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Taco SR504-4 switching relay - first relay makes noise or drops on certain zones


I have 3 zones on a Taco SR504-4 switching relay. One is on Nest with the AC and the other 2 are on Sensi.
When I turned on the heat on the Nest the first relay ( the one not related to the zones) started to drop here and there (maybe when the pump started?). I disconnected the AC from the Nest and now it does not drop anymore but it sounds that is about too (makes noise).
When I turn zone 2 which is on Sensi the relay makes a lot of noise but it does not drop.
When I turn zone 3 which is on Sensi the relay makes no noise.

I can't figure this out. I think is a bad relay which I don't think it can be changed individually. I might have to replace the entire Taco switching box.

Looking for advice.
Thank you


  • Jamie Hall
    Jamie Hall Member Posts: 22,964
    May not be a bad relay. Very often it is simply not enough juice to power the relay. What is the power source for all of this equipment?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Joe Mattiello
    Joe Mattiello Member Posts: 703
    My guess, it is not the control. An easy test is to disconnect the thermostats and place a simple jumper across TT terminals to see if it still makes noises, or act erratically.

    Some of these power robbing thermostats require more voltage than the control has available. I would check with Taco tech support at 401-942-8000. They are better equipped to answer these questions when attempting to trouble shoot a problem.

    I’m curious to know how you make out.
    Joe Mattiello
    N. E. Regional Manger, Commercial Products
    Taco Comfort Solutions
  • SteveSan
    SteveSan Member Posts: 231
    criocin, as Joe suggested I would disconnect the t-stat going to your SR control and jump the R-W on the control to see if the issue is still there. Do you t-stats require a com wire and do you have the com going to the SR control? Because of the draw of all three t-stats you may require an external transformer. The 15va transformer on the control may not be enough. Attached is a wiring diagram to add an external transformer to your SR504-4. You can always call into Technical Services during normal business hours Mon-Fri 8am-5pm EST 401-942-8000 and just ask for Technical Services
  • criocin
    criocin Member Posts: 5
    edited October 2022
    The zone who does not make the relay noise has the thermostat with his own C and power.

    The zone who makes the most noise has the thermostat C from the Taco SR control and has also a Two way zone valve (who uses the transformer on the control) before the pump.

    So I think an external power source for all the thermostats might be the answer:


    I will let you know. Thanks
  • SteveSan
    SteveSan Member Posts: 231
    What type of zone valve do you have? I have wiring diagrams for our heat motor ( 3 wires ) and zone sentry ( 4 wires ) when connecting to an SR control.