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I recently bought a Honeywell wifi thermostat which requires a C wire. Old thermostat didn't have one but there are 3 extra wires that I can use for the C wire. I can't identify C wire in HVAC unit as everything is connected via wire nuts, no terminals. Not sure if correct C wire can be easily identified from pics. Thanks

http://imgur.com/gallery/S7jfxjO
 

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Based on your diagram I would say the B designation (unit) is common. Your thermostat picture shows the the C (can't read tag under the blue wire) terminal is wired to the blue wire, it looks like your already set up with a common in the thermostat.


bcommon.png
 

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Thanks for the replies, I'll get back up and get the model #, it's an American Standard.
The blue wire is connected to the B terminal and the green wire is connected to the G terminal.
 

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Don't bother getting the model number, it's on the diagram. Here is the labeled designation for the low voltage wires. Yes, B (unit) is common. The blue wire in the old thermostat should not have been connected to the B terminal. I suspect you had a short between R and B (stat) that didn't have an affect on the stat operation.


ycz.png
 

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Interesting, so you think connecting the blue wire to the C terminal on the new thermostat would work. Its connected to the B terminal on the old one and the new thermostat has a O/B terminal which I looked up is for the heat pump or valve s
 

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The blue thermostat wire should be connected between the C terminal (new stat) and the B designation (unit). Your new stat should then work fine. I bet when they first connected the old thermostat and tried the heating mode it fried the contact between R and B and blew the control fuse, many years ago, and since the contact got fried a new fuse wouldn't blow. Must of been some butt scratching going on.:biggrin2:
 
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