2 Wire (heater only) Solution...

[PapaBearLodge]

I have a boiler to heat my home and 4 thermostats to heat the 4 zones in the house.     There are no common wires in my home, as I only have two wires to control the boiler within each zone.    I purchased four Gen 3 Nest thermostats and install one of them (which is working nominally).  I know the common wire (not found in my house) provides power to maintain the thermostat, but Gen 3 can work with "most" 2 wires systems.... apparently getting its power from the boiler periodically to recharge.  

Question:   If the Nest I installed is working fine, do I declare victory and install the other three?    Or do I have to wait a week or two to see if it is not getting recharged?

[CoolingWizard]

First, let’s talk a little bit how the nest third generation is capable of using a backdoor approach to getting power to charge its battery. If you think about any electrical items in order to operate, needs two wires. Now, if you take a light you would have a line and a neutral. Now that line in neutral is used in HVAC control systems and is typically called power in and common. Common is the neutral side. If you’re boiler uses a 24 V AC control system, this will allow the thermostat to maintain its charge. What happens is when the thermostat is not calling for heat by using the W wire, going back to the gas valve, it can find the common side of the coil. As long as the thermostat does not draw a large current, the coil will simply be a path, and will not energize the magnet.

know if your furnace is an older style, it will not be using a 24 V AC control system. In fact, the thermostat simply would act as a connection between T1 and T2, and when that connection is made it signals to the gas valve to turn on. You normally find this type of a system, when you see a standing pilot light and not a hot surface igniter, or a spark igniter. 

as long as your next third generation does not complain about not seeing power. You can go ahead and claim victory. However, having said this. Since you have one boiler and four zone valves, the zone valves don’t work the same way and you’re going to need to find a common.

The AC Cooling Wizard

[PapaBearLodge]

Thank you for the response.   I believe I followed virtually everything you said, all the way up until the last sentence.   When I replaced the first thermostat, I verified ~24V in the system, so according your comment the Nest should get its power just fine.    That makes me feel good about installing the next three Nests.    

Your final sentence, however, leads me to believe I need a common if I have four zones??    (all zones are two wire returning to a boiler controller box).   Since each of the 'old' analog thermostats control the boiler independently, I am assuming each of the Nests would also.     Correct?

Thank you for your assistance!

[CoolingWizard]

You are correct, papa, if all the thermostat run back to a zone, controller at the furnace/boiler, and those will all work the same way, so just go ahead and install the other nest and you’ll be fine. Now if you run into a problem, if there’s an extra conductor wire in the thermostat location, it will be possible to go into the zone, controller and hook up a common.

The AC Cooling Wizard

[Patrick_Caezza]

Two-wire-only systems are notorious for causing power problems down the road. Even though Nest has always claimed that a C wire wasn't needed most of the time, in the real world having one is the best practice.

The Google Nest Power Connector is what you need (one for each thermostat) and an external 24Vac transformer.

[CoolingWizard]

The absolute only reason to get a power connector is, if you don’t have enough conductors in your current thermostat cable to carry another signal back to your thermostat, such as a common wire. If you do have an available conductor that you should use available conductor. Since there is a zone controller on the system there’s already a 24 V AC system available and now he just needs to see if there’s an available conductor and attach it to the C inside the zone controller system.

The AC Cooling Wizard

[Patrick_Caezza]

This is a boiler system with zone valve control (probably a taco valve controller) These only use two wires and some have a 24Vac terminal for each valve zone and some do not. The ones that do have a 25Vac terminal never have enough power supplied to them from the common transformer to supply enough power for the thermostats.

[PapaBearLodge]

My Expert friends.....    I have installed all four Nest thermostats.    The hardware installation is a breeze with all the repetition.    Each thermostat appears to be controlling the heating for its zone nominally.    So far, so good.

That said, I had many issues connecting each to the Nest App.   It appears as if I am not the only one to have this issue and that it is unrelated to my particular wiring setup.    I tried all of the workarounds I found to add products to the home including re-boots, resets to factory settings, uninstalling #1 to install #2, and working through several error codes.   At one point I had a thermostat reporting low power such that it had to turn off to re-charge.    I finally resorted to creating four different homes such that each thermostat has its own home and installing each independently.    That seems to have worked.    I, of course, had to migrate my account from Nest to Google to have more than 3 homes.    

I tried to upload a picture of my boiler controller, but I do not have permission to upload images.

I will report back any further issues wrt charging the thermostats if I have any.  

Thank you for your assistance.

[PapaBearLodge]

My controller board

[PapaBearLodge]

Was able to upload picture.   Five lines coming in from the top are the 24V lines to/from thermostats.    Four are from the heater zones in the house and the fifth is from the water heater.    

Lower lines are the high voltage output lines to the valves.  

Even if I wanted to run new lines (which would be a challenge), I am not sure where I'd tap for a 24V common output.

