Furnace Burning Smell in Minneapolis: What Each Smell Means

A burning smell from a furnace is usually dust burning off the burners on the first cold days. A hot plastic, electrical, fishy or exhaust smell means shut the furnace off, and a rotten-egg smell means leave the house and call CenterPoint Energy or 911 from outside. Match the smell first, because the type tells you whether to open a window or shut the furnace off. In Minneapolis, where many basements date from the early 1900s and the mechanical rooms are small, the culprit is often storage pushed against the hot flue pipe: plastic bins, decorations and cardboard will scorch there.

What do different furnace smells mean?

The fishy one surprises people. Some electrical components give off a fishy odor as they overheat, so treat it like a hot-plastic smell. The musty one isn't a burn at all: a furnace humidifier with an old pad or a clogged drain grows mildew, and our humidifier guide covers upkeep.

What to do in the next five minutes

1. Match the smell to the table above. If you can't tell, treat it as the more serious one.
2. Rotten eggs: get everyone out without flipping switches or using the phone inside, then call CenterPoint Energy or 911 from the sidewalk.
3. A CO alarm sounding: everyone leaves and someone calls 911 from outside. Nobody goes back in to look for the cause.
4. Exhaust, hot plastic, electrical or fishy: turn the furnace off at the thermostat, then at the service switch on or near the furnace.
5. Look without touching hot metal. Check for storage leaning on the flue pipe or cabinet, something melted in a floor register, or a filter that has sagged into the blower.
6. Dusty only: put in a fresh filter, open a window and let the furnace run. It should smell less with each cycle.

If you turned the furnace off for anything other than dust, leave it off until someone has found the source. A furnace that smelled electrical and then seemed fine the next morning often has the same loose connection or tired motor waiting for the next cold night.

Minneapolis basements: where burning smells start

A big share of Minneapolis houses went up in the first decades of the 1900s: bungalows, four-squares and story-and-a-halfs from Northeast to Nokomis. Their mechanical rooms are small. The furnace, the water heater, the washer and dryer and years of storage all share one low-ceilinged corner of the basement, and that's where a lot of burning smells come from.

• Storage against hot surfaces. An 80% furnace and a gas water heater both vent through a metal pipe into the chimney, and that pipe gets hot. Plastic bins, holiday decorations, laundry baskets and cardboard pushed against it will scorch. Every furnace lists clearances to combustibles on its rating plate. Keep that zone empty.
• Big floor registers. Houses converted from old gravity systems often still have large floor grates, and toys, crayons and pet treats fall through them onto warm duct surfaces. A flashlight down the register often finds the culprit.
• Lint. A dryer venting poorly, or lint drifting across a small laundry room, collects on the furnace and burns off in odd whiffs.
• The filter itself. A flimsy filter that has sagged into the blower compartment, or one jammed into the slot the wrong way, can rub or overheat. If the filter or the wall around a register is also turning black, our post on black furnace filters sorts soot from dust.

Where does a sharp, chemical furnace smell come from?

When the smell is sharp and chemical, the source is usually inside the cabinet.

• Blower motor. Worn bearings, a clogged filter or a failing run capacitor make the motor run hot, and its winding insulation starts to smell before it fails.
• Capacitor. A failing run capacitor can bulge, leak or scorch. Capacitors hold a charge even with the power off, so leave them to a technician.
• Wiring and the control board. A loose connection heats up at the terminal and cooks the insulation around it. Scorched spots on the board or a brown wire end are the tell.

Turn the furnace off at the thermostat, then at the service switch near the furnace, and leave it off until it's checked. A technician looks for scorched terminals and melted wire nuts, measures the blower motor's amp draw against its nameplate and tests the capacitor against its rated microfarads. Those checks find most electrical smells without guesswork.

What does a rotten-egg smell near the furnace mean?

CenterPoint Energy supplies natural gas in Minneapolis and adds an odorant so leaks smell like rotten eggs. If you smell it, get out first. Don't flip light switches or start a car in an attached garage. Call from outside.

An exhaust smell is different: a stale, stinging combustion smell near the furnace or water heater. In Minneapolis it shows up after furnace upgrades. When an old 80% furnace comes out and a 96% furnace vents through PVC out the side of the house, the water heater is left alone in a chimney that's now far too big for it. It may not draft well, especially on mild days, and exhaust can spill back at the draft hood. The usual fix is a liner in the chimney or a new vent route for the water heater. A damaged heat exchanger can also let combustion gases into the air stream, and our post on cracked heat exchanger symptoms covers how that gets confirmed. With any exhaust smell, working CO alarms matter. Minn. Stat. 299F.51 puts them within 10 feet of every sleeping room, and our CO alarm guide covers placement.

How can you keep the furnace's first-cold-day smell down?

• Put in a fresh filter before the first cold night.
• Vacuum around the furnace and the return grilles.
• Run the heat on a mild October day so the dust burns off with the windows open.

Our guide to turning the furnace on for the first time this fall walks through that first start. A fall tune-up covers the flame sensor, combustion, heat exchanger and venting before the season starts. Our furnace and AC maintenance visits start at $129 for a furnace, or $219 with the air conditioner. Ask us about a maintenance plan if you want the fall visit on a schedule; see maintenance in Minneapolis.

What it costs, and when to call

Finding the source is part of the service call, and the $79 trip charge is waived when you approve the repair. For scale on the larger repairs a burning smell can lead to, HomeGuide's national ranges put a PSC blower motor at $500–$1,400, a variable-speed (ECM) motor at $800–$2,300 and a furnace control board at $300–$650. We don't price from those national figures, and our blower motor cost guide explains what moves them. Plenty of smell calls end with a cheaper fix: a loose wire tightened, a sagging filter replaced, a plastic bin moved off the flue. When an exhaust smell traces back to a water heater left alone in the chimney, the venting fix depends on the chimney itself, so there's no fair number until someone has looked at it.

If the smell is sharp, electrical, fishy or exhaust-like, or if a dusty smell hasn't faded after a day, turn the furnace off and call 612-429-3465. We're open 7 days, 7am–9pm, and a furnace that has stopped heating gets priority. A furnace repair visit ends with a written flat-rate price before work starts; see furnace repair in Minneapolis.

Common questions about heating

Sources

• Minnesota Statutes 299F.51 (carbon monoxide alarms) revisor.mn.gov
• HomeGuide: furnace repair costs homeguide.com