Figure 4-30. - A recycling solenoid valve.

Figure 4-31. - A diaphragm gas valve.

gas supply is allowed to be applied to the top of the diaphragm, forcing it down and sealing the main valve.

PILOT LIGHT. - The gas pilot light in a gas-heating unit is a small flame that burns continuously and lights the main burner during normal operation of the heating unit. It is located near the main burner, as shown in figure 4-27.

The gas flow to the pilot light is, in some cases, supplied by a small, manually operated gas shutoff valve on the main gas line above the main gas valve. In other cases, the gas can be supplied from the pilot tapping on a solenoid gas valve, as shown in figure 4-29. In more expensive heating units, the gas for the pilot light is often supplied by a thermocouple- controlled relay.

THERMOCOUPLE. - A thermocouple is probably the simplest unit in the electrical field that is used to produce an electric current by means of heat. It is constructed of two U-shaped conductors of unlike metals in the form of a circuit, as shown in figure 4-32. If these conductors were composed of copper and nickel, respectively, and are joined as shown in the figure, two junctions between the metals exist. If a flame heated one of these junctions, a weak electric current would be produced in the circuit of these conductors. A series of junctions can be arranged to form a thermopile to increase the amount of current produced, as shown in figure 4-33.

In the heating field, thermocouples and thermopiles are used to produce the electrical current used to operate such units as gas valves, relays, and other safety devices.

The thermocouple is located next to the pilot light of the main gas burner, as shown in figure 4-27. It generates the electric current (usually 50,000 microvolts) which holds open a main gas valve, a relay, or any other safety devices, permitting gas to flow to the main burner. Soon after the pilot light is extinguished, current ceases to flow to these safety devices, thus causing them to shut off the gas to the heating unit. These safety devices will not operate again until the pilot light is lighted and current is again generated by the thermocouple.

THERMOCOUPLE CONTROL RELAY. - The thermocouple-operated relay shown in figure 4-34 is a safety device used on gas-fired heating equipment. The thermocouple, when placed in the gas pilot flame, generates electricity. The electric current energizes an electromagnet that holds a switch or valve in the OPEN position as long as the pilot flame is burning. When the pilot flame goes out because of high drafts or fuel failure, the electromagnet is de-energized, thus closing and preventing the opening of the switch or valve. The

Figure 4-32. - The principle of a thermocouple.