The first genuinely cold night in Boston finds every furnace that spent the summer quietly failing. Most no-heat calls in that first week are not broken furnaces at all — they are a switch, a filter, a blocked drain, or a sensor coated in a film you could wipe off with a dollar bill.
Here is the order to work through, and the point where it stops being a homeowner job.
The four things that are not the furnace
- The thermostat. Dead batteries, a schedule that someone changed, or the mode left on cool. If the screen is blank, that is the whole diagnosis.
- The switch on the wall beside the furnace. It looks exactly like a light switch, it is often unlabeled, and it gets flipped by anyone carrying boxes past it.
- The breaker. Gas furnaces still need power for the blower, the igniter and the board.
- The filter. A blocked filter overheats the heat exchanger, the limit switch cuts the burners, and the furnace short-cycles — heat for two minutes, then nothing.
Confirm the gas is on as well, especially if the furnace has not run since spring.
The flame sensor: the single most common no-heat fault
On a modern furnace the sequence is: draft inducer starts, igniter glows, gas valve opens, burners light — and then a thin metal rod in the flame has to prove they lit. That rod is the flame sensor, and it works by conducting a tiny current through the flame itself.
Over a season it builds an oxide film, the current drops below threshold, and the board shuts the gas valve within seconds because as far as it knows the burners failed to light. The symptom is unmistakable: the furnace lights, runs for three to five seconds, then shuts down, and repeats a few times before locking out.
That is a cleaning, not a part. It is also the reason a maintenance visit before the season costs less than the emergency call it prevents.
High-efficiency furnaces drain, and the drain blocks
A condensing furnace — the kind vented in white plastic pipe through a side wall rather than up a chimney — produces water as a by-product of its efficiency. That water leaves through a trap and a drain line, and when the line clogs with the usual biological sludge, the water backs up into the pressure switch tubing and the furnace refuses to start.
A furnace that will not start at all, with water visible around its base or in the trap, is very often this. It looks alarming and it is one of the cheaper faults there is.
Read the light before you reset anything
There is an LED on the control board, visible through the small window in the front panel or once the door is off. It flashes a code — count the flashes and the pause. The legend is printed on the inside of the furnace door.
Write the code down first. Cutting power clears the lockout and the code with it, and that code is the difference between a technician diagnosing in ten minutes and diagnosing in an hour. One reset is reasonable; a second reset on the same fault is asking for a bigger repair.
Stop and call immediately if
- You smell gas. Leave the building first, then call the gas company from outside.
- The carbon monoxide alarm has sounded, at any point, for any duration.
- There is soot around the furnace, or the flame is yellow and lazy rather than blue and crisp.
- The furnace runs but nothing comes out of any register — that is a blower or a control fault, not something to poke at.
Every home with a fuel-burning appliance should have a working carbon monoxide alarm on each level. If yours is older than ten years, the sensor is past its service life regardless of whether it still chirps.
The visit that makes this article unnecessary
A pre-season service is: burners and flame sensor cleaned, ignition and safeties tested, drain and trap cleared, blower checked, gas pressure verified, filter changed. It takes under an hour in September and it removes most of the reasons a furnace fails in January — when the same call costs more and takes longer to get.
We handle heating repair and maintenance across Greater Boston, and no-heat calls get priority in cold weather. Call (617) 404-9944 with the flash code if you have it. If the furnace is old enough that this is an annual conversation, the replacement options are worth reading before the next failure rather than during it.
