The short answer
A furnace that will not heat usually comes down to the thermostat, airflow, ignition or a safety switch. A gas furnace that clicks and never lights is often the ignition or the flame sensor. Most of these are repairs.
What you can check yourself
- Set the thermostat to Heat, with the set point above the room temperature.
- Check the breaker and the service switch at the furnace.
- Check the filter and open the vents.
- If you smell gas, leave the house and call the gas utility from outside.
What gets checked first
- Thermostat settings and wiring
- Filter, blower and return
- Ignition and flame sensor
- Limit and pressure switches
- The control board
Repair or replace?
Most no-heat calls are a repair.
When to call now
A gas smell or a carbon monoxide alarm means everyone outside first, then the gas utility and 911.