The amount of electrical energy generated by a thermocouple that has a single hot junction and a single cold junction (i.e., approximately 25 millivolts) is considered adequate for most residential heating equipment. However, a few residential furnaces and boilers and most commercial types require a higher voltage. An increase in voltage can be obtained by using a number of thermocouples wired in series (see Figure 5-40). A series of thermocouples in one unit is called a thermopile, pilot generator, or thermopile generator. A thermopile forms a part of a millivolt or self-energizing control circuit.
An example of a thermopile used in gas-fired heating equipment and appliances is shown in Figure 5-41. This particular unit produces approximately 320 millivolts in an open circuit for gas valve control. It is available with an adjustable ignition port and interchangeable orifices for use with any gas.
A thermopile that produces an even higher voltage is the ITT General Controls PG-9A (see Figure 5-42). This thermopile provides a pilot flame for gas burner ignition and generates electricity from the heat of the
pilot flame to operate millivolt gas valves and relays. The replaceable cartridge in the thermopile contains many thermocouples connected in series. The top 3?8 to 1?2 inch of the cartridge is heated by the pilot flame, which produces approximately 500 to 750 millivolts (1?2 to 3?4 of 1 volt) open circuit.
A millivoltmeter must be used to test a thermopile. The meter leads are attached to the valve or relay terminals to which the wires of the pilot generator are also attached. The thermostat must be calling for heat and the pilot burning during a millivoltmeter test.