Hello Outlaw UK,what a heart-breaking story; very sorry to hear so.It's a while ago now that you posted, how are you proceeding?Thank you for providing a well-done problem description, which lets me think of a possible solution.

However, I would appreciate when @jamespa and @editor would give supervision here; I'm no way sure if my contribution can be helpful.When this was my own system, I would proceed as follows, taking single steps, approaching mostly isolated issues independently.

Any of these steps would provide the installer to an explicit, unequivocal mission without charging him unduly a design risk:As measure zero, I would never ever turn down the hall thermostat for a night setback or the like.

Especially the sandstone part of your house will take three days minimum to acclimatise, during which time you will suffer cold feet. When turning down temperature, the building will take again three days to cool down, providing a good feel of sitting in a dwelling done well, but the bad ending is that it will take three days again to get back to setting point (even with a proper heat pump).

The first thing to do for the installer from my point of view would be to take out that unspeakable combination tank from the heating cycle but, since I am stingy, mean and cheap, leave it as a domestic hot water (DHW) cylinder in place.

A manually (yes!) operated three-way valve would serve either DHW preparation or the heating circle with hot water from the heat pump; such concept normally would be automatically driven by the typical heating control system (which includes a weather compensation as well).

This would mean that, some ten minutes or half an hour before taking a shower, I would have to walk into the boiler room, turn that valve to 'DHW' and, later, back to 'heating' again. Since the heating and I are already thick as thieves, this would be no serious issue and, being a temporal solution, neither.

From this measure I would expect serious progress, in front assuming of course that the cylinder will comply with the pipework modification.

First of all I look forward to finally bring the given 45°C to the emitters, thus adding at least 50% more heating power to the house. When further it would turn out that, in the course, the insubordinate outdoor unit gave up its bucking, this would be a great success and spawn an investigation if the 45°C-ban can lifted from that device and if it can get paired with a third party controller, in which case I would seriously consider to keep the green monster in place.

Otherwise it would be time to give it a bullet and call in the knackerman, while putting an order for a new heat pump including a suitable controller and a motor for the manually driven three-way valve. Since the newish R290-pumps are said to be able to supply high flow temperatures without giving up too much COP, they seem the right choice.Next stage then would be to find out if now, with an employable heat pump at hand, the emitters will do a proper job heating the house on the one hand and not sacrificing too much efficiency on the other. When this, as can be expected, will not be the case, at least it will be easy now planning replacements along the visible deficiencies. And the last step would be to finally lay waste to the room decoration while replacing radiators as necessary.

As can be seen, the worst step is the last one and obsolete when the former moves came out well. These again would be either in the boiler room or outside, not much interfering with the use of the building. And none of that will exceed the complete replacement you seem to plan.

As a timeframe I would start immediately with step one and watch the system working during spring and summer, putting step two in place when autumn comes for to approach winter with a performing heat pump. When in January it turns out that further measures will be necessary, do them in 2027.

Posted by: @editor

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The problem is that the technology still gets judged by the worst installs, and there are unfortunately still enough of those around to keep the scepticism alive. I would argue the numbers of poor installs continue to increase.

I think this is an important point to keep repeating, but perhaps with some context:  

I looked around for independent Heat Pump CSAT reports and came across (for a second time) the Nesta one published in 2023, which seems to be the best available.  In summary it says that people are broadly as satisfied with heat pumps as they are with gas boilers, albeit that satisfaction with heat pumps is a bit more polarised.  However the satisfaction with boilers isnt particularly high, only about 70% being either very satisfied or fairly satisfied.  So thats 30% who arent satisfied.  There was no rating 'neither satisfied nor dissatisfied, you had to be one side of the line or the other. 

These people, if they have just had a heat pump installed, will attribute their dissatisfaction to the change in technology, not to the fact that this level of satisfaction is (apparently) representative of our heating industry as a whole!

Hello @outlawuk sorry, I forgot about this, important:if it's not already done (you didn't mention though), consider of insulating your cavity walls. This would be a step into the right direction anyway plus that it could save you from replacing radiators.