What to Do If Your Furnace Won't Kick On, According to an Expert

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Troubleshoot & repair your furnace with advice from an HVAC expert
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If your furnace won't turn on, we can help you identify common causes and fix them yourself. Read on to learn how to troubleshoot and fix your furnace and when to call a professional HVAC technician to repair more serious issues. We'll also share furnace repair and safety tips from experienced HVAC expert, Victor Belavus.

Why won't my furnace turn on?

Common reasons your furnace won't turn on are a faulty thermostat, a tripped circuit breaker, a dirty air filter, or a switched-off gas supply or furnace unit. You can usually fix these issues yourself, but call a professional for more serious problems like a broken capacitor, water leak, or gas leak.

Section 1 of 3:

Troubleshooting a Furnace That Won't Turn On

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  1. 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 display. If it's blinking, faded, or completely off, 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 increasing the temperature a few degrees so it's higher than the current room temperature. This may help the furnace kick on.
  2. 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. [1]
    • 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.
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  3. 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.
    • How to fix : To relight the pilot light, Belavus says to 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.
  4. 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. [2]
    • How to fix : Locate the air filter on the side 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.
    • If you replace the air filter and it still smells like something is burning whenever the furnace blows, call an HVAC repair technician to have a look at the air ducts in your home. They may be clogged with dirt and lint, in which case you'll need someone with experience to clean them.
  5. For a 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. [3]
    • 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.
  6. 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. [4]
    • 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 and power back on, and restart the furnace.
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Section 2 of 3:

When to Call a Professional to Fix Your Furnace

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  1. 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. [5] Hire a professional repair technician to come in and replace it.
  2. If your furnace is part of an HVAC system, it most likely contains a condensation drain pan. The pan collects condensation and funnels it down a drain line so the moisture doesn't ruin the HVAC unit. Over time, the drain line can clog with bacteria, mineral deposits, and other debris, causing the drain pan to pool with water. When this happens, a device called a safety float switch shuts the furnace off to prevent the water from overflowing. [6]
    • A professional can flush the drain line and clean the pan or replace both if needed.
  3. 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 could ignite the gas in your home and start a fire!
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Section 3 of 3:

Frequently Asked Questions (FAQs)

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  1. 1
    Is there a reset button on a furnace? Yes, you can manually reset your furnace by flipping the switch on the side of the furnace or a nearby wall or floorboard. To reset your furnace, flip the switch off, count for 3 seconds, then turn it on again. This will reboot the control board inside the furnace. [7]
  2. 2
    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. [8]
  3. 3
    How do I get my furnace to kick on? Make sure your thermostat is set to HEAT. If it is, Belavus suggests increasing the temperature 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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      References

      1. ↑ https://www.bobvila.com/articles/furnace-not-turning-on/
      2. ↑ https://www.bobvila.com/articles/furnace-not-turning-on/
      3. ↑ https://www.bobvila.com/articles/furnace-not-turning-on/
      4. ↑ https://www.bobvila.com/articles/furnace-not-turning-on/
      5. ↑ https://youtu.be/lG1sIuQUXmY?si=hLG-IXAnDrOZ2vwi&t=592
      6. ↑ https://blog.diversitech.com/what-is-a-condensate-switch-and-how-does-it-work
      7. ↑ https://youtu.be/XHwkKSDkupY?si=583aZ__dIy0bALpl&t=80
      8. ↑ https://www.bobvila.com/articles/furnace-not-turning-on/

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