[CoolingWizard]

Papa, I noticed that your thermostat one cable has two additional conductors available. As for finding the 24 V common normally there would be a connector next to the transformer which is on the left side either above or below it, which would provide a connection to a secondary controller. So you should see a 24 V LINE and COMMON listed on one of those connectors. 

You will need 3 power connectors if you want to get power common to the thermostats. 

The AC Cooling Wizard

[PapaBearLodge]

One of the four Nests has now dropped off the app and wifi due to low power.    This is what I was concerned about.   I am wondering if this is just the first to go and the Gen 3 devices are not drawing power as advertised. 

I will investigate which zones is Zone 1 and the additional conductors.   I will also look for the 24V line potential connection point.  

I may need to learn more about the Power Connector that Patrick recommended too.   

Thanks

[CoolingWizard]

Hello papa, let’s do a couple checks to see if we can find a common in your zone controller. If you happen to have access to a voltohm meter, you would set it to AC voltage, put one of the leads on the red wire, a thermostat, one at the top, and use the other one and touch the nut that holds the transformer on the board. See if you find anything from 20 to 27 V AC. if not, check the white connector above the transformer and see if you can find one of those wires being a common in the showing you 20 to 27 V AC. If not there, check the green one below the transformer and see if you can find the voltage there. If you find the voltage, we need to make sure it’s the common. to do that move the connector on the red thermostat wire to the white wire and see if the voltage goes away. If the voltage drops significantly or goes away, it is indeed the  COMMON you’re looking for. 

if you find the  COMMON, then you are going to need to connect one the two wires of thermostat #1 to the COMMON and at the thermostat connect the same one to the C.

you will also need to order three power connectors for the other thermostats.

Ken, The AC Cooling Wizard

[PapaBearLodge]

Hi Ken, sometimes procrastination works.   I got busy with other activities since my last update with a dying thermostat.  I read your response and was set to be armed with my voltmeter to do some sleuthing, but was busy with other projects.

Sometime over the weekend, the low battery unit stopped exhibiting the error and returned to nominal performance.   No further low power notices.   The hardware is operating as expected and it is now appearing on the Nest App.   My guess is that my controller applies power to each zone squentially as needed and it had to settle in and receive power sufficiently to charge(?).   Regardless, I have not touched anything and all four thermostats are operating properly and none are showing signs of dead batteries.    I will give this some time to see if this is the new normal.

My system still is set up on the App as four separate thermostats in four separate homes, but that appears to be a shortcoming of the app rather than my configuration.

Monitoring for now.

Thanks for your patience.

John 
(Papa)

[Jake]

Hey there,

Thanks for the help CoolingWizard.

Apologies for the delayed reply. I wanted to follow up with you, and ensure everything is good to go from here. Please let me know if you have noticed any trouble with the device, as I would be happy to take a closer look.

Best regards,

Jake

[PapaBearLodge]

The Nest thermostats have been retaining their charge and have been operating nominally.   My only remaining issue is with the app, not the hardware. 

thanks for the follow up. 

John

[CoolingWizard]

Thea NEST Learning Thermostat has the ability to charge its battery using a sort of back door approach.  Imagine an electrical coil that’s designed to engage a contactor to link input power to output power demand, for say a compressor. One side of the coil is going to be connected to the COMMON of the transformer, and the other side is going to be connected to the thermostat. So when the thermostat is not calling for say, cooling or heating, there is an electrical path between the wire Y or W and the COMMON on the coil. The Nest can use this path to reach the COMMON for the purpose of charging the battery. The current draw for that charging circuit is so low that it will not engage the contactor coil.

this works pretty good on most systems however, for the more modern systems that are controlled by a integrated circuit control board, this path sometimes will not work and that’s the reason it’s always better to have a wire running between the C-terminal of the thermostat and the C-terminal of that control board in the air handler.

Ken, The AC Cooling Wizard

[Jenelyn_O]

Hi there,

Chiming in — I hope you get the answer you're looking for. Please let us know if you still have any questions or concerns, as I'll be willing to assist you further.

I appreciate your help, @CoolingWizard, @Patrick_Caezza, and Jake. 

Best,

Jenelyn

[aatienza]

Hey there,

I just wanted to follow up to see if you still need our help. Please let us know as we would be happy to answer any questions you may have.

I appreciate the help, everyone.

Thanks,
Archie

[PapaBearLodge]

Again, my hardware is operating nominally.   My only issue is with the app which requires 4 nests in one house to be treated as 4 separate homes.     The original issue is resolved and this topic can be closed.  

Many thanks to @CoolingWizard who helped me through the process!

PB

[Jake]

Hey there,

I am glad to hear the main issue has been resolved. Per your request, I am going to go ahead and lock the thread at this time. If you have any questions from here, please feel free to create a new post.

Best regards,

Jake