How to Light the Standing Pilot on a Gas Furnace

pilot light
Most modern gas furnaces have a burner that is ignited by an electronic ignition. Older style furnaces with an AFUE of <80% normally have a standing pilot flame meaning the flame is lit at all times. The standing pilot flame is oftentimes referred to as the pilot light of a gas furnace. The main purpose of the pilot light is to serve as a small ignition flame for the gas burner of your furnace.

The furnace pilot and its right hand, the thermocouple, are two of the main players for the proper operation of your gas furnace. Knowing how to locate your pilot light, how it works and how to troubleshoot it when it’s out comes in handy – especially with older furnaces.

How the Standing Pilot Light Works

The thermostat of your furnace sends a signal to your furnace that allows for gas to be delivered from the gas valve to the burner. The standing pilot light ignites that gas to heat the air in the combustion chamber. Without a well functioning pilot light your gas furnace is unable to operate.

You can recognize a properly working pilot light by the colour of its flame. In natural gas furnaces, the pilot light should be bright blue, with the tip of the flame having just a tingle of yellow. If you notice a yellow, split or flickering flame then you need to check if there is dirt in the pilot tube and/or if there is a draft blowing on it. A weak yellow flame will not get hot enough for the thermocouple to heat to its set point.

How the Thermocouple Works

The thermocouple – also called the thermocouple junction – is an electronic device that senses if the pilot flame is hot enough to ignite natural gas or propane fuel to the burner. It contains two metal wires welded at the ends and placed inside a protective metal case. The sensor of the thermocouple is placed in the hottest part of the pilot flame. The other end of the thermocouple connects to the body of the pilot valve.

When the thermocouple senses enough heat from the pilot flame it sends an electric signal to open the gas valve and allows the main gas valve in the pilot assembly to open up and stay open. This results in a constant gas supply to the pilot and to the gas burners (as called for by the thermostat).

If the pilot flame is extinguished or doesn’t produce enough heat then the thermocouple gets cold and ceases producing the electric signals that normally open the gas valve’s solenoid. This causes the gas valve to shut off the gas supply to the pilot and burners.

Relight the Pilot Light on your Natural Gas Furnace

It’s not uncommon for pilot lights on older furnaces to go out and to need to be relit.

Your furnace manual will have step-by-step instructions on how to relight the pilot light on your furnace model and pilot valve type. It’s strongly recommended to consult your furnace manual as the steps vary slightly based on the furnace model and the type of your furnace’s pilot valve assembly.

Most commonly, there are two types of pilot valve assemblies:

  • A red reset button and a gas valve.
  • No reset button and a valve knob that can be depressed.

Either way, the basic procedure to relight a standing pilot light in most older model furnaces is as follows:

  1. Set your thermostat to heat mode and turn it to a setting that will demand heat (e.g. 80 degrees).
  2. Locate the pilot valve on your furnace. 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 normally has a gas cock or valve knob that reads ON, PILOT and OFF.
  3. Turn the knob or gas cock to the OFF position. Once completed, you need to wait about five minutes for any residual gas to clear away.
  4. Locate the pilot. It’s located near the gas burner tube assembly inside the furnace.
  5. Get a fireplace match or butane BBQ grill lighter ready.

To light the pilot with a red Reset Button proceeds as follows:

  1. Turn the gas valve from OFF to PILOT.
  2. Place the lit fireplace match or ignited butane BBQ lighter tip at the pilot, while depressing and holding the button for about 30 seconds (until the thermocouple gets hot enough to open the main gas valve).
  3. Once the pilot stays lit, slowly release the button, and then turn the gas cock from the PILOT position to the ON position.
  4. Once the main furnace burners ignite, adjust the thermostat to the desired setting.
If the burners fail to ignite, then it could be that the thermocouple did not get hot enough to open the gas valve. Wait a few minutes, and then repeat step 1 to 4. This time, hold the red reset button a little bit longer (45 to 60 seconds).

To light the pilot with no Red Button proceeds as follows:

  1. Turn the knob from OFF to PILOT.
  2. Place the lit match or ignited lighter tip at the pilot, while depressing the knob for about 30 seconds (until the thermocouple gets hot enough to open the main gas valve).
  3. Once the pilot stays lit, slowly release the knob, and then turn the knob from the PILOT position to the ON position.
  4. Once the main furnace burners ignite, adjust the thermostat to the desired setting.
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 steps 1 to 4. This time, depress the knob for a little longer ( 45 to 60 seconds)

If the Pilot Won’t Stay Lit

If you’ve followed the previous steps for lighting the pilot and it still does not light, or it will not stay lit, then your thermocouple and/or your standing pilot may need to be replaced.

Our HVAC experts at S&C Mechanical Systems will be happy to inspect your furnace and to give you some detailed and honest feedback about what’s wrong with it.

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