How to Check a Pilot Light on a Gas Furnace

Before the advent of modern electronic ignition furnaces, most gas furnaces featured a small standing pilot light that burned all the time. In such furnaces, the standing pilot works together as a team with the thermocouple (the electronic device that senses if the pilot flame is hot enough to ignite natural gas or propane fuel to the burner) to control gas flow and the ignition of your furnace's burners.

If your furnace has a pilot light, the thermocouple should be kept clean and the flame properly adjusted to ensure energy efficiency and extended furnace life. Check the standing pilot light before each heating season for proper air and fuel mixture and flame color. Here is how you can inspect and repair or adjust your standing pilot light for the best furnace operation.

Inspecting the Pilot Light Flame

The color of a proper pilot light flame can vary a bit but whether your furnace uses natural gas or propane as a fuel, the color should be blue with perhaps a tinge of yellow at the tip of the flame. A bluish-green flame can also result from rust or dirt. To inspect the operation of the standing pilot light.

1. Remove the furnace cover panel, which should expose the burner assembly and pilot. You should be able to see the flame of the pilot light.
2. If the pilot light is out, attempt to relight it.
3. If the pilot light is burning, examine its color. A proper pilot flame should be blue with a yellow tip and should be strong enough to cover about 1/2 inch at the end of the thermocouple tip.
4. If the flame is too strong and improperly adjusted, it will be blue but may be noisy and lift off the thermocouple, causing improper furnace operation.
5. If the pilot light is on, but the flame is a weak yellow in color, it will not get hot enough to heat the thermocouple to the proper temperature required to allow the gas valve to open.

If the pilot light does not light or does not stay lit, then replace the thermocouple. Watch for any live power inside the furnace once the cover is removed.

Adjusting the Flame

There is usually a small screw on the pilot valve body that will adjust the standing pilot flame. You may have to refer to the furnace manufacturer's instructions to find the screw. Turn the screw as needed to adjust the flame throw. There are several flame abnormalities you should look for: 

• A yellow flame is caused by a lack of air and incomplete combustion. It can be caused by a dirty pilot tube tip. You can usually clean this debris away with a small screwdriver or nail tip.
• A split flame is caused by dirt inside the pilot tube. Take a needle or small nail and gently clean the tube.
• A flickering or wavering flame is usually caused by a draft. Check to see if there are obvious sources of drafts in the room. Sometimes this flickering is because the furnace's cover panel has been removed, but it may also indicate a source of the ongoing draft that should be remedied.