Nest thermostat and current HVAC system

Orgchem

I need advice as to whether a nest gen 3 thermostat will work and have sufficient power to run off the TACO switching relay that I currently have. My current system which has a battery operated thermostat is highlighted in blue in the attachment. What I intend to do is run a c-wire from the TACO com on the board to the C terminal on the nest. Thanks in advance of your help.

SteveSan

You are correct. Please pre-charge the Nest t-stat before wiring to the Taco switching Relay.

Orgchem

Thanks for your feedback. However I thought the addition of a c-wire to the nest would energize it immediately. Why does it have to be pre-charged?

SteveSan

We reached out to Nest, because of the internal battery that the Nest has it needs to be pre-charged. If not, the battery when it wears down it will try to "steal power" from our boards and hence the name power robbing t-stat.

Orgchem

Sorry to keep changing the ground rules but I took some voltage measurements today and found out that my current thermostat(RobertShaw model) is actually wired to a 24 volt transformer in the air handler (First Co. 48MBXB-HW). Attached wiring diagram for air handler. I guess my question now is where I actually attach my c-wire from the Nest thermostat when I install.

1. Still to the TACO SR503 24 volt com terminal?
2. To the com terminal on the air handler board?
3. If yes to #2 can I wire cap it to the brown/red wire shown in my first attached wiring diagram (have no free com terminals on the air handler board)
4. Or something else?

Much appreciate your help!

HVACNUT

You can use the common from either the A/H or the Taco zone board. Just remember you'll have 2 reds at the Nest. One for heat (R) and one for A/C (Rc).
I dont think you need to pre charge the battery when its 24v powered. It becomes "power robbing" if there's no common.

Steamhead

Actually, if the Nest is fed from the air handler transformer, and you run the Common wire to the Taco board, it won't work. It has to go back to the same transformer. So the Com terminal on the air handler is the proper place to connect the Common wire. Wirenutting it to the brown wire in the diagram that goes back to the transformer should work fine.