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I bought an ecobee3 smart thermostat which requires the C wire. At my furnace there is no circuit board but there was a diagram and I was able to locate the C - common wire, but it seems to be used by my air conditioner which is outside. I was able to run a wire up to my thermostat to connect the C wire. When I have the A/C and the Thermostat on the same C wire the A/C kicks in all the time. I had to unplug the A/C unit from the common wire and just leave the thermostat for now and it works fine for heat.

How do I get around this problem? why can't the common wire be used for both the a/c and the thermostat at the same time?

furnace model: ruud ubhc-17j14sfa diagram

  • If the outside unit truly uses a C-wire, it does so for some low voltage electronics or on board controller. Unless other wires are mixed up, or the wiring on the thermostat is wrong, nothing you do with the C wire should make the unit come on. The A/C and thermostat can use the C wire at the same time, so something else is going on. Is the wire actually labeled C on the A/C unit or are you assuming the blue wire is C? – JPhi1618 Dec 3 '15 at 19:27
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    There should be no problem using the C wire for both, as it's simply one leg of the transformer. There must be some strange wiring, or you're using the wrong wire. Post the make and model of your unit, some photos of the wiring, and a clear photo of the schematic you're using. Then we should be able to help. As it stands, you haven't offered up much useful information. – Tester101 Dec 3 '15 at 19:30
  • the diagram on my furnace shows the 24v transformer and then the red power wire which goes to the thermostat. There is another wire (brown), the common coming from the transformer, that is what I'm assuming is the C wire. The outdoor unit has 2 wires coming from it, the Y for cooling, and the the common which goes to the C from the transformer. There must be some other wiring issue going on then. – David Dec 3 '15 at 19:32
  • There should be two control wires running to the condensing unit. One is energized when you want the unit on, the other connects directly to one leg of the transformer. – Tester101 Dec 3 '15 at 19:33
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    Are you sure the thermostat is wired and setup properly? Is the thermostat calling for cool? – Tester101 Dec 3 '15 at 21:16

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