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Nest Thermostat and Running a C Wire from A/C Air Handler C Wire

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FredoSP
FredoSP Member Posts: 90
edited February 2018 in THE MAIN WALL
Hello Group!
This is my first post to the site and hopefully you can help me, so please use kid gloves as I'm a new home owner trying to figure things out and learn for myself.

I have a heat only system (Oil) that I've recently replaced an old 2 wire Honeywell Mercury Thermostat with a Nest 3rd Generation Thermostat, Red and White wire only that goes to a Honeywell RA89A switching relay. Everything is working fine and no issues with heat throughout this past winter. I've been trying to read a lot on the web about adding a C-Wire and thus the adventure begins.

My idea, and it could be a total hack, was to run a single wire from the A/C air handlers' C-Wire to the Nest's C-Wire to get the constant 24 volts to keep the Nests' built in battery charged. The house has it's own independent A/C system that was installed three years ago (2015) that goes to a Carrier Cor Wifi Thermostat. The air handler that's in the attic is a Carrier FV4C. These two units would never been on at the same time, so I don't know if that's important but I wanted to mention that.

If that idea is totally off base ... I also purchased a Taco SR-501 Single Zone switching relay to replace the Honeywell RA89A because I thought I could swap it out and use the 24 VAC on the TACO and run that directly to the Nest's C-Wire, but that is only a backup if running the wire from the A/C air handler doesn't work out.

Any thoughts?
Long Island, NY

Comments

  • kalex1114
    kalex1114 Member Posts: 104
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    It might not work as it will not complete a circuit. If you have an outlet nearby where the nest is ,you can get a small transformer and just connect those 2 wires to RC/C on the nest and that will work as well
  • FredoSP
    FredoSP Member Posts: 90
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    Hi kalex1114, thanks for the reply.

    I did see those types of solutions online but don't want to see an ugly transformer on the wall. I'll keep it in the back of my mind though if all else fails.
    Long Island, NY
  • SuperTech
    SuperTech Member Posts: 2,183
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    Put the transformer on the boiler. You don't need a 40VA to power a thermostat. But I would use common from the Taco panel, I think that's the best solution. You should get the panel that has the outdoor reset function so you can upgrade your boiler and save quite a bit of oil.
  • Jamie Hall
    Jamie Hall Member Posts: 23,419
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    The C wire from the A/C has nothing to do with the heat -- so it just won't work at all. Use @SuperTech 's solution.

    What type of heating system is this? You should be aware that the Nests do not play well with steam, nor with any hot water system which uses outdoor reset. They will degrade the performance of either one. They are fine for forced air, which is what they are designed for.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • SuperTech
    SuperTech Member Posts: 2,183
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    Yes, I didn't even consider that its a Nest when I recommended the Outdoor Reset. They two are a bad combo. I would much rather go with a Honeywell Lyric thermostat and the Taco panel with ODR over the nest. You get the Wifi capabilities and the energy saving that outdoor reset provides.
  • ChrisJ
    ChrisJ Member Posts: 15,767
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    This is what I love about the Honeywell Prestige.

    You can power the thermostat from a completely separate source than what you're operating. Not only that, but each device (Furnace, boiler, air handler etc) can each be a separate power source as well. They give you connections for power for each and they're all isolated.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
    ratioCanucker
  • HVACNUT
    HVACNUT Member Posts: 5,860
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    Is the RA89 not for your main zone and therefore not centrally located? If the Nest is near the Carrier thermostat, you can merge them. Just remove the jumper between Rc and Rh.
    Then you can use the Nest for a street hockey puck.
    SuperTech
  • FredoSP
    FredoSP Member Posts: 90
    edited February 2018
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    Wow, lots of responses, thanks! Lets go through each one.

    SuperTech - I think I understand what you are saying with the 40VA. When the call for heat is made with the Nest thermostat it will be closing the circuit and pushing the 24V to the Nest, while at the same time the constant 24V from the C-Wire of the A/C will also flow, so two sources of power probably isn't a good thing for the delicate Nest.

    I also never heard of the ODR, interesting concept that probably has more advantages for colder climates.


    Jamie Hall - We have radiators and baseboard, depending upon which room you're in. I'll post some pictures so you can see. I wasn't aware that the Nest wasn't designed for hot water based systems.

    HVACNUT - Yes, The Honeywell RA89A does 85% of the homes heating and another older thermostat does the other two rooms. I'm 99% sure that thermostat (Which I'm not going to mess with at this time) is line voltage going directly to one of the two circulator pumps.

    I do like your idea of having the Carrier Cor control the heating for at least one of the zones, but I wasn't sure if that was even possible because the A/C system is independent of the heating system. Since the house is a split level, I have easy access to the attic and could easily move wires around.


    The more I'm reading, the more confused I'm getting, don’t get me wrong, the Nest is doing what it's supposed to without a C-Wire, but them comes in the power stealing issue.






    RA89A, currently the Nest is tied to















    This is the Carrier Cor


    Long Island, NY
  • SuperTech
    SuperTech Member Posts: 2,183
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    It's strange to see a Riello burner on that boiler. Nothing wrong with that. I've never seen the backflow preventer on top of the pump either.
    It's a nice boiler. I'd put the Taco control on it and ditch the tankless coil. Make it cold start and add outdoor reset.
  • GW
    GW Member Posts: 4,699
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    You’ve got options. As mentioned you can heat off of the Carrier stat, just grab the rh and w.

    Or do the trans thing and grab rc and c on the Nest

    Does your boiler control or zone control have a C ? It seems like a shame to set up a trans for just a power consuming thermostat. If you can find 24 volts that means you found good power source

    I hear you, the Nest is extremely popular

    Jamie was referring to outdoor reset I’m almost sure. You can certainly run your boiler zone with a Nest

    It’s very unusual to see a line voltage thermostat; I maybe have seen several is 29 years.
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    gary@wilsonph.com
  • FredoSP
    FredoSP Member Posts: 90
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    Hi GW,

    This house was built in 1955, my grandparents being the first owners. I'm now in the process of making it my own and fixing things that were neglected, etc.

    Here's a pic of the thermostat that I think is line voltage. I don't see any transformers or electronics by it.






    Long Island, NY
  • Jamie Hall
    Jamie Hall Member Posts: 23,419
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    Oh my. I haven't seen one of those in years. A simple bi-metal and contacts. May or may not have had an anticipator. It can certainly switch 24 volts AC or a millivolt burner, but I'd be cautious of it on 120 line voltage.

    It needed no electronics or transformers. All it did was close a circuit for the burner control when it got cool, and opened it again when things warmed up. Bullet proof. Dead simple. If it isn't working, clean the contacts and it will be good for another half century or so.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • GW
    GW Member Posts: 4,699
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    Very cool Fredo, same here! I’m raising a family in my grandparents house from the 1950s too. I went a little overboard on the remodel though but I don’t regret it one bit.
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    gary@wilsonph.com
  • John Mills_5
    John Mills_5 Member Posts: 952
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    I did one with an Ecobee where we hooked R, C, Y, G to the air handler and connected to the 2 wires of the boiler's stat to W & C of the thermostat. Then used a little peanut relay to close the contacts to bring on the boiler, just like the thermostat did.
  • FredoSP
    FredoSP Member Posts: 90
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    3/13/2018 update
    Updated pictures of the Taco SR501 install with the Nest. Not sure how to link threads, so go to Controls forum and look for Nest thermostats & Taco SR-504 Switching Relay
    Long Island, NY