@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.

Posted by: @paul1098

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I already have both houses heat loss calcs modelled so will check any quotes against those.

Whenever I see the word modelled I am reminded of the triumph of spreadsheets over common sense. Modelling in this context is no more or less than whatiffery, and to be of any use (and I agree it can be of use, but the word is wrong, it should be 'estimated, as in 'I already have both houses heat loss...estimated so will check any quotes against those', to remind us that the number we have is only an estimate, the word modelled is too concrete), the assumptions, the what-ifs, have to be pretty good, and often they are not. For years, the standard industry assumptions about air changes (what if the air changes are X changes per hour?) were over estimates, resulting in over-estimated heat losses, not that the industry minded, it gave them a (false) rationale to install over-size heat pumps, good for the installer, less likely to underheat the property, but less good for the owner, higher costs. The same thing applies to solid wall U values. In the Freedom Heat Loss Spreadsheet, widely used, the hidden Midea heat pump output data was guesswork (there is a hidden note in the spreadsheet 'the midea figures are mostly made up' ie what if the Midea output is something like this). And so on - it's all just whatiffery.

And then there are typos. My installer's heat loss spreadsheet had a room external wall length entry of 2.2m, when the actual value was 12.2m. Luckily I spotted that in time to correct it. But the general point still applies, anything that requires a lot of numbers typed into it is at risk having incorrect entries. 

In practice, this means it is well worth the effort to try and find alternative ways of estimating heat loss. Actual energy delivered to the house in steady state conditions is the gold standard, and is do-able with daily or more frequent gas consumption figures, see other threads, and of course heat pumps, but by then you are long past the design stage! But not everyone has gas, let alone daily or more frequent energy use data. My own conclusion from looking at my own historical oil use, at a time when I saw no need to have more than just an awareness of my annual heating costs, is that in the vast majority of cases, the available data is just too inadequate.

In such cases we are just left with whatiffery, which can be better than nothing, as long as we are scrupulously careful about our what-if assumptions, we check and double check the entries, and forever remind ourselves that the magic number that comes out at the end is just an estimate, and not a model. In my own case, the Freedom Heat Pump Calculator estimate was around 20% higher than the actual heat loss.          

@paul1098 

@cathoderay is spot on about models, you need to sanity check them if at all possible.

That said models or other ways to estimate loss only need to be good enough to make the key design decisions, which often are quite  coarse grained.  So for example if your house loss is 6kW or less, you are getting a 5-6 kW heat pump simply because it's the smallest generally available and you probably want that capacity for DHW.  Manufaurers often sell heat pumps with finely grained sticker capacity, but when you look under the hood many models are just firmware limited larger models, so apart from a small price difference you may as well get the largest.  For example Daikin sell (or at least until recently sold) 4,6,8,10,12,14,16kW but in reality there were/are only 8 and 16, the rest being firmware limited.  Similarly Vaillant sells 5, 7 and 12, the 3.5 and 10 kW models are simply firmware limited versions of their larger cousins.  Thus your whole house heat loss estimate needs to distinguish between these it does not need to be accurate to even one dp.  Also remember oversized radiators are not a problem, only oversizing matters.

Factoring these practical considerations I can reduce significantly the perceived and actual risk with this part of the process. Nevertheless a sanity check against a measured value is always recommended.

Probably shouldn't have used the word modelled. I have used commercial software with currently accepted U values for walls, floors, windows etc to guesstimate heat loss for each room of the house.
I have very little record of the energy used as the installed smart meter can't transmit any data so is in dumb mode. This is apparently going to be rectified with the install of a new smart meter but I doubt it as there is no 4g or other signal in the village never mind inside the house.

Posted by: @paul1098

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I have very little record of the energy used as the installed smart meter can't transmit any data so is in dumb mode.

Noted.

What do you have.  Even annual usage is better than nothing!

Posted by: @paul1098

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Probably shouldn't have used the word modelled

It's a bête noire of mine, the widespread modern use of the word. I certainly wasn't having a go at you. When I was still in shorts, models were things made by Airfix that you bought in toy shops. The word meant a physical, usually much smaller, representation of a real object. No one expected their AirFix Lancaster bomber to predict whether a real one would fly, if they did try to test it, it would end in tears before bedtime. Later, when I learnt basic statistics and did a scatter plot, and fitted a line by least squares, I called it a regression line, not a model. Then, at some point along the way, everything became a model, and somehow the word assumed a general authority it never really had in such general use. Sure, you can do real modelling eg tank testing of ship hull forms, and suchlike, under strict conditions, and what is going on their is real modelling, but general purpose every day 'modelling' as in I modelled whatever it is that I modelled today is almost always a process of estimation albeit usually based equations (how why they derived? do we trust them?) but the inputs are all too often little more than guesstimates, and sometimes not even that. Being 'professional' is no guard against errors, all the examples I gave earlier came from 'professional' sources. It's why I have a habit of saying from time to time the five most terrifying words in the English language are 'leave it to the professionals'.