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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 at 23:25
This question covers a specific approach than was not covered in the other question. – Tester101♦ Feb 26 at 17:45

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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 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 at 18:41
This seems shoddy and wrong. Why would there be a power surge, that only affects your furnace? – Tester101♦ Feb 26 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 at 17:54

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