thermostat compatibility check

[jianpingjpzhang]

Please let me know if the nest thermostat is compatible to my existing wiring system. Many thanks

Jianping

[CoolingWizard]

We need some more information. Do you have batteries in that thermostat ?

Ken, The AC Cooling Wizard

[jianpingjpzhang]

Yes, Ken. There are two AA batteries in my current thermostat.

[CoolingWizard]

Try this, remove the batteries of your old thermostat, remount it and see if it power up. If it does you have a common wire present.

Ken, The AC Cooling Wizard

[jianpingjpzhang]

no. it doesn't power up after i remount it back w/o batteries

[CoolingWizard]

So that means you do not have a common wire present. And the B wire is for the heat pump reversing valve.

Ken, the AC Cooling Wizard

[jianpingjpzhang]

okay, so the nest thermostat would not work on my existing system, right?

[CoolingWizard]

In certain situation is the nest learning thermostat third generation, has a ability to use a backfeed of one of the unused control wires to keep the battery charged. However, for the most efficient charging capabilities, you could do is still use a nest thermostat would add a nest power connector to make sure you do. You have the power to keep it charged. In some situations there may be an extra unused wire in the cable behind the wall. The thermostat that can be repurposed for the Common.

Ken, The AC Cooling Wizard

[jianpingjpzhang]

by the way, the G nest thermostat package (i haven't open it yet) says it includes two 1.5v AAA batteries. does it mean i don't necessary need a C common wire which is to supply power to the thermostat?

[CoolingWizard]

That thermostat you have uses batteries, which is kind of the traditional way programmable, smart thermostats were powered. So yes, it could be used. You would just have to change the batteries when they get low. That little black wire that has jumper and yours makes me think that there is a black wire, unused inside the cable behind the wall.

[Markjosephp]

Hello folks,

I wanted to ensure that everything was covered here.

Thanks for assisting here, Ken. Big help!

@jianpingjpzhang, I found a helpful article about how common wire, or C wire, works that would help your system to consistently deliver power to the thermostat. Feel free to let us know if you have more questions in mind.

Best regards,

Mark

[Markjosephp]

Hi jianpingjpzhang,

I wanted to see if you still needed help. Please let us know if you have any other questions or concerns.

Thanks,

Mark

[Markjosephp]

Hello jianpingjpzhang,

I'm just checking in to make sure that you've seen our response. Please let us know if you have any additional questions or concerns before I lock this thread in 24 hours.

Best,

Mark