How Gas Furnace Thermocouples Work Boilers and FurnacesHow Gas Furnace Thermocouples Work | Boilers and Furnaces. Furnaces, water heaters, and boilers, especially older gas systems use the standing pilot gas system as an integral part of the furnace’s operation. These systems are best recognized as having a standing flame burning continuously.

It is a basic and time-tested system that provides a reliable way of proving a flame. It employs a thermocouple (some systems can use a thermophile which is a lot of thermocouples embedded in one piece) and a gas valve that has two separate valves inside of it.

How Gas Furnace Thermocouples Work | Boilers and Furnaces

One of these valves is the primary valve which operates directly from the thermostat. It will open and close when the thermostat calls for heat provided the pilot valve is open. If the pilot valve is closed then gas will not be provided to the burners on a call for heat. The pilot valve will remain open as long as there is a pilot flame and the thermocouple/thermophile is in good working order.

The standing pilot is basically a reliable and time-tested way of proving that a flame is present before allowing the main or primary valve to open and introduce a significant volume of gas (natural or propane) to the main burners.

Thermocouple or Thermophile | How Gas Furnace Thermocouples Work

How Thermocouples Work - A thermocouple (or thermophile) is necessary for the standing pilot to function. Technically it is called a Coppel element but most  HVAC technicians refer to it as a thermocouple. It is made of nickel and copper and when heat is applied to it, it produces a very small voltage. These two dissimilar metals create a tiny voltage when they are heated.

In a standing pilot gas system, the gas valve utilizes the pilot valve. What controls this pilot valve? Of course, it is the solenoid that is powered by the voltage produced by the thermocouple. This solenoid requires a tiny amount of voltage to keep the pilot valve open. If for whatever reason, the thermocouple (or thermophile) cools off, then the pilot valve in the main gas valve closes. This prevents gas from moving through the valve and into the main burners.

Why? Because we do not want gas going through the main valve without having a way of igniting the gas. If gas was allowed to escape without igniting it could build up into a dangerous concentration. A potentially explosive concentration. The time-tested and reliable standing pilot system offers a safe and proven flame-proofing system. It is slowly being replaced by more modern systems. However, they do remain as a reliable system to prove a flame for the purpose of igniting the main burners on a call for heat.

Standing Pilot Gas Furnace Sequence of Operation | How Gas Furnace Thermocouples Work

As long as there is a pilot light the following sequence is typical of most standing pilot gas furnace systems. The sequence of operation varies from manufacturer to manufacturer and system to system but the following represents the most common sequence of operation for standing pilot gas furnaces.

Standing Pilot Gas Furnace Sequence of Operation | How Gas Furnace Thermocouples Work
  • with a lit pilot light the thermostat calls for heat and the heating contacts in the thermostat close.
  • the main valve opens and introduces gas to the main burners. The gas touches the pilot flame and is ignited.
  • with the ignition of gas on the main burner heat begins to build up in the heat exchanger. The fan limit control device senses the temperature increase.
  • When the heat in the heat exchanger reaches a designated set point, the fan limit control closes a switch that engages the blower motor.
  • The blower motor introduces warm air throughout the ductwork and the dwelling.
  • The thermostat satisfies as the heat reaches the setpoint. The main valve closes and the main burners cease burner with the lack of gas. The blower continues to run to dissipate the heat.
  • As the heat exchanger cools the device, the fan limit control, which senses heat in the heat exchanger cools also. When the temperature inside the exchanger reaches a certain set point, contacts open and the blower disengages stopping all airflow until the next call for heat the furnace sequence of operation begins anew.

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How Thermocouples Work | HVAC Heating