When a furnace stops heating, one link in its start-up chain has usually broken. On a typical gas furnace the thermostat calls for heat, the small draft inducer fan starts, a pressure switch confirms the vent is clear, the igniter glows or sparks, the gas valve opens, a flame sensor proves the burners lit, and only then does the main blower start pushing warm air through the house. Knowing roughly where the sequence stops tells you, and the technician, where to look.
Safe checks first
- Thermostat. Set it to Heat, a few degrees above room temperature. Replace the batteries if the screen is faint or blank. If the thermostat itself seems dead, see thermostat not working.
- Power. Check the furnace breaker and the service switch near the furnace. It looks like an ordinary light switch and is easy to flip off by accident.
- Blower door. The door covering the blower has a safety switch. If it was taken off to change the filter and is not seated firmly, the furnace will not run.
- Filter. A clogged filter makes the furnace overheat, and a limit switch then shuts the burners off to protect it.
- Gas supply. If your range or water heater uses gas, see whether they work. A closed valve or an interrupted supply stops everything.
- Water at the base. High-efficiency furnaces with white plastic vent pipes make water as they run. A blocked drain line or trap can stop the furnace; pooling water at its base is the clue.
- Pilot light on older furnaces. If a standing pilot is out and you smell no gas, follow the lighting instructions on the furnace label. If it will not stay lit, stop there and call.
Where the start-up stops, and what that usually means
| What you notice | Likely cause | What it usually takes |
|---|---|---|
| Nothing happens at all | No power, a blown control fuse, thermostat or wiring fault, door switch | Electrical checks, often a quick repair |
| Small fan runs, nothing lights | Pressure switch, blocked vent or condensate drain, inducer motor | Clearing the drain or vent, switch or inducer replacement |
| Igniter glows, burners never light | Gas supply, gas valve or a weak igniter | Testing gas pressure and the valve |
| Burners light, then go out within seconds | Dirty or failed flame sensor | Cleaning or replacing the sensor |
| Burners run but the blower never starts, then they shut off | Blower motor, its capacitor or the control board | Motor and board testing |
| Blower runs, air stays cold | Fan set to On, overheating, or a lockout after failed tries | Settings and filter, then testing |
After several failed attempts many furnaces lock themselves out for a while. Switching the power off and back on once can reset it, but if it locks out again the cause is still there and needs a technician.
Leave the house and call when
- You smell gas, the sharp rotten-egg odor. Don’t touch switches or the thermostat. Get everyone outside, then call your gas utility or 911.
- A carbon monoxide alarm sounds. Move everyone into fresh air and call 911.
Turn the furnace off and call when
- A burning smell lasts beyond the first few minutes of the season, or smells like hot plastic or wiring.
- The furnace breaker trips again after one reset.
- You see scorch marks or soot on the front of the furnace, or flames reaching outside the burner area.
- You hear a loud bang or boom as the burners light.
What a technician does
The technician reads any fault code, then watches a full start-up and measures at the step where it fails: voltage at the control board and thermostat terminals, the inducer and pressure switch, igniter resistance, gas pressure and the flame sensor’s signal. A dirty flame sensor or a cracked igniter is often handled on the first visit. Every heating call should also include the safety checks: venting, flame quality and, where it can be seen, the heat exchanger. Repairs use factory certified parts and carry our labor warranty, and you approve an itemized price before work begins. The work itself is described on furnace repair; if your heat comes from something other than a gas furnace, start with heating repair or heat pump services.
Older Marin homes
Many older houses in San Rafael and San Anselmo still heat with an older forced-air or gravity furnace, or with a wall furnace in a hallway. A wall furnace that will not light is often a pilot or thermocouple problem. A very old furnace with a cracked heat exchanger is a replacement conversation rather than a repair, and furnace installation covers that side. Because the heat runs here for much of the year, a fall check under HVAC maintenance is the simplest way to avoid a cold-morning call, and our winter heating tips cover what you can do between visits.