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Can I install the Nest Power Connector with this old Honeywell system?

oldiesystem Member Posts: 2
Hi all, sorry if this has been asked and answered--I've searched and haven't had much luck.

I just moved into a house with what looks like a very old system, and I'm trying to install some Nest thermostats with the Power Controller (since there's no C wire to the thermostats).

Photos here.

Image 1: the control board
Image 2: the box where all the thermostats meet (there are three)
Image 3: a box on the other side of the room with what I think is the C wire

Any and all help would be incredibly appreciated! Thank you!!


  • Jamie Hall
    Jamie Hall Member Posts: 22,100
    You may well be able to MacGyver it. I'd have to be there and check all the wiring to be sure as to what goes where and check the phasing.

    Why not make life simple for yourself (and the next guy who has to work on it?). Get a nice new 24 VAC, 40 volt ampere transformer. Hook the Nests up to that (you'll need three wire minimum cable). Use the Nests to control simple SPST relays. Run the present thermostat wires to the NO contacts on the relays.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 14,495

    I agree with @Jamie Hall

    We get several nest questions a week. All their power connectors and gimmicks are questionable. Either pull a c wire to the thermostats and hook then up with a seperate transformer and relay or skip it and use a regular stat with batteries.

    Hooking a nest up any other way is unreliable, it may work for a while without the C wire and then it will quit. It's all about the internal batteries. See the attached diagram
  • oldiesystem
    oldiesystem Member Posts: 2
    @Jamie Hall @EBEBRATT-Ed thank you both, that makes complete sense. It sounds like I need to think about this a bit more and figure out a longer-term solution.
  • armcomdes77
    armcomdes77 Member Posts: 1
    edited October 16
    I have a Honeywell L8148e1299 on a 24V 2-wire set up. There is a TV and T terminal that are connected to the thermostat. If I am to add a C-wire, which terminal do I use? I tried using a Nest Power Connector, and I read that I should connect the C-wire from the NPC to the Z terminal (which is strap connected to the W terminal. This did not work, and have no power reading at the to wires at the thermostat. So I'm thinking of running a C-wire through the walls but it's not clear which terminal to connect for the Common.

    EBEBRATT-Ed Member Posts: 14,495

    You don't have a "C" terminal and the transformer in your control is likely not large enough to support a Nest.

    The post above yours has an attachment drawing that shows how to add a transformer and relay to power the Nest.
    HVACNUT Member Posts: 5,464
    edited October 17
    Is TV connected to R at the thermostat?
    W-Z is Common, and that is a 40 va transformer, but what else is connected to it?
  • Teemok
    Teemok Member, Email Confirmation Posts: 358
    I get it now