There is a burning smell the first time the heat runs

A smell of gas is not this page. Rotten egg or sulphur anywhere in the house, or a carbon monoxide alarm sounding, means leaving the building and calling 911 or the gas utility from outside. That call comes first.

What separates dust from the rest

Dust is faint and even, arrives with the first cycle of the season and only that one, and fades while the furnace is still running.

Hot plastic or burning wire does not fade. It returns on every cycle and is strongest at the registers nearest the furnace. Overheating motor windings, a bearing dragging, or a loose connection at the blower or the control board all smell like this.

Anything visible — smoke, scorch marks, heat behind the smell — means shutting the system down at the thermostat and then at the breaker.

Why it will not clear on its own

A motor overheats because something is loading it: a dry bearing, a loaded filter, a closed return, a wheel rubbing its housing. The load is still there on the next cycle, and each one takes more out of the windings. Loose connections behave the same way, since the heat they make loosens them further.

The timing settles the question by itself. Dust burns off once. A smell still there on the third cycle stopped being dust two cycles ago.

What to say when you call

• Which cycle it was: the first of the season, or the fifth this week
• What it smells of — dust, hot plastic, burning wire, oil
• Whether it fades while the furnace runs or lasts the whole cycle
• Install year, your address and ZIP code

Common questions

It went after twenty minutes and has not come back. Worth a call? First-of-season dust that clears once and never returns is the ordinary case, and usually needs nothing. If it returned on the next cycle, it was never dust.

Is a burning smell dangerous? Dust is not. Hot plastic and scorched wiring are the front end of a failing motor or a bad connection, and both belong to a visit rather than a wait.

Should the heat stay off until someone comes? If the smell is electrical, yes — off at the breaker. Say on the phone that the system is shut down, because it changes how the call is scheduled.