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A pilot light is a small gas flame, usually natural gas or liquefied petroleum gas, which is kept alight in order to serve as an ignition source for a more powerful gas burner.

Common applications include household water heaters, central heating systems, flame throwers, and hot air balloons. In the past, gas cooking stoves also included pilot lights but electronic ignition has replaced pilot lights in most stoves (and is replacing pilot lights in many of the other applications as well).

In natural gas furnaces, water heaters, and room heating systems, a safety cut-out is usually included such that if the pilot light goes out the gas supply to the pilot and heating system is shut off by an electrically operated valve. This cut-out usually detects the pilot light in one of three ways:

1. Sometimes a mercury filled sensor is used to detect the heat of the pilot light. Expansion of the mercury results in sufficient pressure to operate an electrical switch that interrupts the flow of electricity and shuts off the gas valve when the pilot light goes out.
2. Sometimes a photoresistor is used to detect the light from the pilot lamp. Electrical circuitry connected to the photo-cell provides a current sufficient to keep the gas valve open all the time the pilot light is still burning.
3. Use of a pilot generator or a thermocouple in the flame provides heating appliance safety as it generates enough electrical current from the burning flame to hold the gas valve open. If the pilot light goes out, the pilot generator cools off and the current stops, closing the gas valve.

The above methods are examples of the use of "fail-safe" safety protection.

In domestic heating systems with pilot lights it has been estimated that on average half the total energy usage is from the pilot light, each pilot light using from 240–500 W of gas (8–16 GJ/year).[1] [2] [3] That said, the heat from a pilot light in many appliances (furnaces, space heaters, hot water heaters) is generally released in the same chamber as the primary burner. The heat energy provided by the pilot light in these types of devices provides a minimal amount of heat to the device; the gas burned is not "wasted", since it contributes to the purpose of the device.

An alternative to the pilot light is a system that provides a high voltage electrical arc or “spark” between two electrodes close to the gas flow from the burner that is to be lit. Fail-safe design for such a system requires the burner flame to be detected by passing an electric current through the flame, which is “received” by a control box, whilst the flame is established there will be a flow of electrons through the flame so the control box keeps the appliance working, should the flame extinguish, the electron flow will be broken so causing the control box to shut down the appliance.

A red-hot surface can also be used to provide ignition. Such igniters are often made of silicon carbide, silicon nitride, or another material that is durable under prolonged exposure to extreme heat. Hot-surface igniters are commonly used in cooking ovens and broilers.