A variety of different types of controls are used in gas and oil heating systems to ensure the safe, efficient, and automatic operation of the furnace, boiler, or water heater. These controls function together as a control circuit within the heating system.
The most important functions of the control circuit are (1) to start or stop the gas or oil burner in response to a signal from the centrally located room thermostat and (2) to shut down the burner if an unsafe operating condition occurs.
A complete description of all the controls used to govern the operation of a furnace, boiler, or water heater would be too extensive to include in a single chapter. For that reason, thermostats, limit controls, and related safety and control devices are covered in other chapters. This chapter is primarily concerned with a description of those controls that directly govern the flow of gas or oil to the burner. These are primarily the safety, pressure-regulating, and flame-sensing valves and devices. It also includes descriptions of the operating (ignition) systems.
Caution
Work on gas-fired and oil-fired equipment should be performed only by qualified personnel trained in the proper application, installation, and maintenance of HVAC systems.
Gas Controls
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, trans- former, or safety pilot relay. Some controls may be eliminated, depending on the design and requirements of the system. For exam- ple, a safety pilot relay is not necessary if a nonelectric pilot safety valve is used in the gas line.
Note
The valves and other devices in a control system will vary depend- ing on the type of ignition system.
Gas Control Circuits
The gas control circuits used to operate modern gas-fired heating equipment can be divided into the following three basic types:
1. Low-voltage control circuits.
2. Line voltage control circuits.
3. Millivolt control circuit.
A low-voltage temperature control circuit (see Figure 5-1) uses a step-down transformer to reduce the higher line voltage to approximately 24 to 30 volts. A 24-volt thermostat is used as the controller in most installations.
The line voltage temperature control circuit shown in Figure 5-2 is a 120-volt system. Because the voltage is not reduced, a line volt- age thermostat or controller and a line voltage operator must be used in the system.
A millivolt control circuit (see Figure 5-3) operates on the thermocouple principle. A single thermocouple automatically generates approximately 30 millivolts without the aid of an outside source of electricity. A number of thermocouples used together can generate up to 750 millivolts. This combination is variously referred to as a generator, pilot generator, thermopile generator, thermopile sys- tem, or powerpile system.
Each of the three temperature control circuits described in the preceding paragraphs is also wired into a pilot safety shutoff circuit, generally via a switch-type pilot safety shutoff device. An inline pilot safety shutoff device is also located in each safety shut- off circuit, and these provide complete gas shutoff.