A furnace that refuses to stay like can be pretty frustrating. After all, the point of putting your furnace on is because it’s cold outside and you need to warm your home.

There are several reasons behind why this could happen, some simple and some more complex.

Keep reading while I go through some causes and fixes. Hopefully one of them will help you.

How the Ignition System on Your Furnace Works

First of all, let’s discuss how the ignition system on your furnace works.

There may be unique features on any given furnace but the following steps will be fairly common to any gas-fueled furnace. Seeing the process might help you understand how and where things can go wrong.

If your furnace doesn’t have a standing pilot light you have a direct ignition furnace. In this case, steps 3 through 5 are not relevant.

If you do have a furnace with a standing pilot light, step 4 is irrelevant. This is typical of older furnaces.

  1. Your thermostat relays a call for heat
  2. A draft inducer fan, which is located near the heat exchanger in your furnace, turns on and provides air circulation in the combustion chamber.
  3. The gas valve opens to provide fuel to the pilot burner.
  4. The resulting spark lights the pilot burner.
  5. A flame sensor confirms the pilot is lit and that fire is present.
  6. The pilot light then lights the main burners.

The following is a quick video showing how direct ignition systems work.

6 Reasons Why a Furnace Won’t Stay Lit

Typically, a furnace should cycle on and off about every 10 to 15 minutes. So if yours keeps shutting off within just a few minutes you could be facing a wide range of problems.

The cause of your furnace not staying lit is related to when the flame goes out. So I’ll break this into two sections.

First, if your furnace receives the call for heat from your thermostat, lights the pilot, but then shuts off without firing up the burners it could be one of the following:

1. Faulty Flame Sensor

If your flame sensor isn’t working correctly, it could be telling your furnace that the gas is lit. Even though the pilot light has gone out.

Furnace flame sensor installed
If your flame sensor is faulty, it will automatically shut down the heating unit if it doesn’t detect a flame.

Solution. If you’re handy, you should be able to diagnose this yourself. Here’s a step-by-step guide.

  1. Turn off the power to your furnace
  2. Shut off the gas valve
  3. Find and remove the flame sensor mounting screw
  4. Remove the flame sensor and inspect it, looking for cracks and or corrosion
  5. If there are any cracks in the porcelain insulation or if pieces are missing, the sensor must be replaced
  6. If the sensor simply has some corrosion or soot on it, try cleaning it and see if that solves your issue

Check out the following video on how to remove and clean your flame sensor.

2. Flame Sensor Rod Needs to Be Repositioned

The sensor is simply a thin metal rod. Some of them are L-shaped and some of them are straight and they typically range from 2 to 4 inches long. Regardless, they need to be positioned in the flame to work properly.

Solution. Reposition the flame sensor rod so that it will be covered in flame.

3. Faulty Ignition Control

This is a small electronic board that controls the ignition sequence. However, in some cases, it might stop working.

Solution. If you have a faulty ignitionboard you’ll need to have your furnace serviced by an HVAC professional.

Gas furnace ignition control
The furnace ignition control module provides power to the furnace blower, and is more reliable than the standing pilot light that is found in older gas furnace models.

4. Low Gas Pressure

If your furnace is lacking the required gas pressure its efficiency will drop. And it may be so bad that there’s not enough gas to light your pilot or keep it lit.

Solution. There will be a gas pressure adjustment screw located on the line close to your furnace. Adjust the screw clockwise to increase gas pressure or counterclockwise to decrease the pressure.If you have a two-stage furnace be sure to make adjustments to both low fire and high fire settings.

The second set of circumstances is when your burner does ignite but then goes out within a few minutes. Although your blower motor continues to run your pilot and burners are no longer lit.

5.The Furnace Is Overheating

Yes, your furnace should be hot but there are temperatures that it must remain in.Your furnace has a limit switch that will shut it down if the heat exchanger becomes too hot. This can happen due to dirty filters or blocked supply and return vents. Restricted airflow is often the number one cause for a furnace to overheat.

Solution. Change or clean your filter and make sure there’s sufficient airflow through your ductwork. Having said that, if you do expect your furnace is overheating you do need to call a professional.

6. Faulty Limit Switch

Your furnace may not be overheating by your limit switch which may be damaged or faulty. Oftentimes, if the problem is with your limit switch you may be encountering problems with your blower motor and not shutting off as well.

If you’re dealing with a furnace that won’t stay lit and a blower motor that won’t turn off, it’s almost certainly a problem with your limit switch.

Solution. Limit switches are inexpensive and fairly easy to replace. You can follow the tutorial in the video below.

Conclusion

If your furnace lights the pilot but then shuts off without firing up the burners, you may be facing one of the following:

  • Faulty flame sensor
  • Flame sensor rod that needs repositioning
  • Faulty ignition control
  • Low gas pressure

However, if your pilot and burners do light but then shut off within a few minutes, you may be facing these issues:

  • Your furnace is overheating
  • You have a faulty limit switch

In many of these cases, it’s something you can fix yourself, however, as indicated, there may be some circumstances where calling anHVAC professional is advised.

Hopefully, this information has helped you in finding and resolving the problem with your furnace.

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