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I Purchased the Honeywell WiFi Prog Thermostat to replace my Round Digital Honeywell Thermostat. I have separate Heating and Cooling (Heating is a Weil McLain Boiler for Hot water baseboard and Hot water(no tank) and Cooling is a separate unit that was added later.)

Wires:

RC Cool
R Heat
W Heat
Y Cool
G Cool

3 wires from AC unit and 2 from Heater.

If I were to go the separate transformer route to power the thermostat, where would I land the 2 wires?

C and ???

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possible duplicate of Options for adding "C" wire to thermostat –  Steven Feb 23 '13 at 23:25
    
This question covers a specific approach than was not covered in the other question. –  Tester101 Feb 26 '13 at 17:45
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2 Answers

If you use a 24VAC transformer, you only need to use one of the wires for the C. You can either do nothing with the other wire, or use it for Rh/Rc instead. One benefit to this is that the thermostat is not powered by the furnace, so in the event of a power surge to your furnace, you won't blow your thermostat.

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I've seen that recommendation elsewhere, including the manual. It doesn't make sense to me from a complete electrical circuit standpoint, to leave one end of a transformer floating.. –  HerrBag Feb 25 '13 at 17:46
    
It's not a complete electrical circuit, you're just providing the return (C wire); the power is provided by the furnace's transformer via Rh/Rc. Think of it like stealing a neutral from another circuit. –  Steven Feb 25 '13 at 18:41
    
This seems shoddy and wrong. Why would there be a power surge, that only affects your furnace? –  Tester101 Feb 26 '13 at 17:51
    
It happened to me - power surge (I assume) blew the fuse in the furnace but also blew my thermostat out - burn marks on the back of the LCD panel! For some reason the furnace was the only device damaged - all of my arc-faults tripped too. And of course it happened on one of the hottest days of the summer, on the Sunday before a long weekend when everything was closed :) –  Steven Feb 26 '13 at 17:54
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Bear with me on this story. I believe the above "recommendations" are wrong. Simple reason is you need to have a complete circuit as you stated. I purchased a Honeywell RTH6500WF for installation. Much to my surprise, I found out you had to have a "C" terminal. The HON adviser stated I needed to add an additional 24 volt transformer - which I did, a HON AT72D 1683. I ran it off the 120V for the furnace so that it is off when the furnace was off. Heat and wireless worked great. That was a year ago. Summer came. I turned it to AC and had nothing. The "fan on" also never worked. Using the "R" from the furnace and touched it to the "G" and the furnace fan went on. Touched "R" to "Y" and the outside condenser went on. Problem is in the HON Thermostat!

So I call HON again. Seems that they are not really focused on helping us DIYers. First adviser said he could only help me with programming. Electrical work must be done by an experienced HVAC person. I understand, but I NEED AC. So hung up and called back. Next adviser said basically the same thing. Have your HVAC person add a "C". I said I have a "C" but where does the "Load" or "R" connect. Sorry cannot help you.

So we moved to trial and error based on various videos on U-Tube. The HVAC call out fee is 250 USD so it is more than a new thermostat and it is only 24 V. So here is what I found. The "C" lead from the added 24 volt transformer goes to the "C" on the thermostat base. I then to the "Lead" or "R" from the added 24 volt transformer to the "R" block. I hooked the "R" from the furnace and hooked it to the "Rc" on the thermostat base. Nothing on the thermostat. (Hooked up the other way around gave me heat but no AC). So I said my prayers and found the jumper. I jumped "R" and "Rc" on the thermostat base and EVERYTHING WORKS. I have AC, manual over ride on the fan and heat. Good luck on getting your system up and running.

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-1 This appears to be a story about how you're repurposing other wires to lose functionality rather than a description of the proper method to add a new C wire. Stories rarely make a good answer to the OP's question. –  BMitch Jul 7 '13 at 18:11
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