The thermostat is the basic controller in the electrical control circuits used to operate a heating and/or cooling system. In systems using gas-fired heating equipment, there are three basic control circuits:
• Safety shutoff circuit
• Fan or circulator control circuit
• Temperature control circuit
There are three basic types of temperature control circuits used in heating and cooling systems. These three basic circuits are as follows:
• Low-voltage control circuit
• Line voltage control circuit
• Millivolt control circuit
Typical wiring diagrams for these three temperature control circuits are shown in Figures 4-2, 4-3, and 4-4.
