I have a dual zone HVAC with gas heat. The two units are completely separate and have their own controls. The only difference between the two units is the one upstairs recently had the air handler replaced. I decided about a month ago to replace both the old battery type programmable thermostats in the house. I started with the furnace in the basement. I found that the thermostat did not have a C wire but had extra unused wires in the wall. I picked on and found it in the HVAC unit and connected it up the C terminal in the HVAC unit and the thermostat. Thermostat fired up and has been working perfectly for about three weeks. Late last week I decided to upgrade the thermostat on the second floor (HVAC in attic). No C wire here either but there were wires in the wall. I picked one and used that for the C wire and connected it to the C terminal in the HVAC and the C terminal in the thermostat. The thermostat fired up and I could control the heat and fan but when the system was set to the AC mode, I would get the message "waiting for equipment" for about 5 minutes and then the thermostat would "reboot" (e.g. go back to the "Honeywell" logo splash screen). That is the whole story. I put the old thermostat back on and everything -- including the A/C -- ran fine. The Honeywell support people said that maybe the thermostat was getting some feedback from the C terminal in the HVAC. I confirmed that the thermostat was in working order by swapping it with the one working with the HVAC in the basement and it worked fine. The only thing that I can come up with is that the HVAC people, when they replaced the air handler, may have not wired the attic HVAC back to factory specs -- and maybe they could not as they replaced the motor with a new one. The only other issue could be where I am grabbing power in the HVAC for the thermostat (the C terminal). That really is the only difference. There was (and still is) a wire on the c terminal in the HVAC (going to the outside compressor) but there were several on the C terminal in the HVAC in the basement (one to the outside compressor and one for the humidifier) and that system is working fine. I was thinking that maybe I could provide my own 24 volt power to the C terminal in the thermostat using a transformer. Does that sound like a good idea? If that sounds good, should I fuse the new transformer? Any help would be appreciated.

 

To verify, please list and or take pictures of the thermostat wiring and HVAC wiring.
IE R and Rc = Red

 

Technician found that the wire I was using for "C" was shorting. I should have known as it was the white wire which, if it was not shorted, would have been used for the heat. Technician said that they don't usually vary from the standard colors (yellow=Y; Green=G; White=W; and Red=R) unless one of the wires is bad. Then they pick another color. In my case, he/she swapped the orange for the white. I should have known that something was up when I was wiring the thermostat when the white wire was the longest and, therefore the easiest to use for the C. If you encounter a wire that looks too good to be true (nice and long), it is probably bad. Pick another in the bundle. It will take you a few minutes to strip the wire but it will be worth it in the long run. BTW, the white wire was damaged but not completely cut. There was enough current to power the thermostat. Just not enough to fire up the A/C.