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Twinning disaster. Please help me sort it out.

I don't want to post a wall of text to bore you with so I'll be as brief as possible. Need more info? Just ask.

1. Question: Why do the furnaces and control boards need to be identical to twin them? I'm picturing one light switch turning on two lights. Signal to turn on heat goes to each, each turn on heat. Each board would monitor their own gas, flame sensor and so forth right?

2. I've having to sort out someone else's mess. Two rheem furnaces are twined. Have been for 30 years. At some point, a control board burns out and is replaced by someone. Not both, just one board is replaced. On the new board nothing is connected to the 24v supply terminals. Without low voltage power on those terminals, the board can't control anything right? Motor wire M1 is connected to the 120v supply bus, not the M1 terminal on the board so motor runs all the time. That can't be right is it? The other wires are at least on switched terminals, but they are on the wrong ones according to the furnace schematic and the new board's terminal labels. Are there alternate ways to connect the motor to achieve different.. something? The motor on the second furnace stopped working, and I was asked to replace it and that's how i fell into this mess. New motor installed and it runs. Short on time so I left with the blower needing to be reinstalled. While I'm gone someone else replaces the thermostat. I return, note the motor is spinning the wrong way switch the motor's windings as per motor instructions to reverse rotation. I test it and the motor is turning in the right direction, but now at half the speed. I turn off furnace leave and come back another day. New day, turn furnace on for more troubleshooting, but this time the blower doesn't come on and instead the 24v transformer buzzes and melts. Could someone have miss wired the thermostat in a way that would fry the transformer?

If you can help with any of this you would make my life so much easier.


  • ZmanZman Member Posts: 3,168
    Wouldn't they be staged?

    First, If someone else is also working on the system that is not good. That must stop.

    The idea of 2 light switches and one bulb is a bit flawed. The furnaces are designed to be separate appliances. It may work that way, I would be concerned that the boiler safeties may not work correctly. If your boilers are on separate circuits with separate transformers, you may be generating 48 volts in your 24 volt system. You likely need a separate twinning controller to do this correctly. The manufacture should be able to direct you.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
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