Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.

Taco SR502-4 Switching Relay - Unable to Power Two Ecobee4's?

pburke
pburke Member Posts: 2
edited June 2018 in Thermostats and Controls
Hello,

I tried researching this issue but the solution still seems somewhat unclear to me. I'm in the midst of a major renovation and have had numerous issues with both the plumber and electrician and as such am trying to solve this problem myself.

A new Taco SR502-4 Switch Relay was installed for two heating zones by the contractor. An Ecobee4 thermostat was installed in each zone by the contractor. One thermostat receives power while the other does not. I've traced the issue back to the Taco Switch Relay. On the upper left corner of the circuit board there are two connections to provide power to the "C" wire. One seems to be getting approximately 27 volts while the other connection doesn't seem to be getting any power.




Ecobee support believes that the transformer is not sufficient to power both thermostats and I need a more powerful transformer and an isolation relay. Is this correct? It just seems odd to me that the second terminal is showing no voltage as oppose to low/or insufficient voltage.



Thank you for your time and assistance and I apologize in advance for any improper terminology.

Thanks,
Pat

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 17,841
    If I read that circuit board correctly, one side of that connector is labelled "24 vac" and the other side is labelled "Com". Common usually means a return, sometimes (not always) grounded, while 24 vac means just what it says.

    To what are you connecting the return from the Ecobee which works?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ratio
    ratio Member Posts: 3,050
    As Jamie said, you should be showing (a nominal) 24V between those two terminals, and if you show 24V between the "24VAC" terminal and ground/metal box and appx 0V between "com" and the same place that merely indicates that the common terminal is electrically to the box. If you have an ampy clampy, measure the current on the yaller or red wire from the xfrmr. if it's anything over about ½ amp, it's nearly certainly your issue.

    A 15 VA xfrmr is quite small and I have no trouble believing that it is unable power two Ecobee stats and the SR502. If it were me, I'd just replace the xfrmr with a larger one, but that would not do anything good for your warranty.

  • HVACNUT
    HVACNUT Member Posts: 4,367
    edited June 2018
    What are the Blue (24v) and Black (Com) wires connected to on the Ecobee's?
    You should only need the "Com" wire connected the the "C" terminal on the thermostats. The power (24v) is already there from the Red wires "R" terminals in the relay where the thermostat wires are connected. R-W. Nothing needs to be connected to the 24v terminal.
    Note: For HVAC low voltage control wiring, the power wire (24v) should be Red and Common wire should be Blue or Brown.
    The thermostat wiring should be simple.
    R- 24v
    C- common
    W- to heat heat relay (your SR502)
    G- to fan relay
    Y- to the A/C condenser
    O,B- if it's a heatpump system
    ZmanAlan (California Radiant) Forbes
  • newagedawn
    newagedawn Member Posts: 586
    you need at least a 40 va transformer for that application, maybe even a 75 va transformer, not to familiar with ecobee watt consumption check that
    "The bitter taste of a poor install lasts far longer than the JOY of the lowest price"
  • lchmb
    lchmb Member Posts: 2,997
    edited June 2018


    you may need a resistor
  • Tim Potter
    Tim Potter Member Posts: 272
    try switching places with the blue wires on the 24vac/com termial and see if the other t-stat powers up.
    Winter Park, CO & Lenexa, KS
  • Zman
    Zman Member Posts: 7,139
    @pburke
    HVACNUT said:

    What are the Blue (24v) and Black (Com) wires connected to on the Ecobee's?

    You should only need the "Com" wire connected the the "C" terminal on the thermostats. The power (24v) is already there from the Red wires "R" terminals in the relay where the thermostat wires are connected. R-W. Nothing needs to be connected to the 24v terminal.

    Note: For HVAC low voltage control wiring, the power wire (24v) should be Red and Common wire should be Blue or Brown.

    The thermostat wiring should be simple.

    R- 24v

    C- common

    W- to heat heat relay (your SR502)

    G- to fan relay

    Y- to the A/C condenser

    O,B- if it's a heatpump system

    Like he said :)
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • pburke
    pburke Member Posts: 2
    Thanks for everyone's insightful replies.

