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why are there 3 thermostat wires running to a condensing unit?

Matthew Ajster
4 months ago

I replaced the furnace in the home over the winter. I went to connect the central air unit back in yesterday and noticed that there are 3 thermostat wires going into the unit. There is a step down transformer located next to the contactor. There is a blue and yellow wire coming off the low voltage side of the transformer. the blue wire is connected to a blue wire from one side of the contactor and then connected to the white thermostat wire. The yellow wire coming off the transformer is connected to the red wire of the thermostat wire. There is a yellow wire coming from the other side of the contactor and it is connected to a yellow thermostat wire. Shouldn't the yellow wire from the transformer and the yellow wire from the contactor and the red thermostat wire all be connected so that there are only 2 thermostat wires running into the board on the air handler?

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