Nest/ecobee - do they actually work on SS systems w/o common wires?
October 27, 2014 8:26 PM   Subscribe

We have a single stage HVAC system without a common wire. Reviews online suggest that the nest can be pretty unreliable with our kind of system. Anyone with experience who could provide their perspective? I also like the ecobee, but it appears to not work without a common wire. If neither is workable, is there a good alternative for a SS HVAC without a common wire, or should I wait for the technology to further mature? Thanks!
posted by arnicae to Technology (6 answers total) 1 user marked this as a favorite
 
Note that pretty much every HVAC system ever made has a common terminal, so if your current thermostat lacks a common wire (typically blue) it's probably just because the installer didn't pull a cable that includes one. If there are any unused wires in the cable you can just repurpose one of those as the common (but please label it on both ends for future generations,) or depending on the layout of your house and crawlspaces it might be simple to pull a fresh one to the thermostat location (that's what I did when I installed a Nest.)
posted by contraption at 8:35 PM on October 27, 2014 [1 favorite]


My system did not have a common wire (original furnace was probably 30-40 years old when it was replaced 2 years ago) did not have a common wire. My first gen nest was unusable until a common wire was installed (I am handy and it took about an hour of work after a ½ hour of googling) . I now have a high efficiency furnace installed by pros, and my nest is amazing.
posted by prk60091 at 6:10 AM on October 28, 2014


Hi everyone, this is Mu from ecobee.

I wanted to let you know about the Power Extender Kit (or PEK) that we have for our smart thermostats. As @contraption mentioned, pretty much every HVAC system ever made has a common terminal, if a common wire (C) does not exist at your thermostat location, all that means is the installer didn't run a wire from the furnace location to the thermostat location.

Typically in this situation you would have to call an installer to come in and run a new wire from your furnace to the thermostat location, this is not always easy or cheap. The Power Extender Kit enables you to reliably power your ecobee smart thermostat without having to run a new wire.

You can learn more about the Power Extender Kit and how it is installed by watching this video. If you have any questions, please let me know.
posted by ecobee at 6:25 AM on October 28, 2014 [1 favorite]


Note that pretty much every HVAC system ever made has a common terminal

As @contraption mentioned, pretty much every HVAC system ever made has a common terminal

Can either of you provide guidance on finding such a common terminal? I have a fairly standard oil-fueled steam boiler and can find no evidence of a common terminal on the ignitor box where the other wires connect. Am I just blind or am I looking in the wrong place? This has been the main think that's kept me from getting an ecobee/Nest.
posted by Betelgeuse at 8:40 AM on October 28, 2014


I had my heart set on a Nest for the new house but couldn't figure out how to get it installed, even though I had the builder and photos of the setup on one side and Nest tech support on the other.

The one thing that finally scared me off was the mention on various forums that without an extra (power?) wire, the Nest would cycle the furnace on and off rapidly to draw power for itself, which sounds like a nightmare in the works. Please make sure you're not going to get into that situation - it's "supported" but not a good thing as far as I can tell.

(So I went with a pair of "smart" programmable thermostats, read the cryptic user guide, turned off their WiFi broadcast, etc. Ugh. But this was just before Google bought Nest, so in retrospect, it was a good thing after all...)
posted by RedOrGreen at 9:09 AM on October 28, 2014


> Can either of you provide guidance on finding such a common terminal?

I live on the west coast and don't know oil-fueled systems in particular, but I have attached lots of thermostats to lots of HVAC systems and will give it a shot. You will probably need a very basic multimeter of some sort:

First, find where your thermostat is wired into the boiler. Is there a terminal marked B? If so, that's your common. It's labeled B because the wire is Blue and HVAC installers are not expected to know how any of the stuff they put in works, they just have to be able to identify the color of a wire. This is why I often find myself crawling under houses to inspect the wiring of HVAC systems I didn't install, but hopefully my aggravation will be your gain.

No B for blue (or O for Orange, I guess, though I've never seen that variation in the wild)? That's ok, if your system uses a standard thermostat there is a voltage source at 24VAC somewhere in the system, and the common is just whichever pole of that source is not being used as R (Red, aka Reference Heat.) I was about to do a whole explanation of how to figure out where to splice in, but just discovered this great illustrated writeup on the DIY StackExchange site explaining everything you need to know.
posted by contraption at 7:59 PM on October 28, 2014


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