Installing NEST Thermostat - two wires

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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
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On 12/14/2013 10:52 AM, Ian wrote:

http://www.behvac.com/troubleshooter41.htm
You'll probably need to repull the thermostat wire with at least three wires so you have a common available to power the thermostat; I would repull with 5 conductor in case you think you may want to add central air in the future.
In short, one of the existing wires connects to "R" (if you have "RH" and "RC" then connect to "RH") use "RED" wire for this connection. This one will be the one coming directly from the transformer.
The other existing connection connects to "W" - use the "WHITE" wire for this one.
Finally you will need another conductor from the 24VAC side of the transformer to the new thermostat, that will connect to "C" for Common and if there is a black wire in your cable use that one.
It says here that you "may" not need to repull if you don't have a "C" wire
https://nest.com/thermostat/installation/
Looking at the picture there, it appears the terminals you would use are RH, W1, and C.
here's more info on why you may want to repull
http://support.nest.com/certified/article/Help-with-installation-and-set-up#common
Hope this helps
nate
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On 12/14/2013 11:21 AM, Nate Nagel wrote:

Forgot to add; if you repull then you can also use the fan-only mode. Use the green wire for that function.
nate
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Nate Nagel wrote:

Hi, Why the trouble repulling wires? Some times it is almost impossible pulling extra wires. In that case go wireless.
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On 12/14/2013 12:46 PM, Tony Hwang wrote:

In my last house it took longer to puzzle out the proper connections within the furnace than it did to pull the thermostat wire. Open ceiling in basement, and thermostat wire was clearly visible going through floor, only had to do about 4 feet "blind." If that's the OP's situation there's really no reason not to do it.
Obviously if it's a complete PITA to do so another solution might be preferable.
nate
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Ian wrote:

Hmm, Why are you asking? Read the manual? No? You can download it. Still have questions? Then ask. 2 wires? Where is another one for fan control? Maybe you are future proofing but in this case NEST is way more than what you need.
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wrote:

I don't believe a Nest will work on your system. Did you do fill out their compatibility form?
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On Saturday, December 14, 2013 10:52:40 AM UTC-5, Ian wrote:

What does the Nest install instructions say? Don't they tell you how to hook it up to the most basic two wire install? Normally, comming from furnace, connecting red to white would control it. Does the Next have batteries and need no additional power or does it need an additional common line back to the furnace?
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I've been fighting that battle for months. Thankfully I took still photo before starting and set up a video camera to record the actual steps in the process.
Nest is still not working but at least I could go back to where I started so we have heat.
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NotMe wrote:

Hi, Oh, my, if you read installation instructions and have difficulty understanding what it says, wouldn't it be time for help from pro? While it is being installed, you watch him and learn for next time. Or Nest customer support no good? Or may I ask what seems to be issue to complete installation?
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I had a simple question, emailed customer service and got an answer back right away.
Nest may be over kill for your needs, it can handle 2 stage heating, 2 stage cooling, fan control, and via wifi it can be controlled by your computer, smart phone or tablet, and firmware be automatically updated. If you cannot use these features, it seems to be a waste to use on a 2 wire simple heater.
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why are you such a dick?
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On 12/14/2013 10:52 AM, Ian wrote:

You'll have to read the furnished manual (RTFM). We can tell you the traditional color code, but yours might be different.
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OMG!
Verify that you have the battery powered Nest. If not, exchange the one you have for a battery powered unit.
Turn off power.
Make note of current terminations to old thermostat and use provided wire markers to make the proper designations. The old terminals will have markings that should at least have an R and W on them. If not, no big deal.
Remove old thermostat base and install Nest base, make sure that it is level.
Connect to terminals W1 and RH of the Nest. It is preferable that you match the R and W, but in a two wire system this really does not matter.
Put Nest thermostat on Nest base.
Program the Nest.
Turn power back on and operate the Nest.
Everything should work. If not, turn power off, reverse wiring and turn power back on. I seriously doubt the Nest gives a rip about polarity. This should not be necessary.
To test the wiring (function of furnace), disconnect power. spin wires together. Turn power back on. The furnace should fire up. If it does the Nest is bad, if not, then you have a different issue.
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On Saturday, December 14, 2013 10:52:40 AM UTC-5, Ian wrote:

Yes, it comes with installation instructions, and I have read them, but there is no reference to a simple two wire (unmarked) set up.
And, yes, I know the nest has the capability to control much more than I have, but I want the ability to program, and control remotely. It's only marginally more expensive than the basic ones that provide that functionality.
In the end, I believe the answer is to connect the wires to Rh and W1.
Thanks,
Ian
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Ian wrote:

Hi, Yes, indeed but you have to connect right wire to the corresponding terminal. W1 means 1 stage heat, RH means 24V AC control power lead, H meaning for heating. If you cross this wires you may damage the 'stat. Take a look at the furnace end to see if you can sort out this two wires. If you measure the voltage between furnace frame and this wire, one showing 24V AC or so is one for Rh terminal, the other one is for W1. Some times wires are color coded.
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,

The instructions state that RH and W1 are the terminals to use. Even if the "contacts" are solid state, mixing the wires would not hurt anything.
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Irreverent Maximus wrote:

Hmm, Note the word "may". Until tried result unknown.
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On 12/14/2013 08:25 PM, Irreverent Maximus wrote:

It might not recharge unless the "line" is correctly connected to RH. Otherwise, I agree.
nate
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just try it, I doubt you will damage anything if it doesn't work.
make sure RH is connected directly to transformer
I suspect if you have any issues repulling to add a common will fix.
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