"Regular" Nest Thermostat (2020) with old furnace and 2 thermostat wires

[cbroverman]

Hi, I've spent days trying to figure out if I can use this Nest Thermostat (2020 version, not E, not learning 3/4) with my old boiler system (2 wire T/T) and would appreciate suggestions. I currently have the thermostat hooked up with the red wire in R and the white wire in W, and added an adapter plugged in below the thermostat (one wire is in the C connector of the Nest and the other wire is in the R connector (along with the red wire), per suggestion from another user. I do not have a control board on my old Utica boiler so I cannot use the Google Nest Power connector. It seems to be working without a real C wire. I'm using the external adapter. Has anyone else used this configuration?  How do I know if the batteries are draining and need to be replaced?

[CoolingWizard]

@cbroverman , your nest thermostat will alert you when the batteries need to be changed. And your particular case your furnace is controlled by simply connecting one terminal T to the other terminal T. By attaching one of those T’s to R and the other to W, you should  be able to make it work just fine. 

AC Cooling Wizard

[cbroverman]

Thank you for your reply! I did attach those two wires coming out of my old thermostat accordingly. (those are the ones connected to the T/T contacts. However, there is no constant power being sent to my thermostat since there is no common wire. How long will the alkaline batteries last in the Nest thermostat (2020) since they are what is powering the thermostat (and not the boiler)?

[AxelD · Accepted solution]

Hi cbroverman,

Thanks for posting in the Community! For systems that don't use a C wire, the average battery life is about 3 to 6 months. However, depending on the Wi-Fi activity on the thermostat, the battery life might last longer or shorter.

Let me know if you have any questions.

Regards,

Axel. 

[cbroverman]

Thanks for that info; exactly what I was looking for.