There was a misunderstanding somewhere and it appears that the overall cost for the district heating system would exceed that of the version with a heat pump. I still don't understand this clearly so I obviously cannot explain it. So, it's back to the air-source heat pump, though perhaps not the one by Heliotherm.
I'm looking at some of the results of performance tests of these devices done by independent labs[1] to see if I find a favorite. One new issue that I've become aware of is that our technical room[2] is a little on the small side. The heat pump and its associated plumbing will fit, but we really could use the space if we can save it. Also, this type of installation requires large openings from the room to the outside and because this room is partly underground, construction elements called Licht- or Kellerschächte[3] have to be used. I have a great aversion to these things, partly from the maintenance point of view. To circumvent both these problems, I think a unit that is installed outside the house is called for (we had been considering only internal units so far). I have yet to find any indications that they're less effective than the ones installed inside.
[1] One such facility is in Switzerland → Wärmepumpen-Testzentrum WPZ.
[2] Here's a link to an old sketch → Figure 7g. The lower level.
[3] I'm not sure these things even exist in the same form in the Anglo-world. You can do an image search with the keyword "Kellerschacht" or "Lichtschacht" to see some examples.
