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I recently rewired two thermostats into one Honeywell RTH9580WF1005 Wifi Thermostat. I ran a new wire and hooked up R and W to the oil boiler and Rc, C, G, and Y to the Goodman air handler. The Y is wired directly to the condenser as the control board does not have a Y in the harness. The other wire from the condenser goes to C on the control board.

When I turn on the thermostat circuit breaker, the condenser/compressor immediately starts up. I checked the contact outside and it appears to be working smoothly. With the power on, the contact is depressed. When the power is off, the contact is open and I can press it easily to start it. It seems to me that the Y wire is always energized and thus the condenser won't turn off.

I just replaced the control board thinking that there was a short in the controllers. However, the issue still remains so the board is not at fault.

How should Y be wired when there is no place for it on the control board? How should the thermostat be wired to control two separate systems? Should R and Rc be jumped or not? How can I test the voltage of the wires to verify it was done correctly?

  • Can you use a meter to check for voltage (about 24VAC) from Y to C at the air handler? – ThreePhaseEel Jun 19 '18 at 11:38
  • Yes, Y and C are 27V. Does it matter which of the two wires from the condenser are connected to Y and C? – js1568 Jun 20 '18 at 13:22
  • It does not matter which way around the contactor's coil is connected, no. If you pull the Y wire off the thermostat terminal and leave it unconnected, does the air conditioner shut off? – ThreePhaseEel Jun 20 '18 at 23:13
  • @ThreePhaseEel when removing the Y wire from the thermostat, the condenser still runs. Removing the Rc wire also has no effect. – js1568 Jun 23 '18 at 16:07
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You have a short in your wiring

Somewhere in the wiring for your HVAC system, Rc has shorted to Y. Find the short and fix it (using electrical tape to insulate the shorted wires will work for now given that it's a low voltage system), and the problem should go away.

  • There was a staple through the wire in one location on the ceiling. I removed it and everything is functioning as expected. – js1568 Jun 25 '18 at 0:40

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