Two "C" and "Y" wires

[megeletto]

My current system is using a wifi thermostat and I want to run a hard wire to connect to a Nest thermostat (3rd gen).  Furnace + AC system.

Furnace has these wire connections: C, R, W, W2, Y, G

AC has these wire connections: C, Y,

The current setup includes 2 "C" wires and 2 "Y" wires.

I'm planning to run new wire upstairs to the wall where the nest will be located, but I need to know how to handle the double wires.

Can I connect both C and Y wires to a single wire (i.e. new wire that will run upstairs) and then only have a single wire in each nest terminal on the wall?  Or do I need to handle the splices in a different way?

[Patrick_Caezza]

@megeletto 

Where are you finding the two C and Y wires? Is it at the thermostat or in the furnace/air handler?

[megeletto]

These are the connections at the thermostat.  Should those connections have been made at the air handler instead?

I’m planning to run a new wire upstairs so I can bundle the C (and Y) wires to a single wire on the new run— that way the nest will have a single wire at the thermostat instead.  I just wanted to make sure I wasn’t going to cause trouble with that setup.

[Patrick_Caezza]

@megeletto 

Do both wires go back to the furnace/air handler and terminate at the same location or does one wire go someplace else?

[megeletto]

There are two bundles of wires coming into the thermostat— one from the furnace and one from the AC.  Both bundles have a “C” and a “Y”. 

[johnCNA · Accepted solution]

The typical practice is to have the two C & Y wires connected at the air handler terminals instead of at the thermostat. But it will work the way you have it, although unusual. One set of C & Y go to the air handler board.  The other set of C & Y will run to your outside A/C compressor.  What would be better is to take the 2-wire bundle that goes to the outside A/C and move that to the C & Y terminals on the air handler instead of the thermostat. Then just extend the thermostat wiring as needed.

[Patrick_Caezza]

@johnCNA 

You are correct.

I just stuck on thinking it was a dual transformer system which it isn't.

Good job.

[megeletto]

Hey John,

Is this where the C & Y wires from the AC unit should go? https://freeimage.host/i/2b3JaUl

[Patrick_Caezza]

@megeletto 

Yes, that is the exact spot those two wires should be connected to.

You want to add the C wire to the B/C terminal and the Y wire to the Y terminal.

[Patrick_Caezza]

@megeletto 

Ahhhh!

So you have two separate units?

Are they heat only and cooling only?

Can you post a picture of the original thermostat's wiring here and a picture for each unit showing how they are wired?

Ignore this reply.