The valves used to control the flow of gas through a gas-fired furnace, boiler, or water heater can be divided into two basic categories: (1) manually operated valves and (2) power-operated valves.
The two manually operated valves (gas cocks) used on gas-fired heating equipment provide a backup safety function in case the automatic gas valves fail to operate. One of these valves is located in either the main gas supply riser or the manifold. The other one is located on the pilot gas line.
The manual gas valve installed on the main gas supply line (riser) or manifold is variously referred to as the main gas shutoff valve, manual shutoff plug cock, or simply the gas cock (see Figure 5-7). This valve provides manual control of the gas flow to the main gas burners. It is not used to control the gas supply to the pilot burner, the latter being provided with its own separate shutoff valves.
The manual valve located on the pilot gas line is called the pilot shutoff cock or the pilot gas cock. It is usually the first controlling device on the pilot line (see Figure 5-7). It provides complete gas shutoff whenever it is necessary to remove and service other controls on the pilot line, such as the pilot gas regulator or the pilot solenoid valve.
Power-operated or automatic valves are actuated by some form of auxiliary power such as hydraulic pressure, pneumatic pressure, electricity, or a combination of these sources. The following are the principal types of power-operated valves used on gas-fired heating equipment:
• Solenoid valves
• Direct-acting heat motor valves
• Diaphragm valves
