First: is the fan simply set to ON?
This is the most common non-problem. A thermostat fan setting of ON runs the blower continuously, even between heating cycles, so you feel room-temperature air at the registers most of the time. Switch the fan setting to AUTO and the blower will only run when the furnace is actually producing heat.
If the fan is on AUTO, look at the pattern
| Pattern | Likely cause |
|---|---|
| Warm at first, then turns cold mid-cycle | Burners shutting off early — flame sensor, limit trip from restricted airflow, or overheating |
| Cold from the start, every cycle | Burners never lighting — ignitor, gas supply, pressure switch, or gas valve |
| Brief cool air at the start and end of a cycle | Normal. Blower runs slightly before and after the burners to use residual heat |
| Cold only in certain rooms | Duct issue rather than furnace issue — see heating repair |
| Cold air with a loud blower | Possible limit trip with the blower running to cool the heat exchanger |
Safe checks
- Set the thermostat fan to AUTO and the mode to HEAT.
- Replace a visibly dirty filter with the correct size.
- Open closed supply registers and clear anything blocking return grilles.
- On high-efficiency furnaces, clear snow and ice from the outdoor PVC intake and exhaust.
- Note whether the burners light at all — a blue flame is visible through the sight glass on many models.
- Write down any LED flash code before calling.
Where safe checks end
Never bypass a safety switch, open or adjust a gas valve, disassemble burners or the heat exchanger, jump control board terminals, or work on live wiring. If the safe checks below do not restore heat, stop and call 716-412-2677.
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The flame sensor pattern
If the burners light, run for roughly three to seven seconds, then shut off — repeating a few times before the furnace gives up — that is the signature of a flame-proving failure. The flame sensor is a thin metal rod in the flame path that carries a very small electrical current when flame is present. When it oxidizes, the control board concludes there is no flame and closes the gas valve as a safety measure. It is a common, straightforward repair, and it is measurable rather than a guess.
Why cold air costs more in Western New York
A furnace that keeps cycling without delivering heat still runs its inducer and blower, drawing electricity and moving cold air through the house. During a sustained cold stretch, a home losing ground hour after hour also gets harder to bring back up once the furnace is repaired. Addressing it the same day rather than riding it out for a week is both cheaper and more comfortable.