arg

What was the problem with them?

Initial response from Sunamp to my query about PCM34: "This was not commercialised and only used for a few people by special order." On further conversation, they said that there's now a PCM43 instead (though that is not yet available either). I'm awaiting more info. Given the PCM54 quotes a 65C water temperature to charge it, and an old manual for the PCM34 says 45C, PCM43 will presumably want something like 55C, which is a bit on the warm side for ASHP efficiency.

Thanks. Indeed I am the same @arg as you have encountered elsewhere, but on this forum I fear I'm going to be doing more asking than answering. Having read your trials and tribulations with the PCM54 units I'd concluded they are probably now safe to go with, but was hoping someone had actually had sight of a PCM34. Still, as you say I ought to try talking to Sunamp directly to find out what the official line is. If I learn anything interesting I will report back here.

Is there any sign of the version with the lower transition temperature (ASHP/UFH compatible) becoming available? The standard ones with/without electric element seem to be stocked in several places, but no obvious sign of the other temperatures that were originally promised (the lower temp I am interested in, nor the higher temp one for use with stoves etc). I'm looking at a major refurb of my 80's house (insulation to Passivhaus Enerfit level, MVHR, wet UFH with ASHP, solar PV etc), so I don't have a plant room nor anywhere sensible for large tanks, but I do have a lot of nooks and crannies where Sunamp units would fit. Existing airing cupboard for example has quite a lot of floor area but only quite a small tank for the DHW and no space for a bigger one because the roof slopes down - it could take about 3 Sunamp units if I packed them in! So Sunamp unit(s) heated by solar PV when the sun shines and E7 electricity in the dead of winter seems like a good solution for the hot water, even though the efficiency isn't great. But then I think about the UFH. I know some people here talk about "store it in the slab", but we aren't going to have much of a slab (floors are block-and-beam, with very low ceilings on the ground floor, so it's been a real challenge to insulate the floor and get UFH within the height available), so some storage is desirable and probably needs a buffer tank to stop the ASHP cycling, and it would be nice to have pre-heat for the hot water to get the efficiency of the heat pump. If I had space, a nice big tank with a coil for the hot water to pick up pre-heat on the way to the Sunamp would do nicely, except as above I don't have room for a big tank.... So a combo of the low temp and standard temp Sunamps would be just the ticket if they actually exist.