I installed a Nest at home and it worked OK. Recently, I noticed there was a power issue. Google and others suggested adding a power supply by using the existing G wire because there are only 4 wires. Here is what I am planing to do:

On Nest side:

  • disconnect current G wire (green) and connect it to the C terminal

On furnace side:

  1. disconnect G wire(green) from the G terminal and connect it to the C terminal
  2. Add a jumper wire between Y and G.

However, when I checked my furnace, there is no wire on Y terminal, and there is a blue wire which directly connected to C terminal. But on my Nest side, there is no blue wire, instead, there is a black wire which connected to Y1. If I follow the above step and rewire Green wire to C terminal, what is the next? I tried, if I rewire Green wire to C terminal, when I turned on the cooling, the fan was not on, but the condenser is running.

Please provide you suggestions.

For reference, the current connection on Nest side is:

  • Black wire - > Y1;
  • White wire - > W1;
  • Red wire - > Rh;
  • Green wire - > G;

Furnace wiring

  • Y wire is typically used to turn on an A/C compressor. I guess this is heat only? Do you have a volt meter? – JPhi1618 Jul 3 '19 at 20:09
  • You use the word cooling, so I guess you do have AC. That wire in the picture must have a junction where it splits and goes to the compressor. That could be anywhere, but might be inside the compressor's wiring area (pull the power disconnect on the compressor before taking off the panel to check). – JPhi1618 Jul 3 '19 at 20:21
  • There has to be a junction somewhere in this mess...can you find it and post photos of it? – ThreePhaseEel Jul 3 '19 at 22:22
  • @JPhi1618 I do not know where it is. I am not professional and did not find it. I do not want to break anything. – hong Jul 3 '19 at 23:26
  • @ThreePhaseEel If I switch the G wire (shown as green in the picture) to C, how can I get the fan on when I turn on A/C? Will the jumper between Y and G work? – hong Jul 3 '19 at 23:28

If you only move G to C and jump Y and G, that will do nothing since there's no power going to Y currently. You'd have to "jump" from wherever your black wire (Y1) is actually connected, but if you go to the trouble of doing this, you're better off just running a new common wire.

You could try using Venstar Add-A-Wire, but I think this will be tricky in your case because your furnace isn't controlling your compressor relay, so you'd have to follow R and C and run the Y from the Add-A-Wire to the compressor itself (wherever your black wire is currently connected).

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