Ther are many on here who are very knowledgable about A2A and some of the questions you ask are better answered by them.  I will offer the following.

Posted by: @springswood

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I think in a way that's because the equipment available is just too powerful for my modest and well insulated house.

True I think.  As you say A2A systems are principally used for commercial cooling (typically a small number of large rooms) and I suspect that they have not yet evolved fully for domestic heating (typically a large number of small rooms).  Plus there may be a natural limit on how small a capacity its practical to make. 

Posted by: @springswood

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What does a water heating circuit bring to the party?

A largely silent energy transport mechanism which, in a retrofit, already exists.  Also we in the UK have a bit of an aversion to blown air, although I suspect that could be overcome in time.  Nothing more and nothing less so far as I can see.

Personally I expect A2A to grow in popularity, particularly with the trend towards well insulated open plan. 

Thanks @jamespa. I share an aversion to blown hot air. I finally found a good YouTube video on how to site the indoor units so I'm less worried now Mini Split Location. 

Posted by: @springswood

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I'm very impressed with the inherent efficiency of air to air systems. Its pure heat pumping, using phase changes and latent heat to full advantage. In part it's my physics background that rebels against air to water systems. What does a water heating circuit bring to the party? It's as if back in the 1950s when everyone kept food cool in a pantry someone said, "There's this great new thing called a refrigerator. Just stand this box in the kitchen and you can run a water circuit from it to cool down your pantry." Or you could put the food straight into the box. Not entirely fair I know.

Still one of the attractions of an air to air system for me is being able to rip out all the radiators.

We have experience with an aircon unit that doubles in as a heatpump. As it is 20 years old (R410!), we do not have any monitoring data on consumption, etc. Essentially a mono split for a medium size flat. We use an immersion heater for hot water. 

Installers in the UK familiar with A2A have usually been mostly working with commercial heating/aircon systems and some have posted in this forum. I would run a search as this was discussed a month or so ago. 

My take on why they are not too popular and why A2W is typically being installed for domestic: 

• the concept of continuing to use the rads is appealing, even if often some rads typically need replacing to maximise SCOP 
• able to also provide hot water
• the regulation scheme has until very recently not tried to cover A2A (and this is still WIP by MCS)

I agree with others that given the often lower cost of installation, A2A is likely to grow faster then A2W. 

Posted by: @ashp-bobba

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Ofgem have a very good help line for home owners and can answer your question directly

Useful advice from an expert in A2A ...

Posted by: @springswood

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In part it's my physics background that rebels against air to water systems. What does a water heating circuit bring to the party?

- Comfort and silence (for heating)

- Heat storage with radiant slabs

- Can be maintained, modified, upgraded DIY

- All the tech and regulatory complexity is contained in one plug and play box outside the house, than can be replaced with another when it dies

On the other hand, water sucks for:

- Corrosion

- Flow temperature issues, etc

- Cooling

Basically it's about the logistics of heat transport. Air to air splits use refrigerant all the way, which means refrigerant pipes have to be installed... which brings a regulatory hazard: when/if all the refrigerants that aren't propane get outlawed, and the A2A reaches end of life, what happens?

There used to be a French manufacturer SOFATH who had a genius (at the time) idea: the UFH radiant heating was done with refrigerant directly in thin copper pipes in the slab. On paper that's great, it gives maximum COP. It was in the 1980's, so it ran on R22. When the heat pumps broke down, people who installed  it were pretty much screwed. 

Thanks @bobflux that's pretty comprehensive.

Regulation of refrigerants is a new one on me. I'd imagine that's hard to predict.

I was looking into second hand VW EV's for a friend the other week. When it came finding out if the optional heat pump was fitted you look under the bonnet at the R number. Air conditioning uses R1234Y but the heat pump uses R744 also known as CO2.

Yeah refrigerants are complicated. They need to have the right characteristics (enthalpy, pressure, temperature at the vapor/liquid transition) and also, preferably, non flammable, non toxic, and non environmentally hazardous... First ones were stuff like ammonia (toxic, flammable, corrosive) and ether (highly flammable).

Then CFCs (chloro fluoro carbon) were invented, which everyone loved since they were non toxic and non flammable, so we got R22 (Chlorodifluoromethane).

However, CFCs destroyed the ozone layer so they were banned and replaced with HFC (hydro fluoro carbon) like R407, 410, R32 (difluoromethane).

Then HFC cause global warming so they're in the process of being banned... Gave us hilarious stuff like the commercial brochure of the heat pump saying "R32 environmentally friendly refrigerant of the future" while it's already on phase out lmao

Okay, not so many candidates remaining... time to drop some requirements!

So we get HFOs like various isomers of tetrafluoropropene (R1234yf, R1234ze, etc) which are perfectly safe*, barely flammable*, non toxic*

*may catch fire and release highly toxic products if you crash your car, *may decompose into really uncivilized stuff if released into the atmosphere, *also a PFAS (the now famous "forever chemical pollutants") so meh, at least they don't cause global warming (in theory)!...

