'''Four bedrooms. No chance''. Of what? Something missing? In my very limited experience of trying to get A/A fitted the issue has been finding installers prepared to 'get out of bed' for a relatively small system.

Regarding air to air generally, - IMO yes go for it. My system, not multi split, has been in for 5 years now I don't regret fitting it for a minute. It's performed flawlessly for both heating and occasionally cooling.
I was lucky perhaps, inasmuch as I found a local independent F Gas qualified aircon and refrigeration service engineer.
I opted to do a lot of the tedious legwork that he didn't want. I researched and bought most of the kit, especially stuff that I knew many of the mainstream aircon suppliers didn't supply, but which I wanted as part of the install.
I also did much of the prep, e.g. the outdoor unit base support and loft access points for the pipework, and power cable runs. All this cut down his time on site, and |I was quite happy for him to do the odd half day, leaving him free to do his usual service jobs.

I would aim to reduce demand by adding insulation prior to installing heat pumps. Costs for some people that I am helping have turned out to higher than expected with heat pumps and they are wishing they had stuck with gas.

I liked Owlmans approach

This thread should be required reading for our policy makers and politicians, when you have

Someone keen to improve their property
Willing to pay higher running costs
Accept there is a cost for improvements
Willing to live at temperatures that not only very few would , but would be insufficient targets for rented property.
Has a property that isn’t that easy to adapt
Finds that there are not the companies out there to do the works required / would rather pick the easy jobs.

Surely there must be the realisation that there’s a huge gulf between the goals the nation has set itself and our abilities and financial wherewithal to achieve them.

Hi Gareth - are you sure you need six emitters? Depending on your room sizes, connectedness ie. passageways and doorways, living pattern, the following might be worth considering: Have just one or two fairly central rooms fitted with emitters and keep them a degree or so higher than you might generally aim for, accepting that peripheral rooms will get enough heat via (neccessarily open) doorways. I find a temperature difference of just a couple of degrees generates a miniature stack effect and I've been surprised how effective the resulting gentle horizontal air movements are at carrying heat into the cooler rooms. You could test the idea in advance by turning off radiators in rooms you think might manage with no emitter and see where they end up.

This has been working well enough here with just one emitter, plus the existing lpg ch. retained for cold spells, for 12 years now, except that one end bedroom is used a lot by my wife as a workroom and that does need a higher temperature, so a second emitter there would have been a good idea. The whole thing does rather depend on leaving doors open which happens to suit us fine, and it suits our situation in that we are both here most of the time so the heating is gentle but constant.

Quoting Artiglio above "Surely there must be the realisation that there’s a huge gulf between the goals the nation has set itself and our abilities and financial wherewithal to achieve them." I couldn't agree more. Small air to air heatpumps are a cheap and quick and hassle free way to displace a good chunk of fossil fuel and it grieves me that there is so little awareness of this option, and so much blind adherence to the custom of having a single heating system that does it all. They could be rolled out much quicker than full air to water systems, with little or no need for subsidy, less fear of committing to an entirely new system inexpertly specified and fitted, and far less disruption. They can start displacing fossil fuel now rather than having to follow the standard mantra of insulating and adding radiators first. The insulation can happen later, bigger radiators may never be needed. Then there are those who oppose it because it might be used frivolously for cooling, thus adding to emissions. True, but the net gain is still favourable by a large margin. It is possible to restrict the cooling operation to higher temperatures. I read recently there is a country doing this - India, was it? 28C IIRC. We may also be heading for times when even UK temperatures will threaten survival, when a cooled room would no longer be a luxury.

Edit: India has a default factory setting of 24C, Japan has default of 28C, so they can both be set to cool lower if reset. With its growing properity and stuggling grid India has a particular interest in encouraging careful use.

@ mike7
I agree totally with your comments and they echo my own experience with A2A. As you say, we seem wedded to the single do it all system, CH and DHW, however inefficient each of those functions may be, in terms of finance, material cost, installation upheaval; when the alternative is staring them in the face.
The A2A technology is widely used throughout the rest of the world and from my experience is trouble free. No, we Brits prefer the over complicated A2W system of pumping hot water around the home. Sure that works, in a home designed, or refurbished, for it, but then if doing that you'd design a complex heating system out of the equation altogether,-- but as a retrofit replacement...Bah!

Heaven forbid that the hoi-polloi might cool their homes in Summer when the grid may be awash with green energy, or even if the home has PV to supplement their "extravagant" occasional use.
As for cooling generally; my own experience is that I've rarely had need to use it and only in one room. It's so damned efficient if it's set on manual I find myself switching it off after a very short while sometimes as little as 15-20 minutes but it is nice to find a cool refuge after working outside on a hot day.

@Gareth C
As mike7 says a 6 split may not be necessary, and even if it is then go for two x 2or3 splits, possibly in a stacked arrangement. That give a bit of flexibility, heating and cooling, and servicing.

Posted By: VictorianecoHow would one duct air to air in a larger house?

Using a forced air system. i.e. largish ducts with very noticeable fans. Forced air systems normally run with air much hotter than an air-air HP can sensibly produce, so even larger ducts would be required, or even noisier fans.

Would MVHR be a good system to distribute heat effectively? I think this is where a wet system comes in better with TRV's etc?

Not in most houses. The heat loss limit for a passivhaus (10 W/m² or 15 kWh/m²a) is chosen to be small enough that an MVHR can carry the heat energy required to keep the house warm. Any larger heat loss and an MVHR can't transport enough heat energy/power.

That's why multi-split systems exist; to distribute heat (or cool) over a wider area.