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I have three identical thermostats, model Radio Thermostat CT101. Two of them have C wires connected to the burner and work fine.

The third one controls a heating zone and also central AC. It has R and W wires connected to the burner, and C, Y, Y2, RC, G wires connected to the separate Bryant AC unit. This thermostat operates only on battery power, it refuses to draw power on the C wire - unlike the two other units.

Functionally everything works, it controls heating and AC correctly. But without wired power supply, the device loses the ability to function as a Zwave repeater, and of course it dies if the batteries run out.

I suspect that the thermostat may want the C wire connected to the burner instead, but the manual is sparse with details. How can I find out, short of connecting a C wire to the burner and trying that? (I should note that the cable to the burner only has two wires, so I would have to run a new cable.)

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  • Is there room to mount a standard-issue HVAC relay near the thermostat that's having these issues? – ThreePhaseEel Dec 9 '18 at 6:59

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