Navien NCB-240 & common wire

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Brooklyn NY
I realize this is an old thread, but it remained mysterious for a lot of folks, myself included. After much head-scratching, this solution did work for me. If your thermostat currently does not control a cooling system but only your Navien, this should work: Connect the outer two terminals in the AC24V out on the Navien (see Kurt's image above) to the Common and Rc terminals on your thermostat. Sometimes Rc and R (or Rh) have a jumper on them in the thermostat; if so, remove this jumper. You now have a separate power circuit for the thermostat. I'm not sure this solution would work if you have a Y wire for cooling, as presumably that would close the circuit with Rc. As @jadnashua correctly notes, if you draw power from the R/W line, you will lose the power whenever the thermostat cycles on.
Hey, wait a minute.

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