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Connecting Common Wire on Air Handler

2334 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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What it looks like to me is that someone has hooked the 2 transformers together and knew enough to get them in the same phase. This works however if a transformer is replaced or any wires are changed then you have the penitential to create 48 volts which will cause damage to components attached to the circuit.

The best way to deal with this is to use more conventional wiring methods or make sure that 24 volts is all that is present.

Phasing is simply accomplished by trying different wire combinations from the transformers with a meter until the output is 24 volts. What is happening is that the 2 transformers are sharing the same wire which works until something goes wrong and then it is a bit more complicated to repair.

The short answer to your question is if you take a meter and the potential between the r terminals at the thermostat and the c wire hooked to the transformer in the air handler reads 24 volts it should be fine, however it is risky to do this.
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