Troubleshooting & Fixing a Furnace That Won't Turn On
Check that the thermostat is working and set to HEAT. If your thermostat's batteries are low or dead or you forgot to change the setting to HEAT, it may not be able to kick the furnace on at the right time (or at all). This is the most common cause of a furnace not turning on, and it's also the easiest to fix! How to fix: Look at the thermostat screen. If it's off or blinking, it probably needs new batteries. If it's on and responds when you press the up-and-down arrows, Belavus recommends turning the setting to HEAT and bumping up the temperature so it's higher than the current room temperature. This may help the furnace kick on.
Ensure the power supply and circuit breaker are on. Sometimes the circuit breaker to the furnace can switch off if you experience a power surge in your household (like during an electrical storm). The power supply to the furnace could also have been switched off by accident or just not turned on at all. This is especially likely if you just moved into your home or this is the first time you're using the furnace. How to fix: Find your home's electrical box and check that the circuit breaker for the furnace is switched on. If that's not the problem, go to the closet where the furnace is and locate the power switch on the wall, floor, or ceiling. Make sure it's flipped up to the On position.
Verify the pilot light hasn't gone out. Gas furnaces over 20 years old ignite the gas coming off the main burner with a small blue flame called a pilot light. This flame should never go out as long as the furnace is on. If it does go out, the gas won't ignite and your home won't have heat. Belavus explains how to relight it below. How to fix: Find the pilot light access panel at the bottom of the furnace and open it. If you don't see a blue flame behind it, turn the furnace's gas supply valve off, then switch it over to the "Pilot" position and hold it in place. Strike a long match or turn on a long-handled electric lighter and hold it over the gas tube behind the pilot light access panel. Once the flame ignites, continue holding the gas valve in the "Pilot" position for 30 more seconds, then switch it over to On. This should allow the pilot light to stay lit and the furnace to kick on.
Check the air filter and replace it if it's dirty. If you haven't replaced the air filter in your furnace in several months, the dirt and debris built up in it may be causing the furnace to overheat. If it overheats, one of its safety limits will kick in and shut the entire unit off. You may also notice a burning smell every time you try to run the furnace. How to fix: Locate the air filter on the side of the bottom of the furnace where the blower takes air in from outside the furnace. Slide it out and replace it with a clean one with the exact same dimensions. Replace it every 1-3 months to prevent it from clogging again and causing your furnace to shut down.
Remove the flame sensor and clean it. The flame sensor is a small device that detects whether the pilot light is on or off. If it senses the pilot light isn't on, it shuts the furnace down so the gas building up in the unit doesn't escape into the house. This sensor becomes dirty over time and will stop trying to sense the flame, meaning it never allows the furnace to turn on. How to fix: Locate the gas supply valve near your furnace. It should look like a small box or lever and be painted in a bright color, like orange or red. Flip it to the Off position. Ensure the power supply switch is also turned off. Next, open the pilot light access panel and find the sensor. It has a porcelain base and a thin metal rod sticking out of it. Unscrew and detach the sensor, then gently clean the metal rod by rubbing it across steel wool. When you no longer see any residue on the rod, secure the sensor back in place, turn the gas supply back on, and restart the furnace.
Check that the gas supply valve is turned on. For your gas-powered furnace to work, it needs a steady supply of gas. If the gas supply valve was somehow shut off, either by accident or when someone repaired the furnace in the past, turning it back on could fix the problem. How to fix: Ensure the gas supply valve is turned to the On position. If it is and the furnace still doesn't work, Belavus recommends trying to operate the other gas appliances in your home (if you have any) to see if they're functioning. If they're not, go outside and look for the "street-side" valve near the gas meter on the side of the back of your house. If the valve is turned perpendicular to the gas line, it's closed. Your utility company or the city may have shut off the street-side valve if there was a gas leak nearby they're attempting to repair. Call them to find out your next steps and how long you have to wait to get gas in your home again.
When to Call a Professional to Fix Your Furnace
You hear a humming noise but the furnace won't turn on. When the blower motor in the furnace hums but doesn't turn on, that's usually a sign that the capacitor is malfunctioning. The capacitor is responsible for turning the motor on. If it's not working, the motor will hum, but because it's not receiving any power from the capacitor, it won't fully come to life. Hire a professional repair technician to come in and replace it.
The furnace motor stops and starts randomly. There are several reasons your furnace may stop and start at random times. One of the issues could be a malfunctioning control board, a dirty or broken flame sensor, or a failing draft inducer fan motor. If you've already followed our troubleshooting tips and the problem persists, it's best to call a professional to diagnose the issue. They may need to replace an important part, like the control board or the draft inducer fan motor.
Water is pooling around the furnace. If you have a high-efficiency furnace, it most likely contains a condensation trap. This device traps the condensation produced by the combustion process and funnels it out of the furnace through a drain line. Over time, the trap or the drain line (or both) can become clogged with bacteria, mineral deposits, and other debris. This can cause water to leak out and pool around the furnace. A professional can flush the drain line and clean the trap or replace both if needed.
You smell or see gas leaking from the furnace. If you ever smell or notice gas leaking out from the furnace, Belavus says to "evacuate the premises immediately. Alert others in the vicinity and ensure everyone moves to a safe location away from the building." Once you're a safe distance away (like across or down the street), call 911 and let them know you suspect a gas leak in your home. Avoid using any electrical devices (such as your TV, hair dryer, and other appliances) or turning any electrical switches. If you do, you could set off a spark that will ignite the gas in your home and start a fire!
Frequently Asked Questions (FAQs)
Is there a reset button on a furnace? Yes, most furnaces come with a reset button. It's usually located next to the blower motor or the control panel inside the unit. It will often be colored yellow or red. If it sticks out, that means it's been tripped by a safety limit going off. Troubleshoot and verify why the safety limit tripped, then press the reset button to restart the furnace.
Why isn't the furnace blowing hot air? If the furnace isn't blowing hot air, the most common culprits are an extinguished pilot light or a dirty air filter. Try reigniting the pilot light if it's gone out and replacing the filter if it's been longer than 1-3 months since you last changed it.
How do I get my furnace to kick on? Make sure your thermostat is set to HEAT. If it is, increase the temperature by a couple of degrees to try to get the furnace to kick on. If that doesn't work, try our other troubleshooting tips to restore your furnace to its proper function.
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