C Wire Older Thermostat


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Old 01-23-17, 06:03 PM
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C Wire Older Thermostat

I have been reading several threads on this forum but have yet to find a solution for my home. I have an older Gas Furnace and Air conditioning unit, there is no control board per se for the thermostat, just wires that are connected to other wires.

I am trying to find a way to hook up a C wire, can I simply connect this to Ground in the furnace or is this a 24V supply wire?

The schematic that is attached to the unit
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The unholy wire mess, the brown wire harness on the left goes to the AC unit outside. The brown wire harness on the right is going to the thermostat.
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  #2  
Old 01-23-17, 06:19 PM
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Welcome to the forums.

You have a C connection there.
Do me a favor and carefully pull the wiring apart a little.
I need to see the splices coming in from the A/C condensor.

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Old 01-23-17, 06:41 PM
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Thank you, a diagram I did of the connections, would the red wire to/from the condensor be the C wire?
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Picture close up
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Old 01-23-17, 08:47 PM
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You drew a nice representative diagram but you didn't put any of the furnace nomenclature on it.

For example.... the gas valve has three letters on it.
P = pilot valve
C = common
M = main valve

Notice the red wire from the condensor connects with the C connection on the gas valve which also goes directly to one side of the transformer. That is your C connection.
 

Last edited by PJmax; 01-23-17 at 09:28 PM. Reason: typo
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Old 01-23-17, 09:01 PM
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Thanks, I am still trying to figure out what all the connections are. now I just need to go buy some 18/5 wire. The thermostat is only 10 feet away in a single level home so I just need to run it through the attic.
 
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Old 01-23-17, 09:26 PM
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The heavy yellow wire to the gas valve is common according to the wiring diagram.

If the thin yellow wire lands on the same terminal it will also be common.
 
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Old 01-23-17, 10:32 PM
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The two center taps are joined anc connect to red wire
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Old 01-23-17, 10:41 PM
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Yes.... and they are all common.
 
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Old 01-23-17, 11:15 PM
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Thanks all, looking forward to getting this wired up
 
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Old 01-24-17, 08:37 PM
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Thanks everyone, ran new cable and everything works great.

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  #11  
Old 01-24-17, 08:41 PM
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Nice job..... well done.
 
 

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