WiFi thermostat - help with C wire


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Old 12-27-14, 01:08 PM
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WiFi thermostat - help with C wire

I have a similar problem. Purchased a new WIFI thermostat that requires the "C" wire for 24V power to the thermostat. I only have the red (RH) wire and White (W) wire. I was wondering since the red wire is a 24V feed, can that be jumped to the "C" connector and use it to power the thermostat? Or if I have to run another wire (which is not a problem) where would I hook it to on my Trane XL80 furnace?
 
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Old 12-27-14, 01:37 PM
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Welcome to the forums.

As promised... a new thread.

James... if you go to your air handler.... you should find a C connection with your existing wiring. You can shoot us a pic or supply the full model number of your XL80.
 
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Old 12-27-14, 01:50 PM
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Hopefully the pictures came through.
My furnace is a Trane XL80 - TUD090A936A0

Tstat is a Honeywell RTH6580WF

Both Tstat wires are hooked to a transformer in the furnace (pictured). Not to the control panel.
 
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Old 12-27-14, 04:01 PM
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You cannot jumper R to C.
Do you have a third wire in the brown jacket of your existing stat wire?

The fan center pic is a bit distant.
 
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Old 12-27-14, 04:50 PM
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On that same transformer you should see a W, R, G, Y and C screw terminals. You need to run a new cable and use the C terminal.

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Old 12-27-14, 04:59 PM
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OK, I ran another set of two wires through the wall. I will look for the "C" terminal on the transformer.
Thank you.
I'll let you know if I find it.
 
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Old 12-27-14, 05:07 PM
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Ok, so on the transformer there are the following.

W and R which we know about
The post across from W is Y
The center post is G
The post across from R is B

That is all 5 posts.
I read somewhere that Trane uses B as the common. If this is the case, then I should hook to B?
 
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Old 12-27-14, 05:11 PM
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This is the post I was referring to.


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Welcome to the forum!! Yes, you should be able to wire it as you propose. A Trane/American Standard system is different from others as the B terminal is the common. Make sure you turn off power to the air handler and the hydronic furnace before moving the wires. I suggest you try the set-up on your old stat before wiring up the new one. On your new stat don't forget to remove the jumper between R and RH before wiring it up.


Read more: http://www.doityourself.com/forum/th...#ixzz3N9OuNRnc
 
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Old 12-27-14, 05:19 PM
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Do you have a meter where you can check from B to R. I don't recall ever seeing a B on a basic Honeywell fan relay center.
 
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Old 12-27-14, 05:22 PM
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Yes, I should bet 24VAC right?
 
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Old 12-27-14, 05:25 PM
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I get 27.7 VAC actually.
So this looks like it is correct.
 
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Old 12-27-14, 05:27 PM
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Trane's B is 24 volt common. This is C on almost every other brand.
 

Last edited by Houston204; 12-27-14 at 05:52 PM.
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Old 12-27-14, 05:29 PM
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Ok, going to hook it up that way, B on furnace to C on Tstat. I'll let you know how it works.
 
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Old 12-27-14, 05:54 PM
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Ok, looks like this worked great!!

Thanks for all the help!!
 
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Old 12-27-14, 05:54 PM
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So that would be a custom Honeywell fan center as the replacement for it doesn't show a B terminal.
 
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Old 12-27-14, 05:55 PM
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Great news. Good job.
 
 

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