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I have done quite a bit of research and have not been able to find an answer I am looking for. I currently have a manual digital thermostat that is not programmable made by Braeburn. My system is a Ground A/C unit outside, and my heater/air handler is up in the attic. My thermostat is wired the following way:

  • Jumper wire between R and RC
  • Red wire from wall to RC
  • B terminal on thermostat is empty
  • Blue wire from wall to O
  • Jumper wire from Y to W
  • Yellow wire from wall to Y
  • No wire to W other than Jumper
  • Green wire from wall to G
  • There is a spare wire from the wall harness to nothing as well.

I have not had a chance to go in the attic to check the labels on the air handler, but I am hoping there is an extra wire up there too so I can use it as a C wire. I am curious how, or if I can use a WiFi thermostat or any other programmable for that matter. I have not come up with any leads yet, and don't want to screw things up before I start.

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So I go up and check the wiring on the Carrier F4 air handler and basically find out where the thermostat wiring should be is gone and wires are just spliced together there. So I took pics of everything but am not sure how to post them. According to the instructions shoved in the air handler the thick brown wire is the common for the unit and it attaches to the white thermostat wire. At my thermostat downstairs I have 2 white wires, one hooked up and one extra. Is there a way to test the 5th wire that is not hooked up to see if it has power to be the common wire? I am hoping it is hooked up at –  steve Aug 3 at 16:52

2 Answers 2

The C wire, not to be confused with R or RC is the return path. Think of it as a ground, though technically not. Based on the information above, none of the wires O, B, W, G, R, Y, will work for C. So they can not be jumped over to the C connection. The C wire, usually the brown wire on the transformer, the low voltage (24v) is the common. However, no matter the color, there are only two low voltage wires on the transformer, one power (Red, Blk, Yel), one common. So what ever color not representing the power, is the common C. Of course, AC current flows both directions, but trying to make this simple. Anyway, that wire (the non power wire) needs to come in to the thermostat for the C connection. With an extra wire (any color on the wire loom at the thermostat can be traced back to the 24v transformer and connected to the common, usually color brown wire). That extra wire used, the new color will become the C wire. Just a note, one issue I have had with the WiFi thermostat is the fan ON and Auto don't work independently. So for example, if the thermostat is selected to run the fan ON - no cooling or heating, I've seen that not work. It can be fixed too, just not answering that.

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Typically O, and/or B are used to control a reversing valve in the heat pump. Depending on which is energized, the system either provides heating or cooling. You'd have to check the manual for the specific thermostat you'd like to install, but usually thermostats can be configured to work with heat pumps. You might have to move a few jumpers around in the thermostat to configure it, but the information required to do so should be in the installation guide.

If all else fails, Honeywell has great customer support. I'm sure they'll be able to answer any questions you have, and direct you to the specific product that will work for you.


More information

  • The thermostat gets power from the red wire, which energizes both R and Rc.
  • The G terminal on the thermostat is energized when the fan switch is placed in the FAN position.
  • The Y terminal is energized when the thermostat calls for cooling (temperature above set point).
  • The W terminal is energized when the thermostat calls for heating (temperature below set point).
  • The B terminal is typically energized when the thermostat is set to heating mode (function switch set to HEAT).
  • The O terminal is typically energized when the thermostat is set to cooling mode (function switch set to COOL).

Some thermostats require you to set jumpers to determine whether O or B is energized. In this case, it seems the system requires the O terminal to be energized.

When the thermostat calls for heating or cooling, the system makes the decision of which function to activate based on whether or not the O terminal is energized. I'm not sure exactly how your system is configured, but it seems as though the system works as follows.

  • If O is energized when Y or W is also energized, the A/C comes on.
  • If O is not energized when Y or W is energized, the heat comes on.
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