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 voltage 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 system, 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 shutoff circuit, and these provide complete gas shutoff.