Pulling power for 'C' wire on new thermostat

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Old 08-26-14, 02:59 PM
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Pulling power for 'C' wire on new thermostat

So this is probably a pretty simple question but Ive never bothered with wiring since I've always left it to boyfriends or relatives. I want to run a 'C' wire in order to install a new thermostat. The run to the furnace is pretty simple. I have a two wire cable and already know what terminals to use on the thermostat. Thats assuming I'm running from a separate transformer plugged into an outlet.

I want to know if I can simply run the wire to the transformer on the furnace itself. I'd be sharing terminals and I dont know if thats ok. if I can, which terminals would i use? The pictures shows the transformer. The black and white wires are the ones that currently run to the old thermostat. I need to keep those but also supply 24v constant power to the new unit. Where the black wire is connected seems to be the only terminal with voltage readings. Thankyou so very much for any help!
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Old 08-26-14, 03:08 PM
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Here's the diagram or the transformer itself

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Old 08-26-14, 03:55 PM
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Moved to Heating and Cooling for better help.
 
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Old 08-26-14, 04:31 PM
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Welcome to the forums.

Isn't the screw terminal in the bottom right of your picture labeled as C ?
Sharing terminals ?!?!? That thing looks like a sputnik with wires shared everywhere.

Just make sure to turn power off before changing wiring and don't allow wires to touch.
 
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Old 08-26-14, 04:58 PM
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Originally Posted by PJmax View Post
Welcome to the forums.

Isn't the screw terminal in the bottom right of your picture labeled as C ?
Sharing terminals ?!?!? That thing looks like a sputnik with wires shared everywhere.

Just make sure to turn power off before changing wiring and don't allow wires to touch.
Thanks! Yes that one is labeled C. There doesn't seem to be any voltage off of it though. The only one that produces a reading is the one labeled R with the black wire connected to it. I'm assuming if I don't see voltage off that it won't supply power to the thermostat?
 
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Old 08-26-14, 05:01 PM
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R and C are 24vac. Your thermostat only switches 24vac via the red wire.
The C wire is used ONLY to power the thermostat electronics.
 
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Old 08-26-14, 05:24 PM
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Oh I see! So I would connect one wire to 'R' and the other to 'C' and it should work?
 
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Old 08-26-14, 05:34 PM
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Yes.... if you only connected the R and C to your thermostat..... it would display and show activity but nothing would happen back at the furnace until you connected other wires like W for heat and Y for A/C and G for fan.
 
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Old 08-26-14, 06:28 PM
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What type is the new thermostat?
Geo
 
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Old 08-27-14, 07:54 PM
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Works perfectly thanks! It's a Honeywell wifi thermostat
 
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