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So I'm installing a wireless thermostat on my HVAC. To do this though I have to figure out which wire coming from the HVAC goes upstairs to where the wired thermostat was and which wire goes to the roof to the compressor. I live in a condo so it's not the easiest thing to do.

Currently the wires from the disconnected thermostat are sticking out of the wall upstairs. The problem is downstairs the two sets of wires going into the HVAC are exactly the same, same color and type of wire. Is there a way to trace that wire upstairs to the wires downstairs and differentiate between the upstairs wire and the roof wire? I afraid if I do it by trial and air I could possibly blow the wireless thermostat controller or the compressor or get electrocuted. I really dont know what to do and I'm hoping it's fairly simple. I've tried to get an electrician over but everyone I've contacted is busy for at least a week or more and costs a decent amount of money. I'd love to be able to do this myself.

Thanks so much, all help is very appreciated.
 

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Currently the wires from the disconnected thermostat are sticking out of the wall upstairs.
The problem is downstairs the two sets of wires going into the HVAC are exactly the same, same color and type of wire.
Is there a way to trace that wire upstairs to the wires downstairs and differentiate between the upstairs wire and the roof wire? I afraid if I do it by trial and air I could possibly blow the wireless thermostat controller or the compressor or get electrocuted. I really dont know what to do and I'm hoping it's fairly simple. I've tried to get an electrician over but everyone I've contacted is busy for at least a week or more and costs a decent amount of money. I'd love to be able to do this myself.

Thanks so much, all help is very appreciated.
Power down, label any two wires at the source end, disconnect them and hook them to a battery, then use a voltmeter to find out what two t'stat wires correspond, including polarity. Label those wires. Repeat until all wires are accounted for.
With four wires at the t'stat end you'll need a max of six tests or less for each two wire test.
This assumes that all wires have continuity and no wires are shorted together.
 

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You know I think I was wrong about what is going on. The wires from the HVAC both come out of the unit then go into the wall. One goes to the roof to the compressor which I can not get to and can not disconnect from the power. The other goes upstairs to where the thermostat used to be. The problem is that wire I believe is buried in the wall and the wire that is sticking out is the power that goes to the circuit box. I only needed power because I'm putting in a wireless thermostat. I had that wall painted and the painter patched the wall without leaving those cables exposed. What I need to do now is figure out which wire the thermostat was connected to so I can cut that line and hook it up to the wireless controller device. Does this mean I would have to break into the wall to get that cable to test or is there some other way I could do this? Friends I've talked to seem to have no clue how I could do this with no access to the ends of the wires. Is there a way to tell which one has something connected to it and which one doesn't? If I can't cut the power to the compressor would I get electrocuted if I cut both cables? If the cables were cut would I be able to figure out which is hooked to the compressor?
 
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