The thermostat C terminal provides 24V common return.

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Multiple Choice

The thermostat C terminal provides 24V common return.

Power for a thermostat comes from a two-wire 24V transformer inside the furnace. One side is the hot side (R), which supplies voltage to the thermostat, and the other side is the common return (C). The thermostat uses R as the power source and needs a return path to complete the circuit when it powers itself or drives the control circuits. The C terminal is exactly that return path, the common side of the 24V circuit. That’s why it’s not the hot side, not the earth ground, and not a sensor input—the C terminal is the common return for the 24V supply that powers the thermostat and its electronics.

Power for a thermostat comes from a two-wire 24V transformer inside the furnace. One side is the hot side (R), which supplies voltage to the thermostat, and the other side is the common return (C). The thermostat uses R as the power source and needs a return path to complete the circuit when it powers itself or drives the control circuits. The C terminal is exactly that return path, the common side of the 24V circuit.

That’s why it’s not the hot side, not the earth ground, and not a sensor input—the C terminal is the common return for the 24V supply that powers the thermostat and its electronics.

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