topic Re: seperate heating and cooling systems in Nest Thermostats

seperate heating and cooling systems

jbny — Mon, 30 Oct 2023 19:31:18 GMT

Hi, I have two existing thermostats, one for my heating system which uses a two wire 24 volt line and the other for my Central AC system which uses 24 volt 3 wires (rc, g, y), I have a nest 3rd generation learning thermostat and I have purchased a third party 24 volt transformer so that I will have a c wire available for power. My plan is to do the following:

install the ac adapter to C and RH (leave the existing RH disconnected),
connect the W for calling heat
fan wire in G
Cooling wire in Y (for calling cooling)
What do I do with the existing RC wire?

I am not sure what to do with the existing RC wire, should I connect it to RC on the nest or leave it disconnected?

Any help would be appreciated.

Thanks.

Re: seperate heating and cooling systems

Patrick_Caezza — Tue, 31 Oct 2023 02:29:13 GMT

@jbny

Your plan is close but not right.

Does the AC side have a C terminal/wire in the air handler? Is there a spare wire going from it to the thermostat? If it does use it to add the C wire to the Nest. If it doesn't have a spare wire, you will need the Google Nest Power Connector and would wire it like the following. Substitute Rc for R in the following diagram

If there isn't a C terminal/wire in the air handler you will need the Google Nest Power Connector and a external 24vac transformer and wire it like this. NOTE: subsitute the W wire/terminal labels in the following diagram with Y1 and Rc for R.

On the heat site you would terminate the RH wire to the Nest Rh terminal and the W wire to the W1 terminal.

Re: seperate heating and cooling systems

jbny — Thu, 02 Nov 2023 13:01:27 GMT

Thanks Patrick but I also have a heating system to consider..............do I need the nest power connector if I hook up the external transformer to the c and rh (leave existing rh disconnected), then hook up w, y, g, and leave rc disconnected?