They'll find arguments to ban these, then we're pretty much at the end of the list: propane, water, ammonia, and CO2. So what will we use to heat/cool our homes...

Ammonia is too toxic for residential use.

Water is great for high temperature industrial heat recovery heat pumps but doesn't work below 0°C.

CO2 is nice but it requires enormous pressure, it's great for monobloc systems: everything can be custom designed for it, pipes are short... but for splits, it would get complicated with the liquid line at 60 bar of pressure.

Which leaves... propane. Which, ironically, is a superb refrigerant for a heat pump, even and especially for A2A splits, high performance, non toxic... It would work as retrofit for old R32 splits too, by changing the indoor and outdoor units and keeping the pipes.

I'm sure the bureaucrats will be very very sad about not being able to ban it outright, so my guess is you'll get a mandatory yearly leak test or something of the sort to increase running costs, because as we all know propane is perfectly safe in the rusty camping stove that's been in the garage for 10 years but horribly dangerous when used in a heat pump...

Not to mention the 50+ year old copper pipes connected to a limitless supply of methane. 1kg of propane? Run for your lives!

Posted by: @bobflux

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I'm sure the bureaucrats will be very very sad about not being able to ban it outright, so my guess is you'll get a mandatory yearly leak test or something of the sort to increase running costs, because as we all know propane is perfectly safe in the rusty camping stove that's been in the garage for 10 years but horribly dangerous when used in a heat pump

I strongly suspect that it's industry lobbying which drives the bureaucrats to mandate annual maintenance and the like.  Great money for not a lot of very straightforward work!

Posted by: @springswood

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...

I was looking into second hand VW EV's for a friend the other week. When it came finding out if the optional heat pump was fitted you look under the bonnet at the R number. Air conditioning uses R1234Y but the heat pump uses R744 also known as CO2.

...which, I've found out, is pretty much impossible to get recharged without going to a Volkswagen Audi Group main dealer. I'm actually in favour of heat pumps in cars but I don't like being a captive audience.

@majordennisbloodnok I do sympathise.

On the original question, being naturally stingy I'm wondering what is the smallest system that would give me comfort?

I'd like to get away with a small outdoor unit and just two indoors. A console, i.e. floor level, in the living room and a high level one of the bedroom above. As well as reducing the initial outlay I suspect efficiency would be better, especially in the less cold months when a five port multi split would struggle to modulate its output low enough.

Heat does very naturally work its way into the attic bedroom. Even on the coldest days that tends to be the warmest room in the house. At least when I leave the door open. Although cooling up their would be nice it is only in use 10% of the time and not much of that will coincide with heat waves. So I reckon I can rely on a fan and overnight ventilation.

The biggest problem in terms of heating is getting warm air from the living room into the kitchen. I intend to explore how that might work in practice once the weather gets colder. Meanwhile I'll try to describe the issues. Any suggestions gratefully received. 

First a rough floor plan...

The existing radiators are shown brown, the internal doors hatched blue and my preferred site for a floor console in green (actually a bit forward on the chimney breast). The two sofas are under the window and in front of the radiator, so I think this works well for air flow. It's a bit prominent visually but nothing can be as bad as the gas fire that is currently there.

There's two reasons why I don't think much heat would make it out of the living room door and into the kitchen. I think they're obvious when you see it....

First there's 70cm above the door so I suspect the warmest air will gather there and not make it out at all. A ceiling fan might help with that. Except for the second problem, which is that any warm air that does go through will probably head up the stair well. It's maybe not be clear but there is actually a cupboard above the door next to the coat rack which might help a bit but I doubt by much.

A more extreme idea would be to use a fan at the top of the stair well to drive warm air back down again. The one I got this summer to make my bedroom habitable in the heat waves is too powerful for comfort but might provide a proof of concept.

I could easily get a low level indoor unit in the kitchen. There's a useful 1m high crawl space under the ground floor. I don't favour a high level unit because the ceiling is 2.8m. Plus I do quite a lot of frying and things tend to get greasy on top of the cupboards. However, I'm reluctant to do that because I generally only spend time there when I'm cooking. So I'm not sitting around but I am putting heat into the space from the hob and oven.

The exception is when I have visitors. I suspect it might be more economic to just use a direct electric heater where the existing radiator is. It's at most 20 days each winter and mostly the time the kitchen is toasty because I've been cooking a full roast dinner.

Unfortunately the kitchen is maybe the coldest room in the house. That seems to be down to draughts somewhere behind the kitchen cabinets. I'm hopeful I might have fixed that after I went underneath and blocked where a floor board stopped an inch short of the wall. It's not been cold enough to know if that's fixed it yet.

On a philosophical note it's doubtful whether what I'm suggesting could be called central heating. But if it keeps me comfortably warm that's good enough.

I suppose the reason I'm going into such detail about it isn't merely my personality. I want to be able to get rid of the old gas system altogether so it has to work.