    So based on what I've gathered from everyone's replies and a little deductive reasoning, I ended up connecting both Blue "C" / Common wires to the "COM" terminal on the Taco's board. (Please see picture below). This seems to have fixed the issue as both Ecobee's are now powering up and currently seem to be functioning properly.



    Question being:

    Does anyone see any potential issues with the way I did this?

    Thanks again for everyone's time,
    Pat
    doctorman
  • ratio
    ratio Member Posts: 3,050
    Functionally, it should work; but that xfrmr is very low power. It may not be able to provide sufficient current if e.g. both stats call at the same time. The easiest way to check would be to turn them both down so they call for heat & check the voltage at the 24VAC & COM terminals—you're looking for at least 24 volts at maximum load.
  • hot_rod
    hot_rod Member Posts: 15,440
    Up to 3.5 Va for the Ecobee draw. Plus whatever else is on the circuit.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Alan (California Radiant) Forbes
  • RandyCaruso
    RandyCaruso Member Posts: 1
    VERY similar problem with that Taco board, actually fixed via this info however: When testing for the 24v I got .3v - nothing more ever no matter how I tested. Basic Multi-meter settings but when I hooked it all up: poof! it worked. What the hell, is there a digital switch there I'm not aware of?
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,464
    You shouldn’t use the word 'poof' when discussing wiring and electrical connections. 'Poof' usually means you let the smoke out...lol
    steve
    EdTheHeaterManepmiller
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 2,560
    @STEVEusaPA I thought the same thing.... Poof and my gut went :s
    Edward Young
    Retired HVAC Contractor from So. Jersey Shore.
    Cleaned & services first oil heating system at age 16
  • nh2020
    nh2020 Member Posts: 1
    @pburke I have this same setup and found your comment. Did you ever find a reason that wouldn't work? I was going to try both in together but wanted to make sure it's safe.
  • HomerJSmith
    HomerJSmith Member Posts: 1,421
    15VA isn't a lot of current. Probably just enough to power the circuit board and the relays.

    You can add your own 40VA transformer in place of the 15VA transformer. It is very easy to do and you would have plenty of power and it would be cheap to do.
  • doctorman
    doctorman Member Posts: 109
    pburke said:

    Thanks for everyone's insightful replies.

    So based on what I've gathered from everyone's re
    plies and a little deductive reasoning, I ended up connecting both Blue "C" / Common wires to the "COM" terminal on the Taco's board. (Please see picture below). This seems to have fixed the issue as both Ecobee's are now powering up and currently seem to be functioning properly.



    Question being:

    Does anyone see any potential issues with the way I did this?

    Thanks again for everyone's time,
    Pat

    so just getting the Com connected to the W of the thermostat did the trick , and you can power your thermostat with 2 wires only?

    I have a Nest and same situation
    Taco SR-503-4
    did you upgrade your transformer from 24VAC 15 Va to 40Va ?

    no need for the 1000ohm 1/2W resistor between the COM and thermostat?

  • Steamhead
    Steamhead Member Posts: 15,050
    edited September 2021
    No. The W on the thermostat goes to the W on the relay for that zone. The C wire on the Nest goes to the COM terminal on the board.

    So you need a three-wire cable between the Nest and the SR504.

    I wouldn't trust the resistor hookup. There are way too many electronics in the base of a Nest, and they don't like shortcuts.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • Bucky
    Bucky Member Posts: 33
    Not sure if my question deserves its own thread, plus I'm referencing a post in this thread, so....

    When I replaced my two-wire t-stat with an ecobee, I ran a new 5-wire thingy, and added a 40vA transformer up in the joists above the boiler. Not ideal to say the least. Now I read the above post about replacing the 15vA in the SR-504 with a 40vA. Bingo, I think! But I just went and looked- the xfromer in the 504 is MUCH smaller than the 40vA. And I few other I have also. So much so that it doesn't seem like a 40vA will fit in there as a replacement. Is there some specific model I can get that will fit? Thanks!