01 of 06
The burner in your gas furnace is ignited either by an electronic ignition, as is found in most modern furnaces, or with a standing pilot flame, which common in older style furnaces less than 80% AFUE.
The gas furnace standing pilot (where the flame is lit at all times) is sometimes referred to as a pilot light, but no matter what you call it, its purpose is to serve as a small ignition flame for the gas burner. And when this little flame fails to operate properly or goes out, it's one of the most common reasons a gas furnace fails to operate.
So as they say, good things come in small packages, and the furnace pilot is no exception. This standing pilot flame (and its friend, the thermocouple) makes or breaks your furnace's operation, so it's worth spending some time to learn how it works and how to troubleshoot. Understand the furnace pilot is a crucial part of troubleshooting your gas furnace. The following tutorial will explain how to light an older-style gas standing pilot.Continue to 2 of 6 below.
02 of 06
How the Thermocouple and Standing Pilot Work
The thermocouple is an electronic device that senses if the pilot flame is hot enough to ignite natural gas or propane fuel to the burner. If the thermocouple thinks it's safe, then it keeps opening the main gas valve located in the pilot assembly. If the thermocouple or flame sensor does not sense enough heat from the pilot flame (such as when the pilot is out), then the thermocouple shuts off the gas valve to the burners.
How the Thermocouple Works
The thermocouple (technically called a thermocouple junction) is a device that contains two metal wires welded at the ends and placed inside a protective metal case. The thermocouple sensor is found at the business end of the pilot flame and is designed to be placed in the hottest part of the flame. The other end is connected to the pilot valve body.
As the thermocouple heats up, it produces a small amount of electricity, and when it gets hot enough from the pilot flame, it sends a signal to open the gas valve by using a solenoid operated by a 24-volt transformer. The thermocouple calls the shots, and by converting heat to an electrical signal, it allows the gas valve to open or close.
Once the gas valve is open, gas is then constantly supplied to the pilot and, as required, to the gas burners (as called for by the thermostat). If the pilot goes out, then the thermocouple gets cold and produces no electric signal to open the gas valve's solenoid, and the gas valve shuts off the gas supply to the pilot and burners.Continue to 3 of 6 below.
03 of 06
Preparing to Relight the Pilot
A pilot going out is a very common problem with older furnaces, and is even more frustrating when you don't know how to relight it. It is quite easy, though.
The steps involved can vary slightly depending on your furnace model and pilot valve type, so if possible try and find the instructions adhered inside the furnace door cover, or in the furnace's instruction manual.
Two of the most common types of pilot valve body assemblies will have either a red reset button and a gas valve, or no reset button and a valve knob that can be depressed. But whichever type you've got, if you have an older model furnace with a standing pilot (flame is lit all the time) then this is the basic procedure:
Continue to 4 of 6 below.
- Turn your thermostat to 80 degrees, or to a setting that will demand heat. Make sure the thermostat is in "Heat" mode.
- Go to the furnace and find the pilot valve. The pilot valve body is a box-shaped device into which the main gas line will run; it is located near the gas burners. It will usually have a gas cock or valve knob that reads "On, "Pilot" and "Off." Locate this gas cock or knob.
- Turn the knob or gas cock to the "Off" position and wait about three minutes for any residual gas to clear away.
- Find the pilot. It's located near the gas burner tube assembly in the furnace.
- Get your match or lighter ready. Sometimes the pilot is hard to reach. If possible, try and use a long fireplace match when you light it. If you don't have a fireplace match, then a butane BBQ grill lighter works well too. If you don't have that, then you can fasten a match to the end of a stick when you light the pilot.
04 of 06
Relighting the Pilot
To light the pilot proceeds as follows:
Continue to 5 of 6 below.
If you have a pilot with a red Reset button, turn the gas valve from "Off" to "Pilot." If you have a pilot with no red button, turn the knob from "Off" to "Pilot."
- Place the lit match or ignited lighter tip at the pilot, while depressing and holding the button or depressing the knob as appropriate. Depress the button or knob for about 30 seconds. This maintains gas flow to the pilot until the thermocouple gets hot enough to open the main gas valve.
- Once the pilot stays lit, slowly release the button or knob, and then turn the gas cock or knob from the "Pilot" position to the "On" position. This will ignite the burners and keep the flow of gas supplied as required for the burners when called for by the thermostat.
- If the burners fail to ignite, then it may be because the thermocouple did not get hot enough to open the gas valve. Wait a few minutes and then repeat the above procedure. This time hold the red reset button or depress the knob for about 45 to 60 seconds.
- Once the main furnace burners ignite, adjust the thermostat to the desired setting.
05 of 06
If the Pilot Won't Stay Lit
If you've followed the previous steps on lighting the pilot and it still does not light, or it will not stay lit, then you probably have a problem with the thermocouple, or an adjustment needs to be made to the pilot. You may find that the pilot lights, but that it is an anemic looking flame. That's a different problem we'll look at in the next section.Continue to 6 of 6 below.
06 of 06
Pilot Has a Weak or Irregular Flame
If the pilot lights but the flame is a weak yellow flame, it will not get hot enough to heat the thermocouple to its set point allowing the gas valve to open.
A natural gas flame should be a bright blue with the tip of the flame having just a tinge of yellow. A propane flame should have a bluish green flame with a tinge of yellow at the tip. The flame should be strong enough to hit the thermocouple tip about 1/2 inch from the tip end. If the flame is weak or shaky looking, check to see that a breeze or draft is not blowing on it.
- Adjusting the Flame: There is usually a small screw on the pilot valve body that will adjust the flame. You may have to refer to the manufacturer's instructions to find the screw. Turn the screw as needed to adjust the flame throw.
- Yellow Flame: A yellow flame is caused by lack of air and incomplete combustion. It can be caused by a dirty pilot tube tip.
- Split Flame: This is caused by dirt in the pilot tube. Take a needle or small nail and gently clean the tube.
- Flickering or Wavering Flame: A flame that flickers is usually caused by a draft.