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Pilot light furnace no heat, thermocouple failure, diagnosis and replacement
If the furnace has a spark ignitor for the pilot, click here
Furnaces with a standing pilot have a combination gas valve and thermocouple for a flame safety control system. If the pilot goes out, the system shuts down to prevent the gas valve from opening without a pilot to light the burner.
Thermocouple
The thermocouple consists of 2 different metals. When one end is heated, a voltage is produced at the opposite end called the cold junction. This voltage is very small, usually a maximum of 30 millivolts. For a comparison a 1.5 volt flashlight battery is equal to 1500 millivolts.
It is this voltage (usually in the operating range of 18 to 22 millivolts) that holds the pilot valve open. The video below looks at the inside of the thermocouple.
It is this voltage (usually in the operating range of 18 to 22 millivolts) that holds the pilot valve open. The video below looks at the inside of the thermocouple.
Pilot assembly
The pilot appliance uses a small pilot flame to light the burner and provide for appliance shutdown if the pilot is not present.
Here is how it works.
The pilot burner is supplied gas from the gas valve through a 1/4" pipe (usually aluminum). At the entrance to the pilot assembly, there is an orifice that limits the amount of gas supplied to the pilot. The video below covers how the pilot gas orifice works.
Here is how it works.
The pilot burner is supplied gas from the gas valve through a 1/4" pipe (usually aluminum). At the entrance to the pilot assembly, there is an orifice that limits the amount of gas supplied to the pilot. The video below covers how the pilot gas orifice works.
The pilot burner provides a small flame to warm the end of the thermocouple. The flame also is set near the burners to light the burners.
The thermocouple produces a small voltage that is used to hold the gas valve open so the gas can flow.
If the pilot goes out, the voltage reduces and the safety valve closes. Below are 4 vids on how the thermocouple safety system works.
The thermocouple produces a small voltage that is used to hold the gas valve open so the gas can flow.
If the pilot goes out, the voltage reduces and the safety valve closes. Below are 4 vids on how the thermocouple safety system works.
The video below covers the steps to relight the pilot on a gas furnace. Most of this information is available in the above videos, but this one shows how to do it in an operating furnace.
When troubleshooting, understand you are working with potentially lethal voltages and a highly flammable gas. If you do not have the ability to do these operations safely, do not attempt them.
1. Determine if voltage is present. See home page
2. If voltage is present and unit will not fire is pilot flame on?
No pilot. Is gas available? Gas cock on? Meter open?
2. If voltage is present and unit will not fire is pilot flame on?
No pilot. Is gas available? Gas cock on? Meter open?
Gas cock
When wings of handle of gas cock are parallel with pipe. valve is open.
The video below explains how different types of gas cocks are designed.
The video below explains how different types of gas cocks are designed.
Gas meter
This meter has an inlet valve on the left that can be locked. If locked, gas has been turned off by gas supplier.
Even if the inlet valve is not locked, but is turned off, legally you can not turn it on.
Gas supplier is the only one that can turn it on legally.
Any outlet valves are ok to turn on or off.
Under no circumstances should the meter be bypassed. The pressure in the inlet supply line is far too high for your appliance!
Even if the inlet valve is not locked, but is turned off, legally you can not turn it on.
Gas supplier is the only one that can turn it on legally.
Any outlet valves are ok to turn on or off.
Under no circumstances should the meter be bypassed. The pressure in the inlet supply line is far too high for your appliance!
Combination gas valves
B. If the gas is on and the pilot is out, an attempt to light the pilot can be made.
The pilot is located at the end of an aluminum tube coming from the gas valve.
These are two types of gas valves you may see on your unit.
There is a knob that can be rotated to the pilot position then the knob or a red button is pushed and the pilot can be lit.
When held down for up to 1 minute after the pilot is lit, it should stay on.
The pilot is located at the end of an aluminum tube coming from the gas valve.
These are two types of gas valves you may see on your unit.
There is a knob that can be rotated to the pilot position then the knob or a red button is pushed and the pilot can be lit.
When held down for up to 1 minute after the pilot is lit, it should stay on.
.
The pilot should be blue in color and should be ruffled rather than a lazy yellow flame like a candle flame.
If it is yellow and lazy, the pilot assembly must be removed and cleaned.
If the pilot is clean and the thermocouple end is a minimum of 3/8 " into the flame and still no luck, the thermocouple has probably failed. Thermocouple replacement
They are available from some hardware stores.
Pilot on no main burner home
If it is yellow and lazy, the pilot assembly must be removed and cleaned.
If the pilot is clean and the thermocouple end is a minimum of 3/8 " into the flame and still no luck, the thermocouple has probably failed. Thermocouple replacement
They are available from some hardware stores.
Pilot on no main burner home
Safety: If the aluminum pilot tube is removed, when reassembled it must be leak checked. Soap solution will do. Flammable gas detectors are also available.