Attaching common wire for thermostat on HVAC end

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Old 09-05-14, 07:24 AM
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Attaching common wire for thermostat on HVAC end

Aloha, I have an extra blue wire at my thermostat that Id like to hook up so I can use my new thermostat. I need help as to where to connect the blue wire on my HVAC end. I see a blue wire on the transformer, but its connected to two orange wires and I think its supposed to be orange too, according to the schematic. There's also a spare tab on the transformer that is labelled 208 that the schematic says a blue wire should come out of it. Any clues? Am I even looking in the right place?

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Old 09-05-14, 09:56 AM
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The blue wire connected to the orange wires is the high voltage side of the transformer. You do not want to connect your thermostat to that blue wire!! The other side of that transformer should be marked 24VAC and should have two wires attached to that side. They could be any color but common colors are red and blue. One of the wires is the common wire and one is the 24V wire that goes directly to the R/RC/RH terminal on your thermostat. Find that common wire and that is the wire that the blue wire at the air handler and at your thermostat needs to connect to. You should have two brown thermostat cables: one going from the air handler to the thermostat and one going from the air handler to the outside unit. You need to identify which cable is going from the air handler to the thermostat. In your photo I see a blue wire at the air handler that is not connected. I circled the end in the attached photo. More than likely that is the wire you need to wire-nut to the common wire coming from the common wire from the transformer. To help you identify the common wire from the transformer, find the brown cable that goes outside to the condenser. You should see two wires (more if it is a heat pump) wire-nutted that go outside One of those two wires is the common wire from the 24V side of the transformer. I have attached a diagram and circled in red where you will wire-nut the wires.. I also drew colored lines on the wiring diagram you posted so you can visualize the connections to which I am referring.
 
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Last edited by firedawgsatx; 09-05-14 at 10:25 AM.
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Old 09-05-14, 10:54 AM
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You can probably trace that brown wire in the blue wire nut back to 24 volt common on the transformer.
 
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Old 09-05-14, 01:58 PM
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THANK you so much firedawg for a great response and for preventing me from blowing up my new thermostat.
 
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Old 09-05-14, 02:09 PM
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You are very welcome. Based on my experience, most people, myself included, do better with visual aids. I hope everything works out!
 
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