On Friday, December 23, 2016 at 4:38:29 PM UTC-5, wrote:
We have a Lennox Elite Gas furnance 2 stage and it is hooked up to an EIM - equipment interface module sending two wires RED and White to the thermostat. If i just run C from panel to the Nest will that power up the nest?


Not familiar with the NEst, but for any thermostat that wants power or
where power is an option for the display to light, etc, that is what is
needed, a wire to the common side of the transformer. If you have to
run new wire, I'd upgrade to about an 8 conductor, so you're good for
the future. With an additional wire you could slightly improve the
two stage operation of the furnace. Right now, if there are only two
wires to the thermostat, it's set up as dumb, the system has no way
of knowing if second stage is needed, so it almost certainly is set
up to start on low stage, then if demand for heat isn't met after like
10 mins, then it goes to high stage. With a thermostat that handles
multi-stage, which I think must include Nest, the thermostat makes the
call. If it knows it only needs to go up in temp 1 or two deg, then
it calls for low stage. If it knows it needs to go up 3+ deg, etc,
then it calls for high stage at the start. There is one wire for
stage 1, one for stage 2. Also, typically you have a fan control wire,
so the thermostat can turn on the fan without heating or cooling.
So, to do it right, you'd have:

One wire for heat stage 1
One for heat stage 2
One for fan
One that's connected to one side of transformer
One that's connected to other side (common) of transformer

And if you have AC, then one wire for each stage there.









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