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Honeywell rth9585wf to control Boiler and AC

InternetToughGuy Member Posts: 2
edited March 17 in Thermostats and Controls
I'd like to use a single Honeywell RTH9585WF (Smart Color Thermostat with Wifi) to control both an existing radiator system (boiler) and existing central AC. Wiring in existing honeywell non-smart thermostats:
  • Existing heat thermostat (first image) has wires terminating to W and R only.
  • Existing cool thermostat (second image) has wire terminating to W, Y, G, R, and C. [Rc|R] jumper is connected in both.
How would I wire both of these to the single RTH9585WF (third image)?

I would have thought simply:
  1. Remove the [Rc|R] jumper
  2. Connect one red/power wire to R
  3. Connect the other red/power to Rc
  4. Connect Heat Send to W
  5. Connect AC Send to Y
But the fact that the existing AC thermostat has a W wire connected confused me - not sure why my AC unit has a W connected wire in the thermostat when it doesn't control a heat source. Plus I'm not positive about the R/Rc - does it matter which or do I connect them both and keep them jumped?

Thanks in advance for any thoughts or suggestions!


    HVACNUT Member Posts: 5,828
    It's not a heat pump because there's nothing on O. Can you get the cover off the air handler, or does it go to an HVAC relay? If it's not connected on the other end, you don't need it and just use the W from the heat side.

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,818
    edited March 18
    Insufficient information. Thermostats can be connected to many different appliances. The heater can be Gas or Oil fired or even electric heated boilers. That may make a difference. so I would suggest you add one of these to the transformer on the air handler before it leaves the air handler to feed the thermostat. otherwise you may let the factory installed smoke out of the transformer or perhaps the boiler control.

    The AC system is usually pretty standard. A low voltage transformer usually found in the air handler will send power from the R terminal on the 24 VAC side of the transformer to R or RC on the thermostat. The air handler will have a fan relay that is powered when the R or RC connects to the G terminal on the thermostat sending 24 VAC to the relay and then completing the circuit back to the C terminal on the 24 VAC transformer. The Y terminal on the thermostat will send 24 VAC to the condensing unit contactor to operate the compressor and outdoor fan with a return path to the C on the terraformer. That is all you need to operate air conditioning.

    Since your cooling thermostat has the W connected, then there's a possibility that there is also a heat relay in the air handler. Since you don't use the air handler for heat, there is no need to connect that W wire to the thermostat. And you will want to save the W on the thermostat to connect to the boiler.

    So the new thermostat needs to have the jumper between R and RC removed.
    The air handler gets connected to the RC on the thermostat.
    The C on the thermostat should be connected to the C from the air handler.
    The G on the thermostat gets connected to the Green or Gray wire on the air handler but you need to check that the G wire on the air handler is the wire that operates the fan relay.
    The Y on the thermostat gets connected to the Yellow or Y wire from the air handler. That should be then connected to the compressor contactor.

    No W wire from the air handler gets connected to the new thermostat

    Now the R from the new thermostat must be connected the the boiler thermostat terminal
    Along with the W from the thermostat also connected to the other thermostat terminal on the boiler.

    I hope this helps.

    I am not sure if you have the components as i have illustrated them. There may be other components that I can not possible know about, use this information at your own risk.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • InternetToughGuy
    InternetToughGuy Member Posts: 2
    So I took off the air handler cover and indeed, the white goes nowhere and is just wrapped around the TS. They terminated in the TS just so that it wouldn't be dangling I guess. So I'm good. thank you all very much!