2.12: Pilot Lights
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Many gas-fired systems incorporate a pilot light to ignite the gas when the furnace is first turned on. A pilot light is a steady flame which is always present. It is fed by a normally-closed valve which is held open by a solenoid coil, and allows a small amount of gas through at all times. When the room thermostat engages the heating system and the main gas valves open, the in-rushing gas is instantly ignited by the pilot light flame.
If the pilot light was to ever extinguish, a danger might be presented if the flow of gas was allowed continue without combusting. It is for this reason that the valve is designed to automatically close if the flame should ever extinguish. To achieve this, a thermocouple is used to keep the valve open.
Spark ignition and electronic ignition, sometimes called a glow coil, are alternative methods of igniting the gas used for heating. All rely on the principle of instantly igniting the gas once the main gas valve opens.