Installing NEST Thermostat - two wires

[mrbeldon]

why are you such a dick?

[jasonchu.re]

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?

[trader_4]

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.

[destrowashington]

The yellow=Y.. The green=G.. Blue=B

[david.hariri]

I wrote a blog post explaining how to do this. You do not need to pull a co mmon wire in most cases. Try it according to my instructions first, then pu ll a common wire if you need it, but you really shouldn't since the Nest ha s a feature called Power Sharing which pulses closing of the circuit to dr aw current when it needs to charge itself.

Post with instructions:

[gafuster]

Today I install nest thermostat. I took out the old one.i saw only two wires. White and yellow..so I put those white in W1 and yellow one at Y..am I doing correct..advise me plz

[gafuster]

Same thing happen to me

[Mr Fuxitup]

If this is a 24 volt heat only system, I'm going to *guess* you should use the Rh and W terminals.

Were the terminals labeled on your old thermostat?

If you have a millivolt furnace system, the nest probably wont work with it.

[trader_4]

Good grief. RTFM. The install instructions will certainly clearly explain how to hook it up to a two wire system. IDK how it works now, but years ago there were big problems with using it with two wires.

[Tekkie®]

trader_4 posted for all of us...

+1

[moderator]

gafuster appears to be a driveby shooter.  He prolly won't be back. We'll just close the ticket.

- [e-mail]

[Scott]

bulb furnace thermostat. There are only two unmarked wires. There's 24 V across the wires.

C, etc) they will map to? It has to be the most basic of installations; op en or close the circuit.

I just replaced the same kind of old thermostat with Nest. The Nest info wa s no help for this. I made it work by finding the two wires where they conn ected to the furnace. At that connection there are letters corresponding to the connections on Nest. In my case it was R and W. My old wires were the same color so I could not easily distinguish which was which at the thermos tat end so I ran two new wires (red and white 18ga. solid) by taping them t o the old wire and pulling it through to the furnace. I attached the R wire to the Rh and the W wire to the W1 connections on Nest. R to Rh, W to W1. It works just fine. I'm baffled at all the comments on this thread insistin g it could never work.

[trader_4]

y bulb furnace thermostat. There are only two unmarked wires. There's 24 V across the wires.

, C, etc) they will map to? It has to be the most basic of installations; open or close the circuit.

was no help for this. I made it work by finding the two wires where they co nnected to the furnace. At that connection there are letters corresponding to the connections on Nest. In my case it was R and W. My old wires were th e same color so I could not easily distinguish which was which at the therm ostat end so I ran two new wires (red and white 18ga. solid) by taping them to the old wire and pulling it through to the furnace. I attached the R wi re to the Rh and the W wire to the W1 connections on Nest. R to Rh, W to W1 . It works just fine. I'm baffled at all the comments on this thread insist ing it could never work.

When you have just a two wire thermostat, which wire is which doesn't matte r, it's AC. You'd think Nest would have told you that to avoid all the angst. I hope they work better now, back in the early days there were big, big, problems, especially when using them with just two wires, where they try to power themselves.

[Tekkie®]

trader_4 posted for all of us...

to the old wire and pulling it through to the furnace. I attached the R wire to the Rh and the W wire to the W1 connections on Nest. R to Rh, W to W1. It works just fine. I'm baffled at all the comments on this thread insisting it could never work.

They were acquired by Honeywell.

[opensourcemc]

i can tell you. one of the wires will go to w1 , the 2nd wire will be your common C