Gas Control Circuits

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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.

low voltage control circuit Gas Control Circuits

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.

line voltage control circuit Gas Control Circuits

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.

milivolt control circuit Gas Control Circuits

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.

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