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NEST - 3rd Gen. (Heat Only/Radiator Heat/Tankless Boiler)

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I recently installed a 3rd Generation Nest Thermostat in place of a Honeywell manual mercury thermostat. I have a 2 wire system (the Blue C wire doesn't connect to anything) and I was able to connect it and get the nest up and running. However, it's not getting enough battery. So my system will kick on, but it will not shut off! I have to manually turn off the tankless boiler. If I run an isolation relay to the nest, will this give my nest enough battery to run this system?

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  • Zman
    Zman Member Posts: 7,605
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    If you have a third wire, you should wire it as a common and your problem will be solved. Do you have this 3rd wire?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • vdelbagno
    vdelbagno Member Posts: 3
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    No, the 3rd wire doesn't connect back to the boiler. It's at the thermostat, but when it comes out of the wall there are only 2 wires back to the boiler. When I did try to connect the C wire, Nest didn't recognize it. I have lower power right now with the thermostat saying no power recognized on the W1 slot.
  • Jamie Hall
    Jamie Hall Member Posts: 23,635
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    Oh dear. Well, the first error was to get rid of the old thermostat. I hope you didn't just throw it out; those things are quite valuable.

    But what's done is done. You will have to get power to that C wire somehow. There are a a number of ways, some better, some worse, to do that. I believe that there is a wall wart type of device which is available for the Nest which will accomplish that.

    Once you do that, you will have to experiment with the settings on the Nest to get it to play nice with the tankless/radiator setup. Since this isn't a setup I'd ever recommend, never mind install, I'm not sure what the best settings would be.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • vdelbagno
    vdelbagno Member Posts: 3
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    Nah, I kept the old thermostat and might put it back on the wall if needed. I do like the clean modern look the Nest brings to my house. I live in a historic 1880's home and the modern with some of the old is a nice look. However, things don't always play nicely and this is ever evident with the nest and old wire system I'm working with right now.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,166
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    When the wires that come from the heater are connected by way of the thermostat 2 things happen. 
    1. The heater operates
    2. There is no difference in voltage from the W terminal and the R terminal.  

    When the thermostat is satisfied the connection opens and 2 things happen. 
    1. The heater stops operating 
    2. There is a difference in voltage that you can measure on a meter. About 24volts.  

    It is this difference in voltage that the Nest uses to charge the battery.  SO... As long as the thermostat is satisfied, the battery will be charging. When the thermostat is calling for heat, the battery can not charge.  

    By adding a common wire you will have a return path to charge regardless of a call for heat or not. So, adding an isolation relay probably will not solve your problem. 

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • Jamie Hall
    Jamie Hall Member Posts: 23,635
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    vdelbagno said:

    Nah, I kept the old thermostat and might put it back on the wall if needed. I do like the clean modern look the Nest brings to my house. I live in a historic 1880's home and the modern with some of the old is a nice look. However, things don't always play nicely and this is ever evident with the nest and old wire system I'm working with right now.

    I would. So long as the mercury thermostat is level, the darn things are accurate -- and close to bulletproof. Last forever.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    SuperTechCanucker