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I've been replacing my thermostats with the RadioThermostat CT50, which reports to my home-brew home control/alarm system. I have a 3-year old AireEase 4-ton unit in the shop that has a Comfort Sync 103796-03 smart wifi thermostat. When I removed it, there are only four connected wires, two of which go to terminals marked I+ and I-.

I googled this and found that this is some kind of communications bus architecture that sends heat/cool/fan signals over just the two wires.

The guy that installed it says 'you should be able to convert it fairly easily', but I am sort of stonewalled here. I have plenty of unused wires in the thermostat cable. I was thinking I may have to change out logic or comm boards in both the heat pump and the furnace. Has anyone done this successfully?

Easiest course is to just put the ComfortSync back, admit defeat, and have one 'bastard' thermostat. But I would REALLY hate to do this unless there is lots of $$$ involved.

This is the control board in the air handler:

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The upper photo is the air handler, lower is the heat pump. Could this be simply a matter of installing the thermostat and connecting the W1,W2,Y,G,R leads to the terminal strips in the photos?

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  • Can you post a photo of the wiring at the air-handler in question?
    – ThreePhaseEel
    Apr 28, 2019 at 15:59

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You should be able to wire your thermostat to the conventional terminal block on the air handler

If you look at the bottom edge of your air handler's control board, there is a terminal block to the right of the two ComfortSync connections that has been labeled with conventional thermostat signal labels. This means that your control board should support standard 24VAC thermostat signals instead of a communicating thermostat. You can test this by disconnecting the communicating thermostat from the rest of the system, then jumpering R to G on that right-hand terminal block; when you do this, the fan should come on.

If that is indeed the case, then I would wire the new thermostat conventionally, as it sounds like you have sufficient spare wires in the cable from the air handler to the thermostat to do so. You shouldn't need to touch the wiring between the air handler and the heat pump, by the way, for that matter.

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