Where to install C wire?
On dual transformer systems, Nest expects the common wire to come from the cool side (Nest Pro Guide, bottom of page 20); look towards your A/C not your boiler.

If there is a common terminal on your A/C control board and


18/5 wire bundle - use one of the extra wires to connect the common terminal to your Nest
18/3 wire bundle


Use Venstar Add-A-Wire (or equivalent) or
Run a new common wire (might make more sense to just run a new 18/5 or more bundle)



If there is not a common terminal on your A/C, you'll need to find the wiring diagram and locate the 24v transformer. Pull a common wire off the negative side of the secondary side (24v) of the transformer and connect to common terminal of Nest (or use Venstar Add-A-Wire). You'll need to verify that the added load of the Nest won't exceed the VA rating of the transformer (it shouldn't). If this is over your head - call an electrician.