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How does a relay with integral transformer and only two thermostat wires work?

I've searched all around and I can't find an explanation for this though I have a theory. The relay in question is a Honeywell R841C1169. The primary side switches a 220VAC load such as electric baseboard heaters while the secondary side provides a 24VAC control circuit (2-wire) to your thermostat. When this is hooked up to a "dumb" thermostat, when a call for heat is required it simply shorts L1 and L2 of the 24VAC side and 75 seconds (or so) later the primary side is activated. How does this not just short/burn out the integral transformer?

Thanks in advance.


  • Jamie Hall
    Jamie Hall Member Posts: 23,635
    This is, as it says, a relay. The relay coil is in series with the 24 vac transformer and the thermostat terminals. When the thermostat terminals are open, no current flows and the relay stays normal. When the thermostat contacts close, current flows and the relay pulls in to reverse, which closes the contacts for the main heating circuit.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Big Ed_4
    Big Ed_4 Member Posts: 2,827
    Because there is an load , a magnetic coil that pulls in to make contacts... It is call an relay

    There was an error rendering this rich post.

  • Zman
    Zman Member Posts: 7,605
    I would recommend picking up some of Carol Fey's books available on this website. http://astore.amazon.com/heatinghelp-20?_encoding=UTF8&node=7

    Very few heating guys really understand controls and wiring. Carol makes it super easy to understand.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • chrismaki
    chrismaki Member Posts: 5
    Thanks, and I agree. The reason I'm here is because the electrician I hired to get my non-standard heating issue under control couldn't figure it out and neither could the two other guys he brought in, thankfully at no cost to me. He burnt out multiple relays in the process. I ended up sorting it out with a little research (One Nest, two loads, two circuits) and now I have everything working fine, including having added a common wire by simply moving the transformer to the thermostat away from the two relays. I'm not an HVAC guy, I'm just someone who isn't afraid to take the time to figure something out.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    I wish I could teleport myself.... I LOVE working on "non standard" systems and making it all work and look professional at the same time.

    The 24v side runs a coil which is the fingers that turn on the switch for the 240v load of electric heat. Does that make sense?

    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!