A furnace that will not turn on leaves you in the cold. Before calling for service, check these common issues that have simple solutions.
Check the Thermostat
Start with the obvious. Is the thermostat set to HEAT with a temperature above current room temperature? Try raising the setpoint 5 degrees above room temperature.
Check thermostat batteries if applicable. Dead batteries are a common cause of no response from the system.
Check Power Supply
Furnace switch: Look for a light switch on or near the furnace. Someone may have accidentally turned it off.
Breaker panel: Check if the furnace breaker has tripped. Reset if needed. If it trips again, there is an electrical problem requiring professional attention.
Blower door safety switch: Most furnaces have a switch that turns off the system when the access panel is removed. Ensure the panel is properly seated.
Check Gas Supply
If you have a gas furnace:
Pilot light (older furnaces): If you have a standing pilot light, check if it is lit. Follow manufacturer instructions to relight if needed.
Gas valve: Ensure the gas valve to the furnace is in the ON position.
Gas supply: Verify other gas appliances work. If nothing works, there may be a gas supply issue.
Check the Filter
A severely clogged filter can cause the furnace to shut down on high limit. Replace the filter and wait 30 minutes before trying again.
Reset the Furnace
Modern furnaces have onboard diagnostics. After addressing obvious issues:
1. Turn the furnace switch off 2. Wait 30 seconds 3. Turn back on 4. Wait for the startup sequence
Look for LED error codes on the control board. Count the flashes and compare to the chart on the furnace door.
When to Call for Help
If the above steps do not resolve the issue, professional diagnosis is needed. Ignition systems, gas valves, control boards, and safety switches require trained technicians.
Never attempt to work on gas components yourself. Gas leaks and improper combustion are serious safety hazards.
If you smell gas, leave the home immediately and call your gas utility from outside.