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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Need help determing the common wire on Rheem rbha-14j

Customer Question

Need help determing the common wire on Rheem rbha-14j
Submitted: 1 year ago.
Category: HVAC
Customer: replied 1 year ago.
Need to identify common wire to connect Nest thermostat. Have run new cable to the furnace - 5 wire. Just need to know where to connect additional thermostat common wire to unit /furnace
Customer: replied 1 year ago.
Unit is electric heating
Expert:  Kcrand36 replied 1 year ago.

good morning I am kevin. On the inside panel of your power connection compartment, there should be a wiring diagram. If you can take a picture of that diagram, I'm sure I can help you decide which wires to connect.

Customer: replied 1 year ago.
Hi Kevin - thank you. Here is a picture of the wiring diagram.
Expert:  Kcrand36 replied 1 year ago.

Sorry it took so long, had a few calls to do this morning. Not a very good diagram I might add, but it appears that you may be able to use the chassis of the air handling unit to connect your neutral to power the thermostat. Are there any other diagrams showing the thermostat and wiring to the thermostat?

Customer: replied 1 year ago.
It's a really old unit. I would need to take a photo of the actual wiring but I am out and about at the moment. How would I connect the neutral to the chassis?
Expert:  Kcrand36 replied 1 year ago.

Your R wire is the hot (24V) feeding the thermostat. connect your 5th wire, I think you were saying to metal surface of inside where all the other wires from the thermostat connect. If you can follow it, there is a transformer inside the unit, one wire is showing feeding R to the thermostat, the other goes to a "ground" on the unit itself. If you can connect in the same place that should be sufficient.

Customer: replied 1 year ago.
Thank you. I will give it a try and let you know how it works out.
Customer: replied 1 year ago.
Just to be sure. I should connect the 5th wire from the thermostat to the same place where the ground wire connects from the transformer to the unit chassis - that would then be my common wire. Is that correct?
Expert:  Kcrand36 replied 1 year ago.

Yes, that would be the easiest to determine based on the print you sent me.

Expert:  Kcrand36 replied 1 year ago.

your 5th wire being the "common" to the stat.