or an overheating motor can each cause them. Turn the thermostat to Off and switch the furnace off at its switch or breaker. Check vents for foreign objects only once it has cooled. If the smell has no obvious source, call a technician before using the furnace again.
Musty, damp or sweaty-sock smell
- Replace the filter
A damp, dirty filter is the most common source of musty air.
- Check for moisture
On a high-efficiency furnace look for water in the condensate pan or trap; if you also have central air, the evaporator coil and its drain can grow mold.
- Look at the humidifier
A whole-house humidifier with stale water can smell sour. Clean it or replace its pad.
If the odor comes only from the hot water taps, that is a different issue — see smelly hot water.
Carbon monoxide has no smell
Carbon monoxide has no odor. Keep working CO detectors on every level and near bedrooms. If a detector sounds or anyone has headaches, dizziness or nausea at home, go outside into fresh air and call emergency services. See smoke detector false alarms for the difference between alarm types.
Prevention
Replace the filter on schedule, keep the area around the furnace clear of boxes and chemicals, and book an annual service in early fall. Our winterizing checklist covers the rest.
When to call a technician
Any burning, electrical or oil smell, any recurring musty smell with visible moisture, and any smell after the furnace has shut down on its own should be checked by a professional. For a furnace that is blowing air but not heat, see furnace not blowing hot air.
