Gas wiring systems are complicated but are made of easily understood parts. The system is a combination of switches that control functions and are built as such for safety purposes. The wiring is controlled by a system of components.
The first component is a 120-volt power source that feeds a transformer, which controls low-voltage functions and the 120-volt circulator. Some of these controls include the spill switch, the low-water cutoff, the high-low temperature switch and the damper switches. The spill switch is responsible for turning off gas to stop a consistent backdraft or a flue gas spill. This switch has to be turned back on manually once it has turned off. If it continually switches off, the vent should be checked for problems. If there is not enough water in the water coils, the low-water cutoff switch turns the entire system off to prevent damage or a fire.
A boiler can turn on when the water temperature is at a minimum set by the user and turn off at a similar maximum. The damper switch in a boiler's system does not allow the boiler to turn on unless the damper, which regulates the flow of hot gases, is completely open.