The first cold morning in Sacramento produces a wave of the same phone call: the furnace came on and the house smells like something is burning. Most of the time that is fine. Some smells are not fine, and one of them means you should be standing in the front yard before you finish reading this. Here is how we sort them, in the order you should think about them.
First: the one that means leave now
If the smell is rotten eggs or sulfur, that is the odorant added to natural gas so you can detect it. Do not troubleshoot it. Do not flip switches, light anything, or use a phone inside the house. Leave, take everyone with you, and from outside call 911 and then PG&E at 1-800-743-5000. PG&E’s own guidance is exactly that: leave first, call second. A gas leak is not an HVAC repair until the utility has made the house safe; we come after.
The smells, and what they usually are
- Burning dust on the first heat of the season. Normal. Dust settled on the heat exchanger and burners over the summer and is burning off. It should clear within thirty to sixty minutes with a window cracked. If it lasts longer than a day or comes back every time the furnace runs, it is something else.
- Burning plastic or electrical. Not normal. A blower motor overheating, wiring insulation melting on a hot component, or a plastic object that fell onto the heat exchanger. Turn the furnace off at the switch and call. Running it risks a fire and a dead motor.
- Rotten eggs. Gas. See above.
- Musty or dirty socks. Moisture. On a heat pump or a system with an attic air handler, condensate sitting in the pan or a coil that stayed wet grows a biofilm that smells when warm air passes over it. On any system, ducts that pulled in damp crawlspace air do the same. Unpleasant, not dangerous, and fixable.
- Exhaust, like a running car. Combustion gases where they should not be. A blocked flue, a failed inducer, or a cracked heat exchanger. Turn the furnace off, open windows, and call. If a carbon monoxide alarm sounds at any point, treat it like gas: leave first.
- Chemical or formaldehyde. Rare, and on a furnace it is one of the signs of a cracked heat exchanger letting combustion byproducts into the air stream. Turn it off and call.
- Something dead. Rodents get into attic and crawlspace ducts every winter in the older Sacramento neighborhoods. The smell arrives with the heat and fades when it stops. Not dangerous, but the duct has a hole in it, and that is a repair.
Why the first-of-season smell is worse in some houses
A furnace that has not run since March has a summer’s worth of dust on the burners and heat exchanger. In homes with a one-inch return filter that was never changed, or with a return that pulls from a dusty garage or crawlspace, there is more of it. In the 1950s and 60s ranch homes around Carmichael, Arden-Arcade and Citrus Heights, where the return grille is small and the filter clogs in six weeks, we hear this every October. The fix is a new filter and patience the first day. The longer-term fix is a media cabinet and a look at where the return is drawing from. Our guide to smoke season and filters covers the filtration side.
What we check when the smell does not go away
- Combustion and carbon monoxide. A CO reading at the registers and a look at the flame. Yellow, lazy flames instead of blue ones point to dirty burners or a combustion problem.
- The heat exchanger. Inspected, on most furnaces with a camera. A crack is the one finding we do not repair; we show you, shut the gas to the unit, and quote a replacement. See when to replace a furnace.
- The blower motor and wiring. Amp draw against the nameplate, insulation on the wiring, anything melted or discolored.
- The flue and inducer. Draft confirmed, flue checked for blockage, inducer motor tested.
- The condensate side and the coil. Pan, trap and coil inspected for standing water and growth on systems that also cool.
- The ducts. A camera into the nearest runs where the smell is strongest, for rodent damage or a disconnected run pulling crawlspace air.
When to call and what it costs
Gas smell: 911 and PG&E, not us, until the house is cleared. Electrical, exhaust or chemical smell: turn the furnace off and call us today. Dust smell that lasts past the first day, or a musty smell every time the heat runs: this week. Diagnostic is a flat $69 across the Sacramento area, credited toward the repair. Same-day in Roseville, Rocklin, Sacramento, Citrus Heights, Folsom and Elk Grove. See furnace repair in Roseville, furnace repair in Sacramento, or the full furnace repair page.
If the smell comes with a bang when the burners light, read our guide to furnace noises; if the furnace smells fine but the air is cold, start with a furnace blowing cold air.
Frequently asked questions
Is it normal for a furnace to smell like burning when first turned on?
Yes, for the first thirty to sixty minutes of the season. Dust that settled on the burners and heat exchanger over the summer is burning off. If the smell lasts past the first day or returns every cycle, have it checked.
What does it mean if my furnace smells like rotten eggs?
Natural gas. Leave the house immediately without using switches, phones or anything that could spark, and call 911 and then PG&E at 1-800-743-5000 from outside. This is not an HVAC repair until the utility has cleared the house.
Why does my furnace smell like burning plastic?
Usually an overheating blower motor, melting wire insulation on a hot component, or a plastic object on the heat exchanger. Turn the furnace off at its switch and call. Running it risks a fire.
Why does my heat smell musty?
Moisture: condensate in the pan, a coil that stayed wet, or ducts pulling damp crawlspace air. Unpleasant but not dangerous, and fixable by cleaning the coil and pan and sealing the ducts.
Can a furnace smell mean carbon monoxide?
Carbon monoxide itself has no smell, but an exhaust or chemical smell often comes from the same faults that produce it: a blocked flue, a failed inducer or a cracked heat exchanger. Turn the furnace off, open windows, and call. If a CO alarm sounds, leave first.