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Comment by Someone1234 | Hacker News Reader

You wire a plastic Nest bracket up[0], the bracket redirects the inputs, and when you place the Nest into the bracket it makes the correct contacts with the back of the thermostat.

Wiring up a Nest isn't too bad, you just need the precious C wire. Most of these fancy thermostats require the C wire for electrical power, and many homes lack it. They have a work around, but it requires re-wiring and you lose some functionality (do not recommend).

PS - No idea why nobody offers an AC adapter as an alternative to the C wire. All the C does is provide electrical power anyway...

[0] The middle white thing: http://i.imgur.com/kNUPqvF.jpg

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You don't need a C wire for the Nest, although if you have one it's recommended to use it. The Nest has a small internal battery and can trickle-charge from the R and W wires.

They say it's only "recommended", but my furnace was short cycling without it and required me to run a C wire to it. I'd say that it's pretty much mandatory for a modern furnace.

If you dive into the details there's a ton of disclaimers, limitations, and gotchas running without the C wire. See [0] amongst others. As I said above, I do not recommend.

Just because you CAN do something doesn't mean you SHOULD. This is definitely one of those cases. If you want your Nest to be running problem free on most systems (or any other smart thermostat for that matter), just assume you need the C.

[0] https://nest.com/support/article/When-Nest-needs-a-common-C-...

I've been running mine for a few months without the C wire, but I've definitely had the occasional bit of odd behaviour from it (eg, dead battery), that I believe will be resolved when I get around to running it some 18/5.

As others have mentioned, there can be issues with running without a C wire. Their "power stealing" technique did not seem to be effective for keeping my first-generation Nest from running out of battery. (Although there may be some variables in my situation that contribute to the problem.) I blogged about it here: http://cafbit.com/entry/nest_learning_thermostat

FWIW, the Lux Geo can run on on 24VAC, AA batteries, or micro-USB. I don't know how long the batteries last though.

There are lots of reasons why most fancy thermostats need a lot of power. For example, a fancy screen is hungry, WiFi is an expensive protocol (e.g. as compared to ZWave), etc.. Compare the Lux screen to the Nest, and you'll see at least one reason why they think they can get away with batteries.

Another option, patented by Honeywell I think, is called "power stealing." Here's an interesting article on the subject: https://www.ecobee.com/2014/01/the-problem-with-power-steali....

> They have a work around, but it requires re-wiring and you lose some functionality (do not recommend).

Are you talking about re-purposing the G wire to become the C wire? I had to do that just recently, and I lost the ability to cycle the fan on by itself (without calling for heat or cooling). Not a big loss in my opinion, but my system is dead simple.

The most infuriating thing about it is that the previous HVAC installer cut the fifth wire off instead of sparing it out.

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