The burner in your gas furnace is ignited either by an electronic ignition, found in most modern furnaces, or with a standing pilot flame, which is common in older style furnaces that have an annual fuel utilization efficiency (AFUE) rating of less than 80 percent. A gas furnace's standing pilot, in which the flame is lit at all times, is sometimes referred to as a pilot light.
No matter what you call it, the purpose is to serve as a small ignition flame for the gas burner. When the thermostat signals for gas to be delivered from the gas valve to the burner, the standing pilot ignites the gas flowing to the burners to heat the air in the combustion chamber. When this little flame fails to operate properly or goes out, it's one of the most common reasons that a gas furnace fails to operate.
Because 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. Understanding the furnace pilot is a crucial part of troubleshooting your gas furnace.
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 flowing to the burner. If the thermocouple thinks it's safe, then it allows the main gas valve in the pilot assembly to open up and stay open. If the thermocouple or flame sensor does not sense enough heat from the pilot flame—such as when the pilot is extinguished—then the thermocouple shuts off the gas valve delivering gas to the burners.
The thermocouple—which is 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. When it gets hot enough from the heat of 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.
Once the valve is open, gas is constantly supplied to the pilot and to the gas burners, as called for by the thermostat. If the pilot goes out, then the thermocouple gets cold and doesn't produce an electric signal to open the gas valve's solenoid, so the gas valve shuts off the gas supply to the pilot and burners.
Equipment / Tools
- Long match or wand lighter
How to Relight the Standing Pilot Light
A pilot going out is a common problem with older furnaces, but it's even more frustrating when you don't know how to relight it. The process is quite easy, but the steps involved can vary slightly depending on your furnace model and pilot valve type. If possible, 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 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:
Turn off the Gas
Turn your thermostat to 80 degrees or to a setting that will demand heat. Make sure the thermostat is set to the "heat" mode. Go to the furnace, and find the pilot valve, a box-shaped device into which the main gas line will run. It is located near the gas burners and will usually have a gas cock or valve knob that reads "on," "pilot," and "off." 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 Tube
Find the pilot tube and nozzle located near the gas burner tube assembly inside 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 barbecue grill lighter works well, too. If you don't have that, fasten a match to the end of a stick as you light the pilot.
Set Gas Valve to "Pilot"
If you have a pilot with a red reset button, turn the gas valve from "off" to "pilot." If you have a pilot without a red button, turn the control knob from "off" to "pilot."
Light the Standing Pilot
Place the lit match or ignited lighter tip at the pilot while depressing and holding the reset 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.
Set Gas Valve to "on"
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.
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, 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.
Troubleshooting Pilot Flame Problems
If you've followed the previous steps for lighting the pilot and it still does not light or won't stay lit, then you probably have a problem with the thermocouple or an adjustment needs to be made to the pilot. You may also find that the pilot ignites, but that it is an anemic-looking flame. If a standing pilot refuses to light or won't stay lit, it is possible that it is worn out and needs to be replaced.
If the pilot lights but the flame is weak and yellow, it will not get hot enough to heat the thermocouple to its set point and allow the gas valve to open. If your furnace runs on natural gas, the flame should be a bright blue with the tip of the flame having just a tinge of yellow. If your furnace runs on propane, the 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, which can be corrected by cleaning it.
- Split flame: This is usually 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. Look for sources of draftiness in the room, and make sure the cover on the combustion chamber is properly fitted.
Understanding the Efficiency Rating of Furnaces and Boilers. Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy.