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To understand the functionality of a thermostat, it helps to look at its simple wiring setup. A typical American thermostat uses just four wires: a 24-volt AC feed, a return wire for heating, a wire for cooling, and one for the fan. When you adjust the thermostat, it completes a circuit on one of these three control wires, signaling to the HVAC system to either heat, cool, or circulate air.
Despite its role, it's crucial to note that a standard thermostat has no capacity to regulate how much heat is produced. Its commands are binary—it can only say “HEAT NOW” or “STOP.” Without any inherent knowledge of the temperature context, these appliances remain relatively simplistic devices, relying on the thermostat to dictate their actions.