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I am wiring a second thermostat right next to the original one. The new needs a C wire. Can I share it from the old one?

Old: 8 wires, used to run heat pump (w/ heating and AC) and backup oil. Now only runs AC from heat pump on occasional summer day.

New: Has 2 wires run to it. Controls instant on hot water heater for radiant system.

The new thermostat says, "WILL NOT OPERATE WITHOUT WIRE TO 'C'". Can I take the C wire from the existing thermostat and bridge it to the new thermostat? Can two thermostats share this C wire?

According to Can I piggyback a thermostat with a C Wire hookup on a furnace? I think the answer is "Yes", but when I try it, the new thermostat does not have power.

  • The R wire going to the new thermostat, does it come from the same place as the C wire on the old thermostat? Or is there an R wire from the heat pump to thermostat 1, and an R wire from the boiler to thermostat 2? – Tester101 Nov 4 '15 at 0:45
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You can only share the C wire, if you're also sharing the R wire. If the R wires come from different equipment, the C wires must also come from the different equipment.

  • Is there any reason I shouldn't also share the R wires? Based on diy.stackexchange.com/questions/10482/… I think so, but I wouldn't want one system to start interacting with the other. Just share power. – mankoff Nov 4 '15 at 1:27
  • You should not share the control wiring between separate systems. – Tester101 Nov 4 '15 at 1:32
  • According to graphics in the link I gave above, the loop only closes when W is triggered. It seems like if C and R were each wired to two systems, the circuits would still be incomplete until W triggered, and then only the circuit with the W would be active... – mankoff Nov 4 '15 at 2:15
  • @mankoff The loop from R to C does not rely on W at all. That's why those two are used to supply constant power to the thermostat. – Tester101 Nov 4 '15 at 2:31

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