Making a Nest thermostat work with a two wire system
Posted: Fri Jan 15, 2016 2:42 pm
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:
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
and before you say it - Yes I do know that the baseplate is not level!
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:
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
and before you say it - Yes I do know that the baseplate is not level!