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Slant-Fin Monitron 2 problem

hws Member Posts: 43
One of my customers has a Slant Fin Monitron 2, installed last year by a different contractor, which has been alarming on a fault code. The code indicates Inlet temp sensor - open circuit.

Naturally I first checked for correct field wiring of the sensor into the board, which looked OK, then tested the inlet sensor by measuring ohms, which checked out fine. I called Slant Fin's tech support line, they said if the sensor checks out OK, it must be the digital controller (model em-10). They sent me a new digital controller and I replaced it. I changed some of the factory settings to cancel the pump post-purge (because they don't have a diff. pressure bypass installed, and I noticed the circulator was deadheading).

Next week the customer called and informed me that the alarm was still going off. Having tested all the usual suspects, I called their tech support line again. The guy was stumped, went to discuss it with their engineering department, and got back to me. The engineering department didn't have an explanation either, but they suggested a complete swap-out of the electronic components. I replaced the PC board, digital controller I mentioned earlier, and the wiring harness that connects the two.

I contact the customer a couple weeks later to see how things are going....The alarm is still going off, the fault is still Inlet temp sensor - open circuit.

Anyone have any ideas on what to do next? This is my first issue with a Slant Fin....usually their stuff is pretty reliable. Not sure what to do next.


  • hws
    hws Member Posts: 43
    one other thing to mention

    The digital controller has 8 operational modes. Mode 4 is parallel piping (trunk & branch, supply/return manifold with zone valves), which is what this boiler is operating in. It's worth noting that this mode doesn't utilize the inlet temp sensor, just the supply sensor and built-in ODR. Both of these sensors check out OK too.
  • hws
    hws Member Posts: 43
    new info.

    not sure if this helps, but homeowner informs me that the alarm usually starts up in the morning, when they first turn the heat on for the day. non-programmable stats. Last week, I asked him to try turning on a different zone each morning, leaving the others off (4 in total).

    what we found is that the system alarms no matter which zone calls first. I may come back to take a closer look at the field wiring. I can't see why a thermostat circuit would trigger a "return sensor-open circuit" type fault code.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,510
    Mysterious sensor fault

    Sounds like everything electronic inside the boiler has been replaced, so could it be a wiring problem between the boiler and the panel. These mod cons seem to be very sensitive to electrical noise, and grounding problems (especially when used with cheap generators).

    If that is the case, then the electronics should be programmed to analyze the incoming power to show a power supply problem.--NBC