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Manual High limit trips on Logamatic control

Brian S
Brian S Member Posts: 11
I worked on a Buderus G124x gas fired boiler with a logamatic control w/ OR. Boiler has two zones. Flow valves on supplies and pumps on return. After the thermostats are satisfied and the boiler shuts down at temperature, the temp in the well with the logamatic sensor climbs 40-50 degrees. On colder days its tripping the manual high limit. Any ideas on what might be causing this issue?


  • furnacefigher15
    furnacefigher15 Member Posts: 514
    How long

    How long has it been like this?

    Since day 1, or is this a new development?
  • Brian S
    Brian S Member Posts: 11
    More recently

    Boiler is 4 years old. It tripped once last heating season although I don't think anyone looked into why it happened. In the past week it has tripped 3 times.
  • Tim McElwain
    Tim McElwain Member Posts: 4,593
    If that is a thermistor

    you can ohm it out, at around 70 degrees most of them are around 11,400 ohms, then heat it up cup of hot water and see if it changes, or some ice same thing if not it is gone replace it. The I and O manual should give you some exact numbers on the ohms for your system.
  • Brian S
    Brian S Member Posts: 11
    High limit

    Thanks Tim. I probably should ohm it out to be sure. I did have my probe thermometer in the well and the temperature reading on the Logamatic 2107 control appeared to be right on with my thermomter. I know some temperature rise in the boiler after it shuts off is probably normal. But I watched the burners kick off at 160* and the temp in the well climbed to 210*. 50* sounds like alot. The manual high limit is set at 230*. I'm sure the well would have reached 230* if the outdoor reset was calling for a higher boiler temperature.
  • meplumber
    meplumber Member Posts: 678
    That is a recurring problem with this boiler.

    I have around 20 of these in a retirement community. They are all around 10 yrs old.

    About 3 yrs ago we started having the same problem with many of them.

    After a long process of trying to figure out why these things were overheating, Buderus suggested that we do a flush of the water side of the casting. We isolated the system and flushed out a handful of them and replaced with fresh antifreeze mixtures. That solved the problem. So we did it with the rest of them.

    All of these units are piped primary/secondary as per the engineer. We did find that two of the primary pumps were not circulating to spec. Have your heating professional check primary side pump (if piped primary/secondary) and then give flushing a try.

    Good Luck.
  • Brian S
    Brian S Member Posts: 11
    re: Recurring problem

    Thanks for the info. I did receive some similar input from Buderus just yesterday during another attempt to troubleshoot. Though they didn't exactly come off convincing that this was the solution to the problem. Nor were they willing to offer up much of an explanation towards the cause of the issue, other than "dirty boiler". But with no other leads I did give it a shot. They suggested putting a cleaning solution (I used TSP) in the system for a couple days, then go back and flush it out. This was not an antifreeze system, nor primary/secondary. I will post the results when I go back to flush it out. Glad to hear that worked out for you.

    Did you get any explanation for the cause of the problem?

    Is it an issue with the casting itself? Old piping in the system? etc.
  • meplumber
    meplumber Member Posts: 678
    Recurring Problem

    FamilyB, I don't have an answer for that.

    I have been a Buderus installer for years. I have been trained at their factory. Several times. However, I don't know why that problem only occurs on the 124's. It is essentially the same casting as the 115's. I have posed that question to their engineers. No real answer.

    The flushing will work, or at least it did on the 20 or so of them that I was having the problem with.

    Good Luck. Let us know if it works.
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