DIY Home Improvement Forum banner

Connecting Common Wire on Air Handler

2339 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.
See less See more
1 - 1 of 13 Posts
Will it work? As you described it, and you haven't missed anything, then yes it will work. As it's been mentioned, the 2 transformers are in parallel. I'm on the side of Yuri and Been, and would pull a new cable.

Make sure you don't forget that you have 2 transformers hooked up like this when things don't work. Any pro, or us would need to know that for troubleshooting reasons. You're also more likely to blow both transformers if something shorts instead of just one.

1 - 1 of 13 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.