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Now it all makes sense. I was under the impression that the C wire was a hot wire coming from the unit to power the stat. When instead it is a common (neutral/ground) wire, hence the "C".

Thanks for the input. Looks like the surest/safest thing to do is run the common. I don't have one now so ill have to pull a 7 conductor thermostat wire set from the unit to the thermostat location. I am still curious why using the Rc to also power the stat is not viewed as acceptable by some...

ctfjr, See my other thread about using a single thermostat to replace my two existing thermostats. That will give you a better idea of my existing wiring, and what im trying to accomplish. http://www.garagejournal.com/forum/showthread.php?t=281999 Let me know what you think.

I haven't purchased the thermostat yet, I'm just getting everything in order first (knowledge wise). I would rather not use the fan wire (green in my case) as the common. I like the idea of being able to independently turn on the fan to get air movement throughout the house.

Ok, that makes sense. I had not taken into consideration the current draw of the thermostat and weather there was enough leftover current to tell the unit to start/stop. In my situation I have two separate (simple) thermostats (one for heat, one for cooling) and I'm looking to replace them with...

If there is not a 24VAC "C" wire coming from the heating/AC system , but there is already +24VAC (27VAC in my case) coming to the thermostat via the red Rc wire, why cant you jump the Rc over to the C terminal to power the thermostat? Or can you without any problems?

can anyone provide some assistance?

Alright guys, I have verified the voltages at both thermostats. I'm getting 27 VAC at the heating thermostat & 30 VAC at the cooling thermostat. My research has lead me to the following conclusion. I need a thermostat that I can install in a 5 wire configuration, with the following...

Without measuring the diameter of the conductor, I would estimate the wire including insulation is approx. the diameter of the lead in a No 2 pencil.

How do I verify that the heating system is 24volts? Is it as simple as checking the voltage between the red and white wires at the heating thermostat? Do I also need to verify the cooling system is also 24volts?

Well I cant upload my pictures because they are larger than the allowable file size, so I'll just list the colors of the wires. Heater Thermostat: 1 white wire 1 red wire AC Thermostat 1 white wire on same terminal as 1 green wire 1 red wire I didn't think about checking the voltages until...

brewchief, I'll get some pictures of the wires going to each thermostat and post them this afternoon.

I have quite an old heating/AC system with two individual units/thermostats. The AC unit is all contained outside the home (blower within), with the large inlet/outlet ducts (~12" in diameter each). The Heating unit (electric) is a separate unit all contained inside the home (also with its...