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Author Topic: Nest thermostat power charged (c-wire) through usb?  (Read 20087 times)

Offline RoChess

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Re: Nest thermostat power charged (c-wire) through usb?
« Reply #15 on: December 12, 2014, 12:16:05 am »
Forgot to mention I actually was on the phone with Nest support to double check it would work for me, because I had indeed read the same horror stories.

Support tech was really knowledgable, and verified that if I measured a good AC voltage potential between the two wires that I would be good to go. Grabbed my $5 multi-meter, and measured 28V AC between R and W wires without anything connected. The Nest only draws a few miliAmps, which you could replicate with a variable resistor (potentiometer) to find out at what Ohm level the heater kicks in. That felt like too much work for me, so I just went ahead with the Amazon order.

When it arrived I simply secured the R+W wires with shrinkwrap to feed them through the Nest wallplate without having to pop the breaker; followed by inserting each wire into the Nest backplate, and clicking on the Nest unit itself into the backplate. Display came to live, and I immediatly began to configure it for WiFi/etc.

Inside Vera UI7 -- via the Nest app/plugin -- I can monitor the Nest thermostat battery level in an easy way, and it usually hovered between 85% and 98% during the initial week of messing about. I did forget to check battery level right after unboxing, so by the time I had Vera app configured the battery was already at 100% (possibly already was). Keep in mind that the reason battery dropped so low to 85%, was because I am using an aggressive 60 second poll timer value to update status, and was constantly changing/verifying settings via the LCD display on the Nest (as well as via browser/Vera).

With things settled in now, and relying more on Vera scene control versus manually rotating that awesome control knob, it sits at 100% charged pretty much all the time now (just checked).

I did in the end have to disable the auto-away scheduler of Nest, after 14-days of training it was nowhere near understanding my schedule. This probably saves more Nest battery usage.

Offline Javelin

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Re: Nest thermostat power charged (c-wire) through usb?
« Reply #16 on: June 11, 2015, 06:53:11 am »
Hi wondering if any knowledgable sparkys be able to help me.

I purchased a nest for my Braemar TH325 Ducted heating Unit

Ive currently got the unit wired using R,RH and added a C Wire

It works however even though my wires read 24volts I'm not getting enough juice or battery is deciding not to charge on my nest.

Im only reading a VOC of 14.10 in the thermostat settings on the wires, when it says I need between 20-40 on nest website. So iI presuming my voltage is not right somewhere so I thought id eliminate the furnace leaching from the equation by adding my own transformer.

As im in australia its hard to find a power supply to provide the correct voltage. But I managed to track down this one only question is 1Amp to much or to little for my nest circuit board?


I plan to just replace the 1 wire on the C circuit and see if that works or possible try it on the RC circuit + C wire. Im scared of crossing circuits and damaging my furnance if anyone could advise would be appreciated.

Offline Brientim

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Nest thermostat power charged (c-wire) through usb?
« Reply #17 on: June 11, 2015, 03:27:00 pm »
Have a read here. 

I have central gas heating (in Oz) which has the C but manual thermostat had 2 wires,.  look at the furnace  and you may find you a surprise.

« Last Edit: June 11, 2015, 03:29:16 pm by Brientim »

Offline Javelin

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Re: Nest thermostat power charged (c-wire) through usb?
« Reply #18 on: August 21, 2015, 07:13:01 am »
Can Confirm that the Jaycar Transformer works. I ran the Red wire to Rc and black to C. Then connected my two heater wires to RH and W1. Battery voltage running at full in no time! Vin and Voc both up around 33-37 and 100ma

Now running with clicking sound, full brightness and wake screen on approach. Unit seems alot more responsive aswell.

Both Temps also showing under Ui7 with the new nest plugin Couldn't be happier!