@paul1098, welcome to the forums. Sounds like you’ve already got a useful head start if you’re comfortable with heat loss and pipe sizing.

Your point about installers is fair too. Heat pumps get a lot of attention, but finding good tradespeople can be difficult across the board, and with heat pumps the design mistakes tend to show up more clearly.

The stone cottage, storage heater replacement and National Park restrictions should make for an interesting project. When you’re ready, start a dedicated topic for the cottage with the basics of the property, your heat loss figures and any planning constraints. There are plenty of experienced members here who’ll be able to help you work through it.

@paul1098 — I like many others have an old listed grade II stone cottage in a national park and I had a heat pump installed in early 2022. Getting planning permission and listed building consent was a bit of a struggle but got there in the end. The design was satisfactory, but for various reasons the system under-performed in cold weather initially, but with the help of some very clever people on this forum we worked out what the problem was (bottom line: inadequate secondary flow on the secondary radiator circuit, couldn't transfer enough heat), and now it works fine, and only starts to struggle when it gets down to -4°C outside, which is 2° below the design outside temperature.

Old buildings often have high heat losses, because of the way they are constructed, and the wishes of owners and indeed the sensible wider public not to go around letting green zeal ruin attractive old buildings means they tend to stay that way. But the thing is a heat loss doesn't care one jot where the replacement heat comes from. It can come from anything that produces heat in a form usable for domestic heating, and that includes heat pumps. Yes, your energy bills will be higher, but they will be higher, whatever form of heat generation you use.

One of the secondary benefits which I hadn't appreciated until I had had the heat pump for a while is reduced condensation, which is often a problem in old buildings heated intermittently. Because of the way heat pumps are run (low and steady, on most or all of the time), the house reaches a steady state, without the swings that cause condensation.   

I and I am sure many others will be interested to hear how you get on when you do start in earnest, and will be very willing to help where we can if needed.        

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

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.        

@cathoderay I've noticed SPAB have done work on heat loss thru' solid walls on older properties and found the U value's to be better than RICS would suggest.
The house I'm currently looking at converting  to a heat pump is a holiday cottage and should be simpler. If the process ends up being OK then the more difficult is my main property which is another old building with Oil boiler thermal store and solid fuel stove.

I sized the rads, boiler and piping 35 years ago but things have changed. The individual pipes to the rads are 10mm HEP2O microbore which could prove challenging with the increase in flow rate required for low temp rads. Hopefully the 22mm flow / return mains will be adequate and the 28mm from/to the boiler.
I have seen suggestions that flow rates need to be kept below 1m/s for noise but I believe that is for copper. Because plastic damps noise better I should be able to go to 1.5m/s without undue noise issues, hopefully. I'm expecting to change virtually all my rads to K3 and I can increase sizes vertically without too much issues.
I already have both houses heat loss calcs modelled so will check any quotes against those.

@paul1098 — as we are getting down to the nitty gritty, now is probably a good time to start your own thread, as @editor suggested. There are as you say quite a lot of opinions about pipework sizing, and the various consequences or not as the case may be, and such discussions are best conducted in dedicated threads. I'll check with @editor, normally any migration of existing posts needs his magic touch.