In addition to the room thermostat and the fan and limit control, the basic components of the control system of a gas-fired furnace, boiler, or water heater will generally consist of the following controls:
• Main gas valve
• Pressure regulators
• Pilot gas cock
• Automatic gas control valve
• Automatic pilot valve
• Pilot assembly
The pilot assembly includes the pilot burner and the thermocouple or thermopile (pilot generator) in standing-pilot systems. In more modern systems, the pilot assembly consists of the pilot burner, a spark ignition module, and a flame sensor.
A control system may also include a gas primary control, transformer, or safety pilot relay. Some controls may be eliminated, depending on the design and requirements of the system. For example, a safety pilot relay is not necessary if a nonelectric pilot safety valve is used in the gas line.