When a furnace runs but no heat comes out, the blower is moving air while the burners are not lit, not staying lit, or being shut off early. The usual causes are the thermostat fan set to On, a clogged filter that makes the furnace overheat, a failed igniter or dirty flame sensor, a blocked vent or condensate drain, and a gas supply problem. If nothing runs at all, look at power, the furnace switch and the thermostat first.
Safety first. If you smell gas (a rotten-egg smell), don’t touch the thermostat or any switch. Leave the house and call the gas utility or 911 once you are outside. If a carbon monoxide alarm sounds, get everyone out and call 911.
Which pattern matches yours?
- The blower runs, the air is cool. The burners are not lit. Start with the fan setting and the filter.
- Warm for a few minutes, then cool. The burners lit, then shut down: usually overheating or the flame sensor.
- Nothing at all. No fan, no sound: power, a switch, the thermostat or the control board.
How a gas furnace starts, and where it stops
Most furnaces made in the last few decades run the same sequence every time the thermostat calls for heat:
- The control board gets the call and checks its safety switches.
- The draft inducer, a small fan, starts pulling air through the heat exchanger and out the vent.
- A pressure switch confirms the vent is drawing.
- The igniter glows or sparks.
- The gas valve opens and the burners light.
- The flame sensor has a few seconds to prove there is a flame. If it can’t, the valve closes and the board tries again; after several failed tries it locks out.
- After a short delay the main blower starts and warm air reaches the rooms.
Watch and listen at the furnace: the step where it stops points to the part that failed. Older furnaces with a standing pilot work differently; see furnace pilot light won’t stay lit.
Symptom, likely cause, what it takes
| What you see | Likely cause | What it usually takes |
|---|---|---|
| Blower runs nonstop, air never warms | Thermostat fan on On, or mode not on Heat | Fan to Auto, mode to Heat |
| Inducer runs, no glow, no flame | Pressure switch open: blocked vent, clogged condensate drain, weak inducer | Vent or drain clearing; switch and inducer test |
| Igniter never glows | Cracked or burned-out igniter | Igniter replacement |
| Igniter glows, burners don’t light | Gas valve closed or failed, or no gas supply | Supply check; gas valve test |
| Burners light, go out after a few seconds, repeat | Dirty or failing flame sensor | Sensor cleaning or replacement |
| Burners light, little or no air from the vents | Failed blower motor or capacitor | Capacitor or motor replacement |
| Warm for a few minutes, then cool, then again | High-limit switch tripping: dirty filter, blocked return, weak blower | Airflow correction; limit and blower test |
| Nothing runs, no light on the board | Breaker or furnace switch off, door switch open, blown board fuse, failed transformer | Electrical diagnosis |
| Warm at the furnace, cool at some registers | Disconnected or crushed duct | Ductwork repair |
Read the error code before you reset
Most modern furnaces have a small light on the control board, visible through a window in the lower door. It blinks a pattern, and a label inside the door says what each pattern means. Count the flashes and write them down before you switch anything off, because cutting power usually clears the code.
Checks you can safely make
- Thermostat: Heat mode, set a few degrees above room temperature, fan on Auto. Replace the batteries if the screen is dim or blank.
- Filter: if it is grey and caked, replace it.
- Power: check the breaker and the furnace’s service switch, which looks like a light switch on or near the furnace and is easy to flip by mistake.
- Lower door: the furnace won’t run unless the blower door is fully closed; a safety switch sits behind it.
- Gas valve: on the pipe beside the furnace, a handle in line with the pipe is open. If it is closed and you don’t know why, leave it closed and call. If the range or water heater has no gas either, the problem is the supply; call the gas utility.
- Vent pipes: on a high-efficiency furnace with white plastic pipes, check that the outdoor pipe ends are clear of leaves, plants and nests.
- One reset: turn the service switch off for 30 seconds and back on. If the furnace locks out again, stop: repeated resets don’t fix a failed part.
Never bypass or jump a safety switch, and leave the burner cover on.
When it needs a technician
- Any gas smell (leave first, then call from outside).
- A code points to a part, or there is no code and still no heat.
- Ignition attempts fail again after one reset.
- A safety switch keeps tripping.
- Soot, a yellow flame or a CO alarm, which raise a heat exchanger concern.
A technician walks the same start-up sequence with meters (flame sensor signal, gas pressure, pressure switch, temperature rise, airflow) and checks the vent and heat exchanger before the furnace goes back into service. You get a written estimate before any furnace repair begins.
Repair or replace?
Most no-heat calls end with one part: an igniter, a flame sensor, a pressure switch, a capacitor, sometimes a control board. Replacement becomes a real question when the heat exchanger is cracked, or when an older furnace needs a major part on top of repeated repairs. We lay out both options and you decide; the repair-or-replace guide shows how to weigh them.
Local note
Daly City sits in the fog belt, where heating is needed in most months, so furnaces run many hours and filters load faster. Many sit in ground-floor garages: keep boxes away from the cabinet and the return grille, and keep paint, solvents and gasoline well away from the burners. If your home heats with a heat pump instead, see heat pump not heating.