Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.

If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Need to contact us? Visit

Transformer and relay

Boston_2Boston_2 Member Posts: 107
This may be a silly question, but could someone explain to me why is there a need for both a transformer and relay in my boiler? Is it for the gas valve? We had an outdoor reset installed and now once in awhile i get some chatter from the relay? Is it possible the end switch on the reset is causing the relay to hum/chatter?


  • agelessageless Member Posts: 2
    Most boilers are set up with 24 volt control networks. The purpose of the transformer is to step down the 120 volt supply voltage to 24 volts for your controls. If adding an outdoor reset controller to the existing controls this may add additional load to your transformer. It is quite possible the VA rating on your transformer is not high enough and the contacts on the relay may drop in and out causing it to chatter. The other possibility is you may not have the wiring on the end switch and voltage may cause the the two transformers to buck if you added another transformer with the new outdoor reset.
  • SWEISWEI Member Posts: 7,356
    To add to the above, the relay is used to switch the circulator. If it's chattering, it may be an overloaded transformer as indicated above. It's also possible that either the relay or the transformer is starting to go. A voltmeter will tell you a lot -- put it across the transformer secondary and watch the voltage when the relay pulls in.
  • Boston_2Boston_2 Member Posts: 107
    Thanks both, @SWEI, the circulators are coming of a Taco 3 zone relay panel (120volt output to circulators). The simple wiring diagram is: Thermostats to 3 zone relay pannel> end switch to Taco out door reset> end switch to boiler. I will look at the Transformer VA output but since the outdoor reset has it's own transformer and power source should it be straining the boiler transformer?
  • SWEISWEI Member Posts: 7,356
    Unlikely if it has its own power source, but the presence of two separate supplies in one circuit can get a bit tricky at times. Start with the pull-down test I suggested above.

    Are the boiler, the ODR controller, and the zone relay fed from the same breaker? If not, are they all on the same phase?

    Measure the voltage across each 24 VAC supply, the voltage (if any) between the two common terminals, between each of those and ground, and between the various 120VAC and 24VAC hot leads. I have seen some oddball voltages when transformer secondaries interact, and even stranger stuff when things that are supposed to work together are fed from different phases or subpanels.
  • icesailoricesailor Member Posts: 7,265
    You bet your sweet Bippy it will.
Sign In or Register to comment.


It looks like you're new here. If you want to get involved, click one of these buttons!