Purpose of the Thermocouple

• A thermocouple is a device for measuring temperature. In a furnace, it is an important safety device. When a furnace turns on, it ignites the gas in one of two ways. Either an igniter creates a spark to light the gas, or else a pilot light which is always left on lights it. If the gas does not ignite and continues to flow, it can cause an explosion when it builds up. A thermocouple turns off the gas if the pilot is off. Spark igniter furnaces use a different system.
  
  

Thermocouple Bsics

• When one end of a piece of metal is hotter than another, it produces a current. Different pieces of metal produce different amounts of current for a given temperature. A thermocouple has two different pieces of metal wire welded together at the hot end. The cold end is attached to a circuit that measures the current coming from the thermocouple and determines the temperature.
  
  

How it Works

• The hot end of the thermocouple is carefully positioned in the hottest part of the flame of the pilot light. The flame heats the thermocouple, producing a voltage which flows down the wire to the circuit. If the pilot light is on, the circuit leaves the gas valve on. If the pilot light goes off, the thermocouple quickly starts to cool, and the voltage going to the circuit drops. When that happens, it turns a small motor called a solenoid, which shuts of the gas valve. Usually, the furnace has a button which someone has to then push to manually turn on the gas again and relight the pilot.