A modern gas furnace must be able to turn on automatically when heat is needed and turn off when it's not. To do that, the furnace needs a system that makes it capable of igniting its own burner. A standing pilot light is one
A modern gas furnace must be able to turn on automatically when heat is needed and turn off when it's not. To do that, the furnace needs a system that makes it capable of igniting its own burner. A standing pilot light is one of the oldest solutions to that problem, but it's an inefficient and, in its most primitive implementation, potentially unsafe solution.
A standing pilot gas furnace uses a small, continuously burning flame as a source of ignition for the main gas burner. This flame, called a pilot light, burns a constant flow of a small amount of gas. When the thermostat calls for heat and prompts the main gas valve to turn on, the pilot light ignites the larger flow of gas to the burner from the main supply. If the pilot flame is extinguished, the burner won't light.
The oldest standing pilot furnace models had no safety control that prevented the thermostat from starting the flow of gas from the main valve if the pilot wasn't burning. In such cases, the gas would keep flowing without igniting, which could potentially lead to a buildup of gas in the home or building. In extreme cases, it could cause an explosion or fire. This kind of system, called a "wild pilot," is no longer in use.
In modern standing pilot furnaces, the safety of the system is maintained by an electrically operated switch that turns off the gas supply to both the pilot and the burner when the pilot flame goes out. The system uses a thermocouple to sense the presence of the pilot flame; as long as the flame is present, the thermocouple and an associated solenoid keep the safety switch circuit closed, which keeps the gas valve open. If the pilot goes out, the switch opens, the gas valve closes and the gas won't flow even if the thermostat calls for heat. For the pilot to be relit, the system must be manually reset.
Because the always-on pilot consumes a small amount of gas 24 hours a day, standing pilot furnaces are extremely inefficient compared to furnaces with newer ignition systems. An intermittent pilot system uses a solid-state electronic ignition device that employs a high-voltage spark to ignite either a pilot flame or the burner flame itself. Another type of ignition system uses an electric current to heat an ignition element to the point that it ignites the burner. In both kinds of systems, gas is consumed only when the thermostat calls for heat, so they use much less gas than a standing pilot system.
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