Making a Nest thermostat work with a two wire system

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Mick_VT
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Making a Nest thermostat work with a two wire system

Post by Mick_VT »

So I had a new oil furnace installed this week, to replace an existing forced hot air system. Previously I had had a gen 3 (and before that a gen 2) Nest operating the old system. These are really nice thermostats that not only help save money through things like an "auto away" mode, they also work with the Nest smoke alarms to do cool stuff like shut down the heating if CO or smoke are detected.

Anyhow, we ran into a problem that not only had the heating engineer stumped for a while it also resulted in us (the heating engineer and I) jointly toasting a gen 3 Nest. Good job I still had the Gen 2 as a backup!

The issue is that Nest thermostats (as do all similar alternatives) leach power from the HVAC control lines in order to charge their batteries. This works with (according to nest) 95% of systems. The other 5% need to have a common wire run from the furnace. My new furnace was one of those. We discovered this post install when the oil burner was pulsing when idle as if trying to fire up. What was happening was the Nest was drawing just enough current from the thermostat wires to make the primary think that the thermostat was on. The nest apparently pulses its draw to help avoid accidentally switching the primary, but if the relay is sensitive enough this tactic fails and the relay pulses giving the effect described. The sound of this is distinctive. Imagine a large electric motor connected to a switch, then pulse that switch on very briefly then back off over and over whirr, whirr, whirr whirr.

Nest's offered solution of simply adding a common wire is all well and good if your furnace is able to provide a common wire. The primary on simple forced air systems often does not - the only connections they offer will be the connections for Rh and W1 no others.

The heating engineer said "ok well will need to do that with a separate 24VAC transformer then, but I have not done this configuration before." After fitting the transformer and working through various scenarios and wiring diagrams between us, and managing to fry the gen3 Nest in the process, we were about to give up for the night when I discovered (almost by accident) an article on how to make a nest work with gas fireplaces, which coincidentally are not able to provide the power needed to run the nest.

The article for due credit and reference can be found here: http://www.instructables.com/id/Hack-yo ... ove-or-fi/

This gave enough information to corroborate its strategy with the tech forums at Nest.com (where interestingly there was no similar article.) We gave it a try with the gen2 Nest that I still had in the workshop, and success! No more pulsing, and the heating system was functioning as it should.

here is a picture of the baseplate after install and testing:

therm.jpg
therm.jpg (74.85 KiB) Viewed 63903 times


Not that for heating systems there really is no standard for the wire colors, so here is what you are seeing:

Red is connected to the oil primary and so is white, this is the basic two wire install. The green and yellow wires are the fix. They come directly from the transformer. The green provides the "common" needed for the Thermostat to power itself. The issue we had was "where does the yellow" (other side of transformer) go? Without it there is no potential difference as it is a seperate circuit from the oil primary (red and white). The answer is to connect it as though the nest has an AC unit installed i.e. connect it to Rc. The nest is setup as heat only and only recognizes that it has heating installed as there is no full compliment of AC wiring, but will happily then pull its power from the common wire and Rc line.

Problem solved!

I am posting this here in the hopes that it will save other people many hours of head scratching and perhaps burned out thermostats (or worse oil primary circuit boards). And also for any of you HVAC geeks who might get a kick out of it :crazy:

and before you say it - Yes I do know that the baseplate is not level! :D
Mick...

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Corsetière
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Re: Making a Nest thermostat work with a two wire system

Post by Corsetière »

Bookmarking! Thanks!

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Re: Making a Nest thermostat work with a two wire system

Post by phil »

sounds confusing ! I installed an automated programmable thermostat on my furnace. it has it's own batteries but also somehow it is connected to the transformer at the furnace and my furnace is so old it's aqua marine green. of course the old one was just a mercury switch needing no power. this newer one runs even when I take the batteries out so it is getting power from the furnace and it needed a third wire. I made it work somehow.
I like it , it turns the heat down at night and on before I get up and down when I am at work but it stays warm during the day on weekends. If anyone is cold they can turn it up but it goes right back to it's scheduled temperature when it gets into it''s next time block so it's kind of foolproof.. I never seem to need to change the programming thank god.

I've been having troubles with my smoke detectors. there are 3 and they run on 110 and are linked so they all trigger together. when I turn them off I also loose my attic lights which is a planned inconvenience reminding me to turn them back on. The problem I have is they are too sensitive and go off if we just open the oven. the heat rush sets them off never mind smoke.
I spoke to my bro, the electrician, he said some brands use different wires to trigger each other so make sure I buy three made by the same company. I guess some use the neutral and some use the common wire to trigger each other. He said it is common for them to fail this way even right out of the box and you can't adjust them.
it's been going off at night occasionally so I guess I have to replace all three but I have no idea which are the "best" ones.

