Nest learning thermostat vs. Nest thermostat

[shous77]

Hello -  I purchased a Nest thermostat and got it all hooked up and it appears it needs a C wire...  I am not interested in going that route.  I am in the process of returning it now.  However, it appears if I step up to the Learning thermostat I will be good as is and not need a C wire.  Can someone help confirm this before I go out and buy another one?

Heat Only, 2 Wire (Red & white), Oil, Forced hot water/baseboard.  Previously used 'normal' cheap electric programmable tstats...

Thanks!

[CoolingWizard]

The Nest Learning Thermostat 3rd Generation has the ability  to work without the COMMON wire.  It does this by using a sort of back door approach. It uses the Y wire to travel back to the transformer. If you think about it, the AC electrical contactor has a coil that gets energized by the Y conductor.  One side of the coil is connected to Y and the other side is connected to COMMON.  So, as long as the current is low enough, the contactor will not energize.  The Nest Learning Thermostat 3rd Generation uses a very small current via this path to slowly charge the battery.  For better charging performance, I would purchase the Nest Power Connector kit. 

And now you know,

Ken, The AC Cooling Wizard

[shous77]

Thanks for the quick reply.  When you say "Y wire to travel back to the transformer", can you clarify "Y wire"?  Is this something internal to the thermostat?  Or does Y represent either my Red or White wire?  Just confused as this is only a 2 wire heat only system.

So, the Common wire is only used to charge the thermostat batteries?  I am assuming based on the one I had setup it was not getting enough power from just red/white to function/report correctly? Right?

I just saw a bunch of really negative reviews/feedback on the Nest C wire kit...

[CoolingWizard]

The heating system you have has a step down transformer as well. What the thermostat would do in your particular case is know that you only have a heating system and therefore there would be an R wire and a W wire and perhaps a G wire.  When the furnace is not in a heating call operation, the thermostat will use the W wire to reach the common side of the transformer and perform the same function that it would with the Y wire going to an AC contactor. Use a small current and charge the battery.  

Ken. The AC cooling Wizard. 

[shous77]

OK, makes sense.  So, I guess bottom line is I shouldn't have any issue with 2 wire using the learning tstat as is.  Looks like my issue is primarily only on the 'non-learning' unit...  Thanks.

[EdwardT]

Hi folks,

@CoolingWizard, thanks for the help!

@shous77, how's it going with your Nest Thermostat? Still need our help?

Thanks,

Edward

[Jake]

Hey there,

I wanted to check in and see if you are still in need of any help. Please let me know, as I would be happy to assist, and answer any questions you may have.

Best regards,
Jake

[Jake]

Hey there,

I wanted to check in, and let you know that I will be locking the thread in 24 hours.

Best regards,

Jake

[shous77]

Got it, thank you!

[Jake]

Hey shous77,

I am glad to be of help. With everything being good to go, I am going to lock the thread at this time. If you have any questions from here, please feel free to create a new post.

Best regards,

Jake

[CoolingWizard]

The Nest Thermostat has to have special hardware and software to be able to use the back door approach I described.  I know it works with Cooling Systems and the Nest Learning Thermostat 3rd Generation, however, I am not sure it works with heating only systems.