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How Do I hookup Honeywell RTH8500 thermostat with No C wire with wires RC, RH, G, W, Y currently used? I have an external wall adpater transformer but don't know where to connect it to. I have separate Heating and Cooling (Heating is a Weil McLain Boiler for Hot water baseboard and Hot water(no tank) and Cooling is a separate unit that was added later.)

RC Cool

RH Heat

W Heat

Y Cool

G Cool

  • What model of thermostat are you dealing with, and can you post photos of the wiring at each end? – ThreePhaseEel Nov 27 '16 at 21:48
  • Honeywell RTH8500 – ajb1974 Nov 27 '16 at 21:50
  • It does not answer the question on where or what to do with the second wire from the external transformer. – ajb1974 Nov 28 '16 at 0:29
  • @Machavity -- not a dupe -- he is using a two transformer configuration vs the normal single transformer config – ThreePhaseEel Nov 28 '16 at 1:18
  • Can you post pictures of the wiring at the boiler end? – ThreePhaseEel Nov 28 '16 at 1:18

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