How to Inspect a Gas Furnace Pilot Light

Repairman with digital infrared thermometer checks gas furnace output temperature

BanksPhotos/Getty Images

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 to control gas flow and ignition of your furnace's burners. The thermocouple is the electronic device that senses if the pilot flame is hot enough to ignite natural gas or propane fuel to the burner.

If your furnace is of a vintage that includes a pilot light, it should be kept clean burning and properly adjusted for energy efficiency and extended furnace life. Check the standing pilot light before each heating season for proper air and fuel mixture and flame color. This tutorial will describe how to inspect and repair or adjust your standing pilot light for best furnace operation.

Inspecting the Pilot Light Flame

The color of a healthy pilot light flame can vary a bit depending on whether your furnace uses natural gas or propane as a fuel. 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.

To inspect the operation of the standing pilot or pilot light, proceed as follows:

  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 following the instructions found in steps 3 and 4 of How to Light a Gas Furnace Standing Pilot.
  3. If the pilot light does not light or does not stay lit, then replace the thermocouple as described in Replacing a Furnace Thermocouple.
  4. If the pilot light is burning, examine its color. A proper pilot flame should be blue with a yellow tip and strong enough to cover about 1/2 inch at the end of the thermocouple tip.
  5. 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.
  6. If the pilot light is on, but the flame is a weak yellow flame, it will not get hot enough to heat the thermocouple to the proper temperature setpoint required to allow the gas valve to open.

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. There are several flame abnormalities you should look for: 

  • A yellow flame is caused by 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 merely because the furnace's cover panel has been removed, but it may also indicate a source of the ongoing draft that should be attended to.