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Troubleshooting our Buderus Logamatic 4000 based heating system, part I (introduction)


This story is about my journey as an IT professional into the realms of heating control systems, where I had basically zero experience. I was learning the guts of such a system at my own expense, and today I am confident in changing even major parameters of my pretty complicated setup (for a house anyway).


My motivation for writing this story is 1) to document it for myself and 2) to make it useful for those who are fighting similar issues. I am an engineer, but not a mechanical engineer: I had no clue about pumps, valves and control systems. Yet, I was able to decompose and understand what I found and came up with solutions to my problem(s). So here it goes.

How it all started


We've recently moved into a new house with one nightmare scenario: it's not going to be warm enough. I was telling my wife that this was impossible: just look at the oversized boiler in the basement. 55kW, it's HUGE. No way, we'll have issues with temperature. But at the end she is always right :)








When we took over the keys of the house, I was instructed by the seller how the heating needs to be turned on. The house has not been used for 1.5 years at that point, it was tempered at 16°C the last winter... We kicked the boiler on, turned on heating zones and left the house humming at 16°C as it was becoming colder at that time.

A week later, we moved in. I increased thermostats to 22°C, which we were using in our previous house.

At the same time of us moving in, we started a quite sizable remodelling work (I know, we should have done that _prior_ to moving in...), so we moved to a small subsection of the house and concentrated the heating there, while setting the rest of the house to lower temperatures. So far, so good.

The problems started when we needed stuff to dry out (e.g. estrich) a couple of weeks later, so we needed to heat some of the rooms that were previously cold (e.g. down at ~16°C), once we started doing that, we found that either the rooms we stayed in were warm enough or the new rooms could be heated, but not both at the same time.

At this point, I had a choice to troubleshoot the system myself or find a contractor who would do that. I didn't have too much faith in contractors, because I was feeling the system is too complex and the issues too subtle, changes were all slow. Also, due to the remodelling work we had a lot of external influences (doors left open for long, etc). I anticipated a contractor coming in, "needing" to replace expensive stuff while the problem wasn't solved.

Overview of the system


Some technical details might be interesting, especially for those who have similar issues:

  • our boiler is a Buderus G234 X, 55kW, which is responsible for creating hot water both for domestic hot water (DHW, an acronym I recently picked up) and for heating
  • the boiler and the heating zones are controlled using a Buderus Logamatic 4000 series system (I think it is 4321) with 2x FM442 and 1x FM441, totaling to 5 heating zones plus hot water. This system is ~20years old, but is a completely digital, modular system. (which I grew to like eventually :)
  • we are using 4 of the 5 available zones + hot water, one of which is the house heating.

The house heating is comprised of 3 circles:

  • inside the boiler (kesselkreiss): regulated by the Logamatic system based on heat needs in the various heating zones.
  • primary house heating circle: regulated by the Logamatic system to somewhere between 30-40 degrees, depending on external temperature
  • secondary house heating circle: connected to the primary through a heat exchanger, producing hot (~30 degrees) water for wall/ceiling/underfloor heating in the house.

The reason for having 3 circles as I understand is to produce relatively constant, low (~30-35°C) temperatures for under-floor and wall heating.

Fortunately, so far, all of the above turned out to be in solid, working condition. The system as a whole didn't operate the way we expected though, so I started to understand and troubleshoot pieces, but more on that in the next part of this series.

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