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I purchased a Nest Thermostat E and found that I don't have C-wire. With the research, including adding venstar to HVAC and Youtube video by Venstar, I tried to add c-wire to my Nest Thermostat E with the Venstar Add-a-wire (ACC0410). However, I can't find exact match with my HVAC control board system. So, I hope that someone experienced with these can help me what to do.

As you see below, it is different from Venstar Youtube video setting. The video doesn't have lines from compressor but mine does. Could you please help me how to connect my Venstar ACC0410 to my control board and what to do with Nest Thermostat E setting?

First, here is my current HVAC control board setting as below.

my control board

And, here is my current setting with my Nest without C-wire:

Nest Thermostat E setting

The final implementation is summarized below.

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Just follow the yellow brick road to your C terminal

Based on the fact that you have a two-wire cable apparently running out to your compressor based on the fact one wire of the cable is landed on Y, we can find your C terminal easily. In order for the air conditioner to come on when called, its contactor coil has to connect between Y and C as the thermostat applies 24VAC to Y to call for cooling. So, the other wire from the compressor cable must connect to C, and looking at the labeling on your control board, lo and behold, it does! Furthermore, HVAC terminal blocks are designed to have 2 wires under one screw, on opposite sides of the pressure plate, so the fact there's a wire already there is no obstacle to your plans.

From there, it's no different than how the videos describe it.

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  • Thank you @ThreePhaseEel for answers. I updated my post with diagrams based on your answer. As you see in the last diagram, at C, there will be 3 wires and at Y, there will be 2 wires. Is it ok? – Jong Lee Oct 7 '20 at 17:45
  • @JongLee -- ah. 3 wires under a single screw terminal will probably still work (it's a trifle naughty, but in the grand scheme of things, nobody really cares about thermostat wiring) – ThreePhaseEel Oct 7 '20 at 23:44
  • Thank you @ThreePhaseEel. I will try it this weekend. Also, If found a good entry about how to add 3 wires to 1 terminal. diy.stackexchange.com/questions/80263/… – Jong Lee Oct 8 '20 at 19:20
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This is the summary of the answer to my question. Thank you for @ThreePhaseEel.

I depict the diagram based on ThreePhaseEel's suggestion below. The first image is the my current setting, and second image is the one with add-a-wire by ThreePhaseEel's answer.

enter image description here

enter image description here

Also, I had a question about three wires on terminal C. According to the @ThreePhaseEel's answer and the post (How many wires can be connected to the C terminal at the furnace?), I implemented the setting and tested. Heating, Cooling, and Fan all works well.

The following image is the final result. Note that I did a pigtail with 3 wires at C terminal.

enter image description here

Thank you again, @ThreePhaseEel

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