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why 3 thermostat wires instead of 2?

126 Views 2 Replies 3 Participants Last post by  eggs

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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You probably don’t need that transformer anymore because you got new furnace. Transformer was sending power to the house to thermostat or
Something. Take it out and just connect 2 wires from furnace one on one side of contactor and the other one on the other side of contactor
From the looks of your picture - - The Transformer was used to power the AC condenser and they brought back both the 24 volt hot and common along with the controlled switched power. You would need to remove the transformer from the controls. Then power the contactor from your new furnace. While you are at it, it might be a good idea to replace the contactor and possibly the capacitor. Both look to be in ruff shape.
Specs on the contactor would be 30 amp with 24 vac coil. You would need to look for the capacitor spec on it. Good Luck.
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