Electrical Circuits
Each gas-fired appliance is wired according to the specific make and model. A wiring diagram is included in the appliance manufacturer’s installation and operation manual. All electrical connections must be made in accordance with the manufacturer’s installation instructions. Read these instructions and follow them carefully.
Caution
Only a certified HVAC technician or someone with similar qualifications and/or experience should attempt to wire a gas-fired appliance.
Note
All local codes and regulations for wiring gas-fired appliances must take precedence over the instructions in the manufacturer’s installation and operation manuals. In the absence of local codes, all electrical wiring and connections should conform with the appropriate instructions and provisions found in the latest edition of the National Electrical Code.
A description of the electrical circuits used to operate modern gas-fired heating equipment is included in Chapter 5 (Gas and Oil Controls).
Automatic Controls
The automatic controls are used to ensure the safe and efficient operation of a gas-fired appliance. They are mentioned only briefly here because detailed descriptions of gas system controls are found in other chapters of this volume. These controls can be roughly divided into the following six broad categories:
• Room thermostats
• Ignition (lighting) devices
• Main gas valves
• Flame-sensing devices
• Pressure regulators
• Safety valves and switches
A gas heating system is controlled by a centrally located room thermostat. The thermostat sends a call for heat to the furnace or boiler when the temperature of the air in the room reaches the set- point (heat setting) on the thermostat. This occurs automatically when the temperature falls below a preselected heat setting, or manually when the temperature adjustment dial or lever is moved up to a warmer heat setting. When the heat setpoint is reached, it closes an electrical circuit between the thermostat and the furnace or boiler, which, in turn, activates the furnace or boiler control circuit. (See Chapter 4, Thermostats and Humidistats, in V olume 3 for a description of these thermostats.)
The main gas valve controls the flow of natural or propane gas to the burners when the thermostat calls for heat. In most heating systems, the main gas valve is combined with a pressure regulator to form a combination gas valve. Main gas valves are covered in Chapter 5 (Gas and Oil Controls)”of this volume.
Another important control is the device used to detect whether the gas in the burners has successfully lighted after the call for heat by the room thermostat. Some systems use a thermocouple to verify that a pilot burner is lit. If it can detect the pilot flame, it will open the main gas valve, allowing gas to flow to the main burners where the pilot flame will ignite it. If it cannot detect a pilot flame, it will not open the main gas valve. The flame sensor of an electronic ignition system performs the same function as the thermocouple in standing pilot systems, but it looks for the flame in the main gas burners. See Chapter 5 (Gas and Oil Controls)” for a description of flame-sensing devices.
The fan and limit control is another device that ensures the safe operation of a gas appliance. It controls the blower in a forced warm-air furnace. If the blower fails to operate during the heating cycle, the fan and limit control will shut off the main gas burners to prevent the furnace from overheating. Read the appropriate sections of Chapter 6 (Other Automatic Controls)” of this volume for a more detailed description of fan and limit controls.
The pressure regulator, which is commonly combined with the main gas valve in the form of a single combination, controls the amount and pressure of the gas used by the gas-fired appliance. See Chapter 5 (Gas and Oil Controls).
A wide range of safety valves and switches have been created by different gas furnace and boiler manufacturers to ensure the safe and efficient operation of their appliances. Pilot safety valves, pressure switches, and other devices are described in the appropriate sections of Chapter 5 (Gas and Oil Controls)”of this volume.