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.