Phil

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Re: Making a Nest thermostat work with a two wire system

Post by phil »

about these nest stystems... today there is quite an alarming report about Google-nest shutting them down or "bricking them" ( breaking them unnecessarily) Not very good advertising for the technology.

http://www.cbc.ca/news/technology/revol ... -1.3521927

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Re: Making a Nest thermostat work with a two wire system

Post by Shrimpdip »

We tried about every combination of wiring to get our boiler working with it. Even with nest tech support on the phone we couldn't get it working without the pulsing issue. In the end I had to add a relay to electrically separate the nest from the boiler. Nest and separate 24v transformer on one side of the relay and the thermostat wires from the boiler on the other. So far so good.
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Making a Nest thermostat work with a two wire system in AC mode

Post by malcolmh »

OK - so I have a zoned house (3 zones) and the 2 wire solution worked great for me during the winter with a Nest thermostat in each of two zones and a "dumb" thermostat in the 3rd zone.

Now we're moving to summer and I need to use the thermostats to control my AC. I have a manual switch by my furnace to move the whole system into AC mode so all I really need to do is have the nests change from heating mode to cooling mode.

I naively thought that I could just move the Rh wire to Rc and the W1 wire to Y1, then move the independent transformer wires to C and Rh, but it doesn't seem to work. Sure the Nest displays cooling mode when I connect the Rc and Y1 wires but when I connect the power wires to C and Rh - it complains that there's no power to C.

Any ideas?

Thanks so much.

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Re: Making a Nest thermostat work with a two wire system

Post by colincurwen »

I'm having a hard time trying to get my nest thermo to work in our offices - we're in an old building where there's a central boiler and our office suite is heated by radiators that operate on a single zone valve control. There was an old two-wire mercury bulb switch thermo that simply opened the zone valve to heat the place.

basically, there's a single pair wire (red/black) running from the transformer at one end of the office, and another pair coming from the zone valve. In the box the black are connected in a nut, and the red were connected to either side of the thermo. When the mercury connects the red leads, it closed the circuit and the valve opens.

I connected the Gen 3 Nest by putting the red leads into W and Rh, but got an error E72, so I did some searching and concluded that I needed to add a wire to connect the Nest's common to the common side of the transformer. This got rid of the error and I figured I was good to go.

However, whenever I set the Nest to heat, I get a E102 error. Can anyone tell me what I am doing wrong? Nest support sez I need to contact a nest pro installer but I feel like it's a simple fix (or at least confirm that it WON'T work at all). Anyone able to help me? I've enclosed diagrams of what I have, as well as the wiring diagram from the original thermo to illustrate the way it was wired before.

Thanks in advance for any help.
Attachments
what I currently have
what I currently have
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original thermo install
original thermo install
wiring.png (51.01 KiB) Viewed 47962 times
what I did to date
what I did to date
setup2.png (25.43 KiB) Viewed 47962 times

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Re: Making a Nest thermostat work with a two wire system

Post by PeterK »

I installed a Nest on a very old two-wire fuel oil furnace that had a mercury bulb thermostat. I got one of these kits and ran the power from an outlet on the back side of the wall, leaving the small board inside the wall. It’s been working great.

https://www.amazon.com/Venstar-ACC0436- ... B00755BZZC

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Re: Making a Nest thermostat work with a two wire system

Post by ProfMikeT »

I have a Nest E thermostat that I installed in place of an old 2-wire, mercury bulb switch system, with one wire connect to W1 and the other connected to R. I have the wires from a 24 volt transformer that I would like to connect to the thermostat so that the battery is kept charged. The remaining unused terminals are Y1, G, C, and *OB. The Nest technician I talked to said that none of the other terminals on the Nest E could be used to form a complete circuit to the transformer. Anyone have alternative advice?

Thanks,
Mike
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Re: Making a Nest thermostat work with a two wire system

Post by Mick_VT »

ProfMikeT wrote:I have a Nest E thermostat that I installed in place of an old 2-wire, mercury bulb switch system, with one wire connect to W1 and the other connected to R. I have the wires from a 24 volt transformer that I would like to connect to the thermostat so that the battery is kept charged. The remaining unused terminals are Y1, G, C, and *OB. The Nest technician I talked to said that none of the other terminals on the Nest E could be used to form a complete circuit to the transformer. Anyone have alternative advice?

Thanks,
Mike


Mike are you sure that your nest won't charge from the two wires already there. Mine did on my original furnace, it was only with the update to a new burner type that issues happened
Mick...

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