Half the no-heat calls we run on the first cold night come down to a switch, a filter or a breaker. Work through these before you pay for a diagnostic.
The first genuinely cold night of the year is our busiest call of the season, and a good share of those calls end with us flipping a switch someone didn't know existed. None of what follows requires tools or any business being near a gas line — if you get through all six and still have no heat, that's a real fault and worth a tech.
Stop here if you smell gas
Rotten egg or sulfur smell means leave the house first and call the gas utility from outside. Don't flip switches, don't check the furnace, don't look for the source. That one isn't a troubleshooting situation.
1. The thermostat, properly
Set it to Heat, not Auto or Cool, and push the target temperature at least five degrees above the current room reading — a two-degree bump may sit inside the deadband and never call for heat. If the display is blank or dim, replace the batteries even if you think they're fine. If it's a smart thermostat, check it hasn't quietly reverted to a schedule or an away setting after a firmware update.
2. The breaker
Find the furnace breaker in the panel. A tripped breaker doesn't always look obviously off — it can sit in a middle position. Push it firmly to Off, then back to On. If it trips again immediately, stop and call. That's a fault, not a nuisance trip.
3. The furnace switch
There's a switch on or beside the furnace that looks exactly like a light switch, often at the top of the basement stairs or on the unit's side panel. Painters, house guests and people looking for the light switch turn it off constantly. This is the single most common cause of a dead furnace we find.
4. The filter
A clogged filter chokes airflow, the heat exchanger overheats, and the high-limit switch shuts the burner down to protect itself. Classic symptom: the furnace fires, runs briefly, shuts off, then tries again a few minutes later. Pull the filter out and hold it up to a light. If you can't see light through it, that's your answer. In our dust, three months is realistic for a one-inch filter — not the twelve months on the box.
5. The gas supply
Check that the gas valve at the furnace is parallel with the pipe, which means open. Then confirm other gas appliances still work — if the stove won't light either, the problem is upstream of the furnace and it's a call to the utility, not to us. If you've recently had work done or the meter was accessed, this is worth ruling out before anything else.
6. The condensate drain
High-efficiency furnaces produce condensate and drain it away. If that line clogs or the pump fails, a float switch shuts the furnace down deliberately to avoid an overflow. Look for standing water in the drain pan or a full condensate pump. Clearing it sometimes brings the furnace straight back.
What the blinking light is telling you
Most furnaces have a small LED behind the access panel that flashes a diagnostic code, and the legend is usually printed on the inside of that panel. Counting the flashes and reading the legend takes a minute and tells a tech more over the phone than any description of the noise it's making. If you call us, that code is the single most useful thing you can have ready.
When it's time to call
- The breaker trips again after you reset it.
- The furnace ignites and shuts down repeatedly with a clean filter.
- You hear a loud bang on ignition, which can indicate delayed ignition and needs looking at promptly.
- The blower runs but never produces heat.
- Any gas smell, at any point.
We run heating calls around the clock through the winter, including nights and weekends, because a furnace that dies at 2am in January isn't a problem that waits until Monday. And if one of the six items above fixed it — good. That's a call you didn't have to pay for.
