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setting up nest

suelesko
Community Member

Do I need a C wire to install

12 REPLIES 12

straitshooter
Community Member

It's best if you do but, according to the specs you can install and run your hvac  without.  It will use its internal battery to control the system.  The downside to this is, you will need to occasionally recharge the battery.  

BrKorb
Community Member

NB: "occasionally recharge the battery" will turn out to be quite often. I recommend an external 18V dc power supply.

Negatory on "an external 18V dc power supply".  Assuming we're discussing a Nest Learning Thermostat in the USA, its specs indicate 20-30V AC.  Nominal voltage via a system transformer, the control circuitry is 24V AC.  Dunno if maybe there is a different voltage thermostat for a non-USA application.

Thinko. Yep: 24V dc. Still, the batteries die too fast. My initial install used the 3 connected wires ("C" not connected) and it was less than a month. I was able to find the common connection in the FAU and wire up a "C" connection. That lasted about 8 months. Still not ideal, but I was willing to live with it. The next battery pair lasted 2 days, so I may yet go get a 24 (TWENTY FOUR) volt power supply.

Interesting.  Not sure I can offer much more help.  It would seem to me that if the thermostat has been connected to a "C" wire (mine is), batteries are no longer needed; except for retaining settings in the event of a system power failure.  If you are replacing batteries (such as AA, AAA or otherwise) you have a Nest thermostat that I'm not familiar with.  Batteries in mine are non-replaceable in the conventional sense.  I never have to replace any battery so long as I am connected to the "C" terminal.  My thermostat and system are nominal 24 volts AC current, not DC.

That behavior was my expectation when I got the beast. Mine takes two AA batteries.

Markjosephp
Community Specialist
Community Specialist

Hello folks,

 

Thanks for sharing your knowledge here, @straitshooter.

 

@suelesko and @BrKorb, Thank you for taking the time to let us know about your inquiries. I'd love to know more about this.

 

Could you please tell us what type of Nest Thermostats you have? I found a helpful article about how common wiring works. Please let us know if that helps.

 

Regards,

Mark

I have a 3G learning Thermostat controlling a gas fired forced air furnace & air conditioner.  I also utilize the * connection to govern a plenum mount fan powered flow-thru humidifier.  This required the addition of a 24vac spst relay to cycle the humidifier when the thermostat sensor calls for humidity.  All works flawlessly. 

Hi there,

 

Sounds great! Thanks for letting us know what you've done to have it sorted out, @straitshooter. I hope this will help other users as well.

 

@suelesko and @BrKorb, I wanted to follow up and see if you are still in need of any help. Please let me know if you are still having any trouble from here, as I would be happy to take a closer look and assist you further.

 

Regards,

Mark

Hello folks,

 

I'm checking back in — how's it going? Still need our help? Let us know so we can assist you right away.

Thanks,

Mark

No progress. No helpful information about how to chase down the cause of my battery issue. My issue is better explained here:

https://www.googlenestcommunity.com/t5/Nest-Thermostats/Very-low-battery-replace-now/m-p/341032#M728...

EdwardT
Community Specialist
Community Specialist

Hi folks, 

 

I'm just checking in to make sure that you've seen our response. Please let us know if you have any other questions or concerns as I will be locking this in 24 hours.

 

Thanks,

Edward