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Have an older round Honeywell Stat with 5 wires, G,Y,W,R, RH. I have hot water heat (no fan) and a Central AC with air handler.

New stat has terminals for G,Y,W,R,RC. Question is does the RH from the old stat go to R or RC on the new stat? I assume that R would go to the remaining one. I tried R to R and RH to RC and it's not working.

  • Can you include the model number of the new thermostat? Would help us find manuals for it. – Machavity Aug 16 '17 at 12:07
  • New Stat is Honeywell CT87N – BrianP58 Aug 16 '17 at 12:12
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I'm assuming that R, Y, and G come from the air handler. While Rh and W come from the boiler.

That would mean that you need to:

  1. Remove the jumper between R and Rc
  2. Connect Rh from boiler to R on the thermostat
  3. W from boiler to W on thermostat
  4. R from A/C to Rc on thermostat
  5. Y from A/C to Y on thermostat
  6. G from A/C to G on thermostat
  • I went to look at the wiring to the boiler. There is the white wire which is connected to W on the old stat. There is a blue wire which is connected to R on the old stat. The one wire currently connected to RH appears to run to my AC. Confused as all works okay with the old stat. – BrianP58 Aug 16 '17 at 14:33
  • @BrianP58 We're going to have to know more about the wiring to answer this definitively. Please show photos of the wiring as it was connected to the old thermostat, and as it is connected in the boiler and the air handler. – Tester101 Aug 16 '17 at 15:27
  • My mistake the wiring I was looking at was electrical and not for the stat. Traced the wires from the boiler and the air handler and all is described as above by Tester101. Should be good to go. Thanks!! – BrianP58 Aug 16 '17 at 19:45
  • One more question. There are two dip switches. It says it you have steam or gravity heating both switches should be "on". I assume hot water baseboard heat would be the same as steam heat?? Another option mentions hot water but almost mentions warm air and obviously there is no fan for heating purposes. – BrianP58 Aug 16 '17 at 19:53
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According to the manual, on page 3, the RH wire should be attached to the R terminal as well, not the RC. Leave the R to RC jumper in, since you do not have an RC wire.

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    That's what I read as well. However, called Honeywell this morning and they said R to RC and RH to R and remove the jumper. Not sure what to do as I don't want to damage a transformer. – BrianP58 Aug 16 '17 at 12:31
  • That's odd that they would contradict their own manual on this. Not sure I can advise you beyond what the manual says – Machavity Aug 16 '17 at 12:32
  • OP does have an Rc wire, it's just not labeled as such. Rc is the R wire from the cooling system, while Rh is the R wire from the heating system. – Tester101 Aug 16 '17 at 13:40

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