Posted by: @paul1098

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I must admit that I've always been wary of heat pumps in old properties as I thought you'd need to increase the insulation and there is no way external or internal insulation is being added to the walls. I'm very comfortable with my solid walls breathing as they should to avoid rising damp.

I think the sensible route is to do what you sensibly and sensitively can. External insulation is out of the question for aesthetic reasons, and internal insulation is unhelpful in small rooms. What you can do, and I have done, or am in the process of doing, is things like unobtrusive secondary glazing (PET-G clear plastic sheets held in place by magnetic strips (which is OK in listed buildings), helps seal draughts as well, and easy to do yourself provided you are prepared to accommodate the idiosyncrasies of old window frames, mine are all over the place!) and other anti-draught measures like closing door gaps etc and of course loft insulation, but again you need to careful about ventilation in the roof space.

The unavoidable fact is that old buildings are always going to be leakier than modern purpose built low heat loss buildings, that's just the way it is. Yes, we could cover everything in some hideous modern material, but the cost in aesthetics is to my mind far to high, and then, as you rightly say, you may well end up suffocating the buildings ability to breathe. We who chose to live in old attractive buildings just have to accept that as well as the rewards that come with living in such a building, there are also costs, and one of them is higher heating bills.

Posted by: @paul1098

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Was your system designed from scratch or did it replace a gas/oil boiler?

In practice, both. It replaced an old oil boiler, but the lower flow temperatures meant the existing radiators were too small, so in practice the system was designed mostly from scratch, apart from using the existing radiator pipework, which was of adequate bore, at least where it could be seen or inferred. The workaround I used to get adequate radiator outputs was to use K3 (three panel) radiators, and while these do tend to look very bulky in isolation, once in place, they blend in sufficiently (nearby furniture will typically stand out more). The 'wall print' (area taken up) is significantly less than a similar output K2 radiator, and this is useful in small rooms with not a lot of wall space.

At the time I was getting quotes (2021), I did have some understanding of heat loss calculations, having done them for my oil system, as well as the coal fired system that predated the oil heating. Some of the heat loss estimates that accompanied the quotes were absurd (a common problem, as you probably already know) but I could weed them out, and in the end went with a heat loss that made sense to me at the time (just over 12kW). That was based on a spreadsheet (what I call whattiffery - what if the wall heat loss is X etc etc) method, and in practice, measuring the heat loss using the heat pump (easy to do with monitoring which I have) the actual heat loss is a bit less, around maybe 10-11kW. One well fingered cause of over-estimates of heat loss is too many air changes per hour, but for old stone buildings there may well be another, over-estimates of the U values for stone walls locked into the whatiffery in heat loss spreadsheets. SPAB did, and I believe is still doing, some research on this, see here.