Adding C to my Nest thermostat

[Crewel]

hi all,

I currently only have 4 wires running to my nest. I see that my furnace supports what appears to me as a common wire. It is currently being used by the AC unit outside. One wire on "Com 24" and the other on "Y"

My question is: can I add the unused blue wire from the thermostat wire (you see it wrapped round the outside) to the "Com 24" on the PCB? I am afraid to do this because the AC unit outside is already plugged into it. I wasnt sure if that would mess it up.

Any thoughts?

 

[Reach4]

Yes.

 

[Sponsor]

 

[Stuff]

No problem. Done all the time. Look how there are two wires on Y.

 

[Jadnashua]

A typical thermostat is just a fancy on/off switch...it only connects the hot lead to the one needed to either turn on the a/c unit, or the heating unit...the other side of the power (always takes in/out), is after the control in the device (the furnace or a/c unit). The Nest thermostat (and many other smart ones) work best if they can get external power...that takes both in/out. The common is the other half of all of the control lines. As long as the furnace's transformer is big enough to handle the load (and pretty much all of them are), it can power your thermostat.

 

[Crewel]

Thanks all. I tried it and it worked as expected. Nest now sees the C wire supplying 25VAC.