A Few Seconds, Then the Burners Go Out
The furnace lights, runs briefly, and the flames disappear. The usual reason is that the control never proved the flame. A flame sensor is a small metal rod in the flame. It carries a tiny electrical signal. A coated rod, a cracked igniter, or a sensor that does not sit in the flame makes the control shut the gas off.
The furnace may try again, then lock out. A blinking light on the board is a code. The chart is on the blower door. Count the blinks before you cut power, because killing power can clear the code.
Cleaning that sensor means opening the burner area. That is not a filter change. Leave the burners, the gas valve, and the sensor to furnace repair.
Several Minutes, Then the Burners Stop
The burners run long enough to make heat, then stop, while the blower keeps going. That pattern is the limit switch. It opens when the heat exchanger gets too hot, shuts the gas off, and leaves the blower on to carry heat away. When the furnace cools, it may start again.
The usual reason is not enough air across the heat exchanger. A filter you cannot see through, supply registers closed for the winter, or a return grille blocked by stored boxes will do it. Closet furnaces are prone to this because the return often comes through a louvered door.
A thermostat mounted above a supply register, in a sunny spot, or next to a lamp can also end the cycle early. The furnace is obeying a sensor that thinks the room is already warm.
Checks That Stay on the Safe Side
- Set the thermostat to heat, and set the fan to Auto rather than On. On runs the blower with or without the burners.
- Turn the furnace off at its switch before you pull the filter. If you cannot see light through the filter, replace it with the same size.
- Open supply registers. Move boxes, beds, and curtains off return grilles.
- If the furnace lives in a closet, make sure the door louvers are not covered.
- Write down a blinking code before you turn the power off.
Stop there. Do not bypass a limit switch, disconnect a safety, or take the burner cover off to scrub a sensor.
Gas, Carbon Monoxide, and the Reset Button
If you smell gas, or a carbon monoxide alarm is sounding, leave the building. Call the gas utility or emergency services from outside. Do not hunt for the leak, and do not keep starting the furnace.
Use a reset button once. If the burners shut off again, leave the furnace off. Resetting it over and over can push unburned fuel into the burner.
Radiators Are a Different System
If the heat comes from radiators or baseboard and there is no blower, this is not a furnace problem. See a boiler that runs while the radiators stay cold.
Closet and cellar gas furnaces in the Bronx, including houses in Van Nest and Crotona Park East, shut off for the reasons above. The repair page is gas furnace repair. If the filter and the registers were not the cause, call (914) 573-5224.