enter image description hereMy nest was installed with no c wire and it lost power. At this point the tech looked at the wiring diagram in a/c unit outside but I do not know what he did. A/c worked but later nest had no power. I had turned off the pilot from furnace. I wonder if this affected.

Then a 2nd tech came and connected the brown wire to c (which was previously not connected) and the yellow wire to C also. Tech did not do anything to outside a/c and I am wondering if he needed to adjust the wiring at outside unit so that a/c works. Fan works, heater works but a/c does not turn on.

Thanks for any help figuring this out!

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  • Can you get us a better shot of the wiring at the furnace/air-handler end, especially one that shows the wires going to the A/C as well? – ThreePhaseEel Jun 20 '19 at 22:24
  • I will try. I am out of town but will get back next week and will try to get more pictures. If the yellow wire is the one that goes to the condenser, then the yellow cable should be connected to the y terminal from the furnace/air-handler right? – Polina Jun 22 '19 at 1:32
  • Can you post a photo of the wiring from the outdoor unit to the air-handler? Something's missing here... – ThreePhaseEel Jun 23 '19 at 15:42

I agree with @RetiredMasterElectrician that if the repairs were attempted by licensed HVAC contractors, they should have no problem diagnosing and fixing this and should do so free of charge since it will have taken them 3 tries to do so.

If the yellow wire at the control board was previously connected to Y, I have no idea why he would've removed it. That will be your call for cooling (turns on the compressor) and probably why your A/C stopped working (but fan/heat work which are controlled by G and W respectively).

Yellow should be reattached to Y, leave brown connected to C, and everything should function. Do not jumper Rh to Rc. The Nest has an internal jumper when it is required. Plugging into the Rh terminal is appropriate in your case.

Nest 1 stage heat, 1 stage cool wiring

Before you had a C wire, the Nest will use the furnace's R/W wires to charge its internal battery. After you turned off the pilot light, it was probably only running on battery and couldn't maintain its charge. Using a C wire should fix that issue and give the Nest constant power.

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I am really not sure you have asked a question. Your heading mentions a diagram but you have no diagram just pictures. So when you send in better pictures per @ThreePhaseEel request you might see if you could post up some sort of diagram.

If you real question is, Why isn't my cooling coming on? Then one of the reasons is that you haven't connected you're control power to Rc which should be jumpered from Rh (which may be internal the t'stat base, but no diagram). Unless your heater and ac have two different power sources.

My question is, if you have two people that came out and worked on your units and didn't fix a fairly simple T'stat control. How can you call them "Techs"? If these people came out and work for a licensed HVAC-R contractor, you need to call the owner and have him send someone out there who knows what they are doing and make it work. No reputable contractor should leave you in this condition. It should be covered in his warranty.

Hope this helps.

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  • Thank you both for the input. I replaced the picture but that is the only thing I have right now because I am not at the house. I am flying back to the be home when the technician/owner of the company comes back to repair the wiring. I am trying to understand as much as possible to make sure this time they leave the unit correctly wired. Basically, yes, my question is Why isn't my cooling coming on? – Polina Jun 23 '19 at 15:26

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