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Folks, I am having problems connecting my Nest thermostat to a boiler. Given Laars Mini-term JVS series with Temperature Control PCB Module (pictured below)

RE2344300

When I short Com call and Heat call wires the boiler starts working. Being a naive person I would expect to detect 24v AC, however my multi-meter shows only 2.6v which is not enough. I have only 2 wires running from this controller to a thermostat.

Question 1: How do I connect my thermostat to something that does have standard R,C, W1, W2, etc. terminal?

Question 2: Is it possible to supply required voltage via these 2 wire to power off the thermostat and without damaging the controller (additional gadgetry is OK)

Question 3: Looking carefully in the middle of the PCB controller legend shows "hopeful" R,C,W2,W1 PCB connector which must be on the opposite side of the device. It is not easy to reach and therefor can't be a standard interface to the controller. Am I correct?

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I found one solution that I don't like but it will probably work. enter image description here

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2 Answers 2

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With only two wires between the thermostat and the boiler, you're not going to be able to power the thermostat, and call for heat. You'll need at least three wires for that.

The C terminal on the controller is likely the COM terminal, though I can't say for sure. According to the schematic, pin 2 of the PCB connector is C. But I'm not sure where it's routed from there on the board.

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My first attempt to conceptualize my own answer. Please let me know if it is a bad idea. I want to use Aube RC840T-120 relay. Please let me know if it is a bad idea.

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    You have the wrong relay for the job -- use a straight 24VAC relay instead of the relay/transformer combo you're currently specifying\
    – ThreePhaseEel
    Commented Oct 26, 2016 at 0:24
  • 1
    Agree 100%, Just ordered homedepot.com/p/…
    – bioffe
    Commented Oct 26, 2016 at 15:27

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