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The 24-volt control circuit: transformer, fuse, thermostat, and the loads it switches
A transformer steps line voltage down to 24 VAC. R runs through the board fuse to the thermostat, which switches R to G (blower), Y (contactor coil at the outdoor unit) and W (furnace heat). Each load returns to C, the common side of the transformer. A float switch in the Y circuit opens on high water.
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