On Monday, August 10, 2015 at 11:11:45 AM UTC-4, wrote:
On Saturday, December 14, 2013 at 7:52:40 AM UTC-8, Ian wrote:
I am installing a new NEST thermostat, replacing an old two wire mercury bulb furnace thermostat. There are only two unmarked wires. There's 24 V across the wires.

Can anyone tell me which letters on the NEST base (W, Y, O/B, AUX, E, G, C, etc) they will map to? It has to be the most basic of installations; open or close the circuit.

Thanks,

Ian

I am considering buying a NEST thermostat today. I have one of the basic two wire thermostats where I have a white and a red wire. It's the type with the mercury bulb.

The guy came around from the installers and tried installing a nest three months ago and told me it would not work on a two wire system. I have heard different stories about this. Some say yes it does work, other say it won't work without pulling through a common wire (whatever that is, I am no electrician) My two wires are red and white BUT I also have two other wires that are there, a black and a green one that weren't connected at the thermostat. I am wondering if I could use one of those wires as a common if need be. I would imagine they are for a future install of a heat pump.

Can anyone advise me on this?.

Thank you.

On Saturday, December 14, 2013 at 7:52:40 AM UTC-8, Ian wrote:
I am installing a new NEST thermostat, replacing an old two wire mercury bulb furnace thermostat. There are only two unmarked wires. There's 24 V across the wires.

Can anyone tell me which letters on the NEST base (W, Y, O/B, AUX, E, G, C, etc) they will map to? It has to be the most basic of installations; open or close the circuit.

Thanks,

Ian

Assuming those additional wires run all the way back to the furnace,
which is likely, then yes one of them could be used as a common to
supply power. If the installer was at all competent, he should have
been able to deal with that the first time around.


I looked into the Nest a few years ago and from what
I saw, there were a huge number of problems, with most of them involving
trying to steal power with 2 wire installations. Things like it
shorting out, forcing the heat or cooling to full on, regardless of temp.

I think the Nest sucks for other reasons too. Lots of stories of
people saying that they can't control it, it just does what it wants,
etc. That's how it's marketed, isn't it? I saw people complaining
that if they were home from work, sick in bed, and wanted it to just keep
a set temp, it would just keep reverting back to figuring out that because
it didn't see movement for awhile, it thought you'd left the house.....
I don't need that. For me a Honeywell VisionPro that's programmable
works just fine. I would get internet connectivity in my next one,
so that I could control the heat from my phone, turn it up when I get
back to the airport, etc. Could get a VP that does that for $150.

Also, don't be fooled into some incredible claimed energy savings.
Those savings are mostly due to what you can do with any programmable
thermostat.