Most gas furnaces contained a small standing pilot light that burned continuously. If your furnace is of a vintage that includes a pilot light, it should be kept clean and properly adjusted to help ensure energy efficiency and extend the life of the furnace. Heres is how to light pilot light on furnace for best furnace performance.
Standing pilot basics
The burner in your gas furnace is ignited either by an electronic ignition , as found in most modern furnaces, or by a standing pilot flame, which is common in older-style furnaces that have an AFUE (annual fuel utilization) rating of 80 percent.
A gas furnace standing pilot (where the flame is lit all the time) is sometimes called a pilot light, but whatever you call it, its purpose is to serve as a small ignition flame for the gas burner. When the thermostat signals that gas is to be delivered from the gas valve to the burner, it is the standing pilot that ignites the gas to heat the air in the combustion chamber. And when this little flame doesn’t work properly or goes out, it’s one of the most common reasons why a gas furnace doesn’t work.
As they say, good things come in small packages, and the oven pilot is no exception. This standing pilot flame (and its friend, the thermocouple) makes or breaks your oven, so it’s worth spending some time learning how it works. Understanding the furnace pilot is an important 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 the natural gas or propane fuel flowing to the burner. If the thermocouple thinks it is safe, it allows the main gas valve in the pilot assembly to open and stay open. If the thermocouple or flame sensor does not detect enough heat from the pilot flame (e.g. when the pilot is off), the thermocouple shuts off the gas valve to the burners.
How the thermocouple works
The thermocouple (technically called a thermocouple junction) is a device containing two metal wires welded at the ends and placed in a protective metal case. The thermocouple sensor is located at the 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 housing.
As the thermocouple heats up, it produces a small amount of electricity, and when it gets hot enough from the heat from the pilot flame, it sends a signal to open the gas valve using a solenoid operated by a 24-volt transformer. The thermocouple calls the shots, and by converting heat into an electrical signal, it allows the gas valve to open.
When the gas valve is open, gas is then continuously supplied to the pilot and to the gas burners as required by the thermostat. If the pilot goes out, the thermocouple becomes cold and produces no electrical signal to open the gas valve solenoid. The gas valve stops the gas supply to the pilot and also burners.
Preparing to turn the pilot back on
A pilot going out is a very common problem with older furnaces and is even more frustrating when you don’t know how to turn it back on. It’s pretty easy though. The steps involved may vary slightly depending on your furnace model and pilot valve type, so find the instructions taped to the inside of the furnace cover or in the furnace’s owner’s manual if possible.
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 button that can be depressed. But no matter what type you have, if you have an older model furnace with a standing pilot (flame on all the time), this is the basic procedure:
- Turn your thermostat to 80 degrees or to a setting that requires heat. Be sure to set the thermostat to HEAT mode.
- Go to the furnace and find the pilot valve. The pilot valve housing is a box-shaped unit where the main gas line will run; it is located near gas burners. It will usually have a throttle or valve knob that reads ON, PILOT and OFF. Find this gas valve or knob.
- Turn the dial or throttle to the OFF position and wait approximately three minutes for any remaining gas to dissipate.
- Locate the pilot tube and nozzle. It is located close to the gas burner tube assembly inside the oven.
- Get your match or lighter ready. Sometimes it is difficult to reach the pilot. If possible, try to use a long fireplace when lighting it. If you don’t have a fire match, a butane barbecue grill works well too. If you don’t have it, you can attach a match to the end of a stick when you light the pilot.
Turns the pilot back on
To turn on the pilot, proceed as follows:
- If you have a pilot with a red Reset button, turn the gas valve from OFF to PILOT. If you don’t have a red button pilot, just turn the control knob to PILOT.
- Place the lit match or ignited spark plug tip at the pilot while holding down the reset button or holding down the button, as appropriate. Press the button or button for approx. 30 seconds. This maintains steady gas flow to the pilot until the thermocouple gets adequate hesat to open the main gas valve.
- With the pilot on, slowly release the knob or knob, then turn the throttle or knob from the PILOT position to the ON position. This ignites the burners and maintains the flow of supplied gas as needed to the burners.
- If the burners do not ignite, it may be because the thermocouple did not get hot enough to open the gas valve. Repeat the above procedure after waiting for a few minutes and then. This time, hold down the red reset button or press the button for approx. 45 to 60 seconds.
- Once the main furnace burners are lit, adjust the thermostat to the desired setting.
If the pilot does not turn on
If you have followed the previous steps to turn on the pilot and it still does not light, or if it does not stay on, you probably have a problem with the thermocouple or the pilot needs adjustment. Or you may find that the pilot lights up but is an anemic flame. This indicates another problem.
If a standing pilot refuses to light or won’t light, it is possible that it is worn and needs to be replaced.
If the pilot has a weak or irregular flame
If the pilot lights but the flame is a faint yellow flame, it will not get hot enough to heat the thermocouple to its set point and allow the gas valve to open.
A natural gas flame should be light blue with the tip of the flame just a tinge of yellow. A propane flame should have a blue-green flame with a hint of yellow at the tip. The flame must be strong enough to hit the thermocouple tip approx. 1.30 cm from the tip end. If the flame is weak or shaky, check for a breeze or draft.
- Adjusting the flame: There is usually a small screw on the pilot valve body that adjusts the flame. You may need to refer to the manufacturer’s instructions to locate the screw. Turn the screw as required to adjust the flame throw.
- Yellow flame: A yellow flame is due to lack of air and incomplete combustion. This can be caused by a dirty pilot tube tip, which can be corrected by cleaning it.
- Split flame: This problem is usually caused by some dirt in the pilot tube. Take a pin or a small nail and gently clean the tube.
- Wavering or flickering flame: A flickering flame is usually caused by a draft. Look for sources of draft in the room and make sure the combustion chamber cover is properly fitted.
When you stand in front of a gas furnace for the first time, you are often unsure how a pilot light works. Turning on a gas furnace is not difficult, nor is it dangerous. But when there’s a problem with the pilot light, you should read in detail in this article how to do it correctly, what to think about and what to avoid if possible.