topic Re: Gen 2 worked. Will replacement Gen 4? in Nest Thermostats

Gen 2 worked. Will replacement Gen 4?

Dpinton — Sun, 23 Nov 2025 00:35:56 GMT

My Nest Gen2 two worked fine as wired above with my baseboard heat. Before ordering the replacement Gen 4, I did the compatibility check and it appeared to be fine. When it arrived, I scanned the barcode on the unit with the app and it showed as incompatible. I have a RH and a W1 wire - it’s my Gen 2 wiring - nothing has changed.

Will it work or will I blow up this new unit?

Re: Gen 2 worked. Will replacement Gen 4?

Etrotochaud — Sun, 23 Nov 2025 00:44:47 GMT

I’m following because I am now showing that I need a C wire when I did not before.

Re: Gen 2 worked. Will replacement Gen 4?

Dpinton — Sun, 23 Nov 2025 03:24:38 GMT

Hi - OP here. I hooked up my Gen 4 using exactly the same wiring as my Gen 2 (Rh and W1 wires only). It did not blow up. 😬

But I did find a Reddit thread (and Google’s Ai also backed this up) that says you can run the thermostat with 2 wires for heating only. But you do need a C wire in order to run your COOLING system AND your heating system. Hope this helps.

Re: Gen 2 worked. Will replacement Gen 4?

jenniffert — Tue, 25 Nov 2025 17:37:45 GMT

Hi @Dpinton,

Thanks for reaching out to the community! I understand you were concerned about the compatibility of your new Nest Learning Thermostat (4th gen) using the same wiring as your previous model. You mentioned receiving an incompatibility message in the app after scanning, despite the initial compatibility checker tool showing it was fine.

I appreciate you sharing this important update on the case and for your input to help our community members with their concerns about the C wire. I'm glad to hear the new Nest Learning Thermostat (4th gen) unit is working correctly using your existing Rh and W1 wiring.

If you need more assistance, please let me know.

Best regards,

Jenniffer

Re: Gen 2 worked. Will replacement Gen 4?

jenniffert — Tue, 25 Nov 2025 18:06:00 GMT

Hi @Etrotochaud,

Thanks for posting in our community! I understand your concern about why now your current setup indicates a C wire is required for your Nest Learning Thermostat (4th gen), which was not the case previously. I can definitely help you explain why it is necessary in some cases.

Some systems—including heating-only, cooling-only, zone-controlled, and heat pump systems—require a C wire or a compatible power accessory, like the Nest Power Connector, to consistently deliver power to your thermostat. The thermostat typically uses power sharing to power itself. If power sharing doesn't work well without a C wire or power connector, the thermostat relies on its internal rechargeable battery for power and might lead to a faster decline in battery health. To learn more, you can see these articles: Learn about the common or C wire and Nest Power Connector.

Keep me posted.

Best regards,

Jenniffer