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Buderus Logamatic & pool heating and a "dumb" thermostat

I had a number of posts about our Buderus heating system in the past, and I had a recent project I wanted to document, both for my future self and for those poor souls who seek solution for this or a similar problem.

So my Buderus has a heating circuit that heats a pool and the "need for heat" signal is provided by a simple relay in the pool system: when the relay is closed, that indicates it needs heat. When it's open, heat is not needed.

This is basically the same you would get from a "dumb" thermostat, whenever temperature is below the threshold the relay is closed, otherwise it is opened. Furnaces I've encountered in the past always used signals like that.

Not with Buderus though, in it a "temperature controlled" zone needs a MEC2 or a BFU installed, both Buderus products for serious $$$. These devices operate on a 1200 baud serial bus and are actually digital devices that connect to the Buderus control system.

Buried within the various Heating Circle options however I finally found a solution (in the not-so-great Buderus manual, chapter 15).

The elements of the solutions were:

  • I am using the day/night mode of the circuit, in day mode it is running, in night mode it is shut down.
  • To achieve that set setback type to "shutdown" (chapter 15.9), e.g. the circuit shuts down in setback (e.g. night) mode.
  • Then, set the "External changeover mode" (chapter 15.23), to "Day/night via terminals WF1 and WF3", this means that if the terminals WF1 and WF3 are closed (via the relay), the circuit switches to day mode, otherwise it remains in night mode.
  • The last change was that I had to disable frost protection (e.g. set the frost protection temperature to -20 degrees), as I didn't have a temperature sensor on this circuit, which means the assumed temperature is 0 degrees. This means that the circuit would remain running even if the relay is open.
That's roughly it. Took me a while to figure out, especially the last point. Now, I'll just have to install an Arduino/RPi to control that pin, but that's a different chapter.
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