Why Is My Furnace Not Turning On?

Just as you would if you found your AC blowing hot air, check the thermostat if your furnace isn't coming on when you think it should. Someone may have switched it to the wrong setting, such as cooling instead of heating, and that's an easy fix. You might also discover that the thermostat batteries are dead, which is another easy fix.

If the display on your thermostat is malfunctioning, you may need to replace it. Another common thermostat problem is related to calibration issues — for example, if the thermostat says it's a cozy 74 degrees but it's actually chilly in your house, the gauge needs calibrating. These are repairs you can do yourself if you're comfortable.

Furnaces typically require some electricity to power on correctly. Conduct the following tests to ensure there isn't a problem getting power to the furnace:

• Check the circuit breaker. If it has tripped, switch it back on.
• Check the furnace power switch. Someone or something may have bumped it and turned it off.
• Check for blown fuses or loose connections. Change fuses or shore up connections to ensure your furnace has uninterrupted power.

Gas furnaces require a pilot light and an ignition to power up. If the pilot light goes out for any reason, your furnace won't work properly.

To check the pilot light, first consult the manual for your furnace so you know where the pilot light is located. Typically, it's behind a removable panel, and you can carefully take that off and look for a small blue flame.

You can relight the pilot light as long as everything else is working properly. If the thermocouple or gas valve are bad or there is a clog in the pilot light area, it won't relight.

You'll need to follow the manufacturer's instructions to relight your pilot light, but some basic steps include:

• Turning off the thermostat and the furnace.
• Flipping the breaker to the furnace to ensure no power flows to it.
• Waiting around 10 minutes to ensure all gas in the area has dissipated.
• Locating the knob that controls the pilot light and the end of the gas line that comes from it.
• Holding a small flame to the end of the gas line while pressing the starter button for the pilot light — you may need to set the knob to "pilot" before doing this.
• Releasing the starter button and ensuring the knob is set to "on" and that the flame stays once you remove the source you used to light it.

Newer furnaces have ignition sensors. When these go bad, they can prevent the furnace from starting even if the pilot light is lit.