A furnace's gas valve controls gas flow to provide a steady stream of gas while controlling pressure. The valve also includes a safety feature that cuts off the gas supply if a certain temperature is not reached to prevent leaks.
A gas valve for a furnace is made up of four major parts that include a manual valve, a regulator, a solenoid valve and a pilot safety. The manual valve is turned by hand to shut off gas supply to the furnace completely. In older furnaces, this valve is a component of a gas chain, and it is open when the handle aligns with the gas pipe. The regulator reduces gas pressure from a gas line to a more manageable pressure for the manifold to distribute to the burners.
The solenoid valve opens when the thermostat signals that heat is needed, allowing gas to flow to the burners as long as the other valves are open. A pilot safety is held open by a voltage created by a burning pilot flame. When the pilot is not lit, the valve shuts off the supply to prevent gas from leaking into the outside environment. It may include either a button or an adjustable knob that allows gas to flow to the manual valve when pressed or turned.