enter image description hereI need to validate my understanding for the installation of a WiFi thermostat that requires 24volt continuous power to operate in a constant state. I am DIYer who is computer literate.

Like a number of homeowners in the Northeast, we have a natural gas furnace for heat and hot water that operates 12 months a year. Our AC system is separate and is electrical,and operates only during the warmer months, The location of the furnace is the basement while the AC is in the attic. We have three heat zones; basement, first floor and floor. Two AC units one for the first floor and one for the second floor.

The current thermostats are located on each floor and are programmable. They are Honeywell 7 day schedule with batteries, consequently no c wire was present.

We purchased a Honeywell WIFI thermostat that requires a c wire for a constant 24 volt.

The gas heating system located in the basement is easily accessible and we able to run a cable to create the c wire. The AC system is located in the attic and although accessible is shut down during the cooler months, about 6-7 months a year. Since the AC is shut down 24 volt power would not be available from the AC systems to owe the thermostat. The more practical approach is to use heating system 24 volt transformer common for the c wire.

I shut off power to the furnace and AC while connecting or disconnecting the wires to the thermostat.

With this approach I connected the R and RC to the respective terminals on the new thermostat. The instructions in the Honeywell setup manual states that if you connect the R and RC you should remove the jumper cable. I believe that this is Honeywell assuming the heating and AC are combined units that share a 24volt power source. I removed the jumper loop on the thermostat

I introduced the C common wire by connecting it to the common terminal on the control board of the gas furnace. I powered up both the furnace and AC. The Thermostat powered up.

I followed the prompts for set up and was able to connect to my WiFi. The internet connection was made. The heat commands functioned perfectly. The AC however would not respond. I checked to insure the AC was on a the junction panel.

I thought that installing the jumper loop would allow the two system, heat and AC, to rely on the the c wire as common wire from the furnace. This would be more practical since the first furnace is on 12 months a year. I turned off both heat and AC, Installed the jumper loop and turned the power on to both systems. Everything appears, heat and AC, to function without it issue. Even the fan function was operative.

I called Honeywell to verify my approach, but the person on the phone was unable to answer my inquiry. He had to the speak with another CSR after each question. I then wrote to Honeywell providing a schematic and narrative but no response.

If anyone can validate this approach we would be grateful. We. have disconnected the the thermostat until we can validate our approach. We would like to purchase a second WiFi thermostat but will not until we can validate or installation.

  • Can you post the model number of your new thermostat as well as photos or a diagram of all the wiring involved? Nov 13, 2017 at 4:35
  • 1
    Can you provide the schematic here as well? Without seeing the wiring, we'd just be guessing. Is this thermostat controlling all zones, or just one heating and one cooling zone? Please also include make and model numbers of all equipment in use (furnace, air handler, any zone controllers involved, zone valves, etc.).
    – Tester101
    Nov 13, 2017 at 11:55
  • Yes we need a some what detailed schematic. How many thermostats are currently used? How many transformers are currently used? How many air handlers are used? How many condensers are used? Are there any unused wires at the thermostat locations? How many zones of heat? Zones of cooling? P.
    – Paul Logan
    Nov 14, 2017 at 8:22
  • Model of thermostat is RTH9585WF.
    – Vet 1968
    Nov 14, 2017 at 20:27
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    Schematic added to original question. Hand drawn hope you can read. The common wire c would be shared by three thermostats connected to heating system viaTACO switch 24 volt transformer. No common wire available to the AC. Using jumper loop to share common with thermostat. Otherwise AC appears to be inoperative due to incomplete circuit.
    – Vet 1968
    Nov 14, 2017 at 20:30

1 Answer 1


This will not work. You need to pull Common from the AC/air handler, remove the jumper, then connect the baseboard heat (boiler) to Rh and W.

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