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I currently have two thermostats. 1 for heating and 1 for cooling. The heating one stopped working recently and now that winter is coming, I am interested to combine them into one. Here is what I have right now:

It is my understanding (based on things like: https://www.reddit.com/r/Nest/comments/6p93b4/wiring_nest_with_2_wire_dry_contact_boiler_24v + https://i.imgur.com/QnNPAVW.png) that I should be able to have the nest trigger a simple relay that then turns on/off the furnace + circulator pump. Looking for guidance on how to wire wire that with the existing thermostat that controls the AC system (Learning - 3rd gen)

3 Answers 3

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Rh and W1 to the red and white from the heat would be worth a shot.

Not clear what the black wire is up to, (you could investigate with a voltmeter, or show how it's connected in the old thermostat) but connecting red and white makes heat on all the basic two-wire thermostats I know.

There's already a simple relay triggering that function "in the system" for the standard dumb thermostat to work. The nest just needs to connect them to trigger that relay.

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  • Yeah, Rh and W1 will get the OP up and going provided their Nest can be configured for two-transformer operation Oct 2, 2023 at 2:07
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You can find the installation instructions here, along with what each terminal on the back of the thermostat is supposed to be connected to.

https://nest.com/support/images/misc-nest-thermostat/nest-thermostat-gen3-install-guide-US.pdf

Connecting the white wire of your heating system as drawn to W1 on the thermostat should allow the Nest to control your heating system. Connecting the red wire to Rh should allow the thermostat to be powered by which ever system is active (cooling or heating).

The system should work with C not connected to the heating system (or either system based on the manual), but if you experience issues disconnecting C from the cooling system and possibly connecting the heating system black wire to that terminal may help.

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Looking for guidance on how to wire wire that with the existing thermostat that controls the AC system

http://support-assets.nest.com/images/pro-faq/Nest-Pro-Installer-Guide.pdf

see page 11 (pdf page 12) Conventional 1 Stage Heating, 2 Stage Cooling

Nest page 2 The Nest Thermostat does not use jumper wires, it will automatically jump terminals for you. Do not connect jumper wires to the Nest Thermostat. And it has a built in li-ion battery which is why it does not require power from the transformer (via common wire).

connect heat system red @ ancient stat to NEST Rh. Connect heat system white @ ancient stat to NEST W1. If heat system black wire per your pic is at 24V off the load side of that transformer (assuming your pic is technically incorrect) then you could connect it to NEST C (common) where the blue common from AC is going to, but per your pic it insinuates black is coming from the 120VAC line side of the transformer in which case if it is do not connect it to the NEST (and it should also not be connected to the ancient thermostat). But only connect one common wire from either of the two transformers to the NEST, do not connect both common wires (from heating & cooling) to the C terminal of the NEST.

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