Pilot light

Merker gas fired water heater from the 1930's, with pilot light clearly visible through the aperture in the front cover. The large opening allowed for the manual lighting of the pilot light by a lit match or taper

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.

Origin

During the American colonial period, people kept a small flame called a "pilot" burning in order to quickly light a fire for a fireplace or for a cooking stove.^{[citation needed]}

Uses

Common applications include household water heaters, central heating systems, flamethrowers, and hot air balloons. While most commercial kitchens still rely on pilot lights for burners, ovens, and grills, current residential systems typically use an electrical ignition. More commonly known as standby on modern remote control fires.

Safety protection

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

1. A sensor filled with mercury is used to detect the heat of the pilot light. Contraction 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. A photoresistor is used to detect the light from the pilot lamp. When the pilot light goes out, electrical circuitry connected to the photoresistor shuts off the gas valve.
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.

Energy waste

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 between 240 and 500 Watts of gas power (between 8 and 16 Gigajoules/year). 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.

Modern alternatives

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 boilers.

A disadvantage to modern alternatives requiring high voltage is that the appliances become useless during a power outage. Pilot light solutions work independently of the electrical system.