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Connecting Common Wire on Air Handler

2341 Views 12 Replies 5 Participants Last post by  supers05
Hi everyone. I am trying to connect the Common wire to my Nest thermostat, so it charges the battery. Fortunately, the therm wire has a 5th wire running from therm to the air handler. It's currently disconnected at the therm. What is confusing me is that this 5th blue wire is connected to the R terminal on the air handler.

There is a second wire also connected to the air handler's R terminal, which travels a bit and then connects to the Rh line from thermostat and the R from the boiler at a wire nut.

Outline of the Thermostat Wiring

G (therm) connects to G (air handler)
Rh (therm) connects to R (air handler) and R (boiler). These three Rs connect in the 'junction' photo I've linked to below.
C (therm) was connected to the R (air handler); I disconnected the side at the thermostat; other side is still at R on air handler
W (therm) <--> W (boiler)
Y (therm) <--> Condensation pump's Red. Condensation pump's White goes offscreen to A/C compressor Red. Compressor's white connects to air handler's Common terminal. From what I understand, this is done so that if condensate tank overfills, 24vac is removed from the A/C compressor.

(I'll post a link to photos of the wiring in a minute.)

I've been cautioned that this situation is not optimal because the boiler R and the air handler's R are connected to each other and go to a single Rh at the thermostat. That's worked well because there isn't a Common.

So the main question is how to determine whether I can disconnect the Blue 5th wire that runs to the thermostat from the air handler's R terminal, and move it to the C terminal. Then attach the other side to the C terminal on the Nest. Or would this compromise the systems because the boiler and air handler would now both be connected at the C and R.

Thanks so much for the help.
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Here is a Photobucket link with images of:
Air handler connections and notes
Junction (air handler, boiler, thermostat lines) with and without notes

s482.photobucket.com/user/jstricks/library
It does, but I don't have separate lines from the boiler and air handler running to it. The house was bought with the two systems' reds connected, and then a single red line run to the thermostat connected to Rh. That's been the setup for years with no problems. Just trying to confirm that connecting the Common line won't upset the balance.
Thanks Yuri. I have traced the wires, maybe these images and mores help.

Here is a Photobucket link with images of:
- Air handler connections and notes
- Junction (air handler, boiler, thermostat lines) with and without notes

http://s482.photobucket.com/user/jstricks/library
Sorry, meant 'notes' not 'mores.'
Over and over again, I read about examples where the boiler and air handler share a connection at Rh. Nest even directly addresses it here: https://nest.com/support/article/Ho...rmostat-had-a-short-wire-connecting-Rh-and-Rc

How could that be the case unless the boiler R and air handler R were connected together?
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