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How can I safely wire Nest to this transformer?

ericmhurtadoericmhurtado Posts: 53Member
Hello,
I recently discovered two unconnected extra wires, red and white, that are part of the set of wires connecting my Nest to a HONEYWELL RA89Aswitching relay. Please see attached pic.
As you can see from the other pic, for some reason, these red and white wires are not wired to anything. I assume at one point that they were wired to that transformer that you can see on the lower left side of the pic.
My question is how can I safely wire the Nest to the transformer using these two red and white wires? Thanks for all the helpful advice.

Comments

  • icy78icy78 Posts: 251Member
    Does it work like it is? If it does leave it as-is.
  • ericmhurtadoericmhurtado Posts: 53Member
    No it’s doesn’t work great. It exhibits some power stealing such as a sign showing “delayed for x:xx”, turning on the boiler when there is no call for heat, etc.
  • HVACNUTHVACNUT Posts: 2,172Member
    That is not correct. You can use the transformer and RA89 but you need to add an isolation relay. See pic.
    The reason is, theres also a transformer in the RA89 (that does not provide external 24v power), so if you used the separate transformer as well, there would be two separate (butting) 24v power sources at the Nest. And that will probably burn out everything.
    I dont know the model # off hand but look for a RIB relay. Easy to install. You need: single pole, single throw, normally open, with a 24v coil.
  • ericmhurtadoericmhurtado Posts: 53Member
    I’m trying to understand. Should I use another RA89 to serve as the isolation relay? There has to be an easier way to add a “c” wire to the Nest even with my setup. Or should I just buy a bare bones, battery operated, ~$20 Honeywell thermostat?
  • Jamie HallJamie Hall Posts: 10,453Member
    If you insist on using the Nest, Nest does have available an external power adapter which you could use. I would still use an isolation relay -- not another RA89, just a plain vanilla SPST normally open 24 VAC coil relay. In any event, you need a 24 VAC supply to the Nest.

    Or, as you say, you could use a programable, batter powered Honeywell -- there are a number of different models -- which, if your system is either hot water or steam will play much better with the system anyway.
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • ericmhurtadoericmhurtado Posts: 53Member
    I tried to find a 24v spst isolation relay and the nest power adapter online but could not find it. This is the frustrating part as well as understanding how to wire it. That’s why it’s easier for me to go the battery powered, no “c” wire required, Honeywell. It’s available everywhere and it’s a quick fix.
    However, I’d like to get this Nest working properly.
    Can you please send me a link to a spst isolation relay and to this nest power adapter? Thanks. I appreciate the time and the advice.
  • ericmhurtadoericmhurtado Posts: 53Member
    If I use this Nest power adapter, wouldn’t I be in the same situation as HVACNUT described when I mentioned that I’d like to wire the nest directly to the transformer?
  • HVACNUTHVACNUT Posts: 2,172Member
    Google R.I.B relay.
    Relay In a Box.
    You seem like you can handle the wiring. Pretty easy.
    Just wire nut each unused wire separately.
    I spent 7 hours on that diagram. Do you understand it?
  • JohnNYJohnNY Posts: 2,254Member
    For installations, troubleshooting, and private consulting services, find John "JohnNY" Cataneo here at :
    "72°F Mechanical, LLC"
    Or email John at [email protected]
    John is a professional Master Plumber, licensed by The Department of Buildings of The City of New York, and works extensively in NYC while consulting for clients in and out of state.
  • ch4manch4man Posts: 134Member
    your other option would be to replace the RA89 with a honeywell fan center. it has all the components that you need less a 4X4 J-box to mount it

    but the RIB relay/transformer combo will be quieter and draw less power
  • ericmhurtadoericmhurtado Posts: 53Member
    I’m going to try the isolation relay. HVACNUT, I’m afraid I’m having a bit of trouble understanding the diagram so please bear with me. Does the R wire from the transformer need to go to the Rc or Rh terminal on the thermostat? Where do I connect the R wire on the transformer? Thanks for all the help.
  • sputnik99sputnik99 Posts: 6Member

    I’m trying to understand. Should I use another RA89 to serve as the isolation relay? There has to be an easier way to add a “c” wire to the Nest even with my setup. Or should I just buy a bare bones, battery operated, ~$20 Honeywell thermostat?

    The simpliest solution for me was to buy a plug in 24v transformer to power my Ecobee thermostat. I replace the old two wire with a five wire cable and fished it back to the thermostat. This has been running fine going on two heating seasons now.
  • apraetorapraetor Posts: 15Member
    edited January 18
    @sputnik99 The Nest doesn't require a c-wire like an Ecobee. It's supposed to be able to charge the internal battery by leeching power; the few mA it draws was intended to be low enough that it wouldn't cause the furnace relay to close, and the battery tides it over while the furnace is running.

    The Ecobee on the other hand comes with an adapter set so it can be installed using 2 wires, though you do need to have the C-wire available at the furnace end. For users who can easily fish a c-wire it isn't needed. The adapter worked perfectly for me, until my wire failed during boiler replacement. Ran a new 3-conductor cable and wired it directly.
  • Jamie HallJamie Hall Posts: 10,453Member
    The Nest isn't supposed to require a c-wire. Like so many things from Silicon Valley, when reality strikes... if you want the blessed thing to run reliably, a steady power source is essential. This is particularly true for systems which may require less frequent, but longer runs, as the internal battery just don't have what it takes.
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • ericmhurtadoericmhurtado Posts: 53Member
    The isolation relay would seem to work IN this situation. But my situation has changed as I need to replace my ra89a with a taco sr501 relay and I’ll use its 24vac/com to run a common from the nest. Thanks to all for the helpful advice on this discussion.
  • SuperTechSuperTech Posts: 938Member
    edited February 13
    You could have just wired 24 volts from the transformer to RC and common, just as long as you plan on using the Nest for heating only. Remove any jumper between RC and RH, most thermostats are designed to switch power from two different sources, such as a boiler and central air system. No isolation relay is needed. I have my T6 pro at home wired like this.
  • ericmhurtadoericmhurtado Posts: 53Member
    Hi @SuperTech. @HVACNUT mentioned earlier in the thread that because the Nest is already receiving 24v from the transformer in the ra89a relay that the wiring you mentioned would cause issues. I’m a bit confused. Is this correct? Thanks
  • SuperTechSuperTech Posts: 938Member
    It would only cause issues if there is a jumper between RC and RH. I've done this enough times that I can say without any doubt that it works.
  • HVACNUTHVACNUT Posts: 2,172Member
    > @ericmhurtado said:
    > Hi @SuperTech. @HVACNUT mentioned earlier in the thread that because the Nest is already receiving 24v from the transformer in the ra89a relay that the wiring you mentioned would cause issues. I’m a bit confused. Is this correct? Thanks

    Do what you posted at 12:17.
    Replace the RA89A with the Taco SR501. That will give you the 24v you need.
    FYI the reason you cant use the RA89A is you only have access to the Common side of the transformer. The 24v power is internal to the control relay. When you test TT without wires connected, it will read 24v only because its getting voltage through the coil. But TT is just an on, off switch. That's why Nest says you dont "need" a Common. Uh huh.
  • ericmhurtadoericmhurtado Posts: 53Member
    Oh ok I get it. The nest is not getting a constant 24v. I assume the taco sr501 would solve this issue?
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