Ait to air heat pump/aircon

Friend of mine has. The living room is single brick, south west facing and takes time to get warm and too hot in summer, so he stuck something on the wall outside that blows hot and cold inside. He is well pleased with it.

Ah. Ny friends unit is precisely what you cant implement, Bugger. I suspect that your preferred solution will not be a 'drill ten holes and bugger off' but a few days of custom work, and very expensive.

The only good news I can offer is that it should work. if you can implement it.

No, but looking at air2water cooling.

Beware that that may require planning permission, especially if pitched or you're raising the height of the building. Outbuildings are typically single storey because that's permitted development, where putting an aircon unit on the roof would blow through the maximum height budget.

There is no strict need to have the units back to back - just think about what happens in commercial premises (although the larger buildings pump around chilled water not refrigerant).

However the 'easy install' units are pregassed and there is only enough gas for a certain length of pipe - check the installation manual for how much that is. If going beyond that length it'll need more gas, but that should be straightforward for an F-gas installer to do, which they would have to do anyway for a regular (non-'easy') unit. There may be a certain maximum amount of gas in the system the unit can cope with - check the manual.

OTOH the longer the pipe run the more losses you get from it (even insulated), so you lose a bit of efficiency when doing so. Also, you will need to arrange a condensate drain from the indoor unit: do you have any convenient wastewater pipework, or can you drain it outside? There are condensate pumps so it doesn't strictly need to be downhill, although the pumps make a noise and are another moving part to maintain.

Theo

Some manufacturers apparently sell pre-gassed, self-seal, extension pipes.

I have a 2.5kW Mitsubishi split system. The inside unit is on the landing, with the inverter drive outdoor unit wall mounted on brackets at waist height for convenience of access. The sound levels are insignificant and only really noticeable when standing alongside the outdoor unit on full power. It has cooling function which is the reason I wanted it, plus the bonus of heating. It was a godsend during the exceptionally hot days last summer. The connecting pipes (1x1/4" and 1x3/8" plus 20mm condensate drain) and cables pass through one hole in the wall then down to the outside unit and a drain point. The condensate is handy for filling the steam iron.

I'd have thought it would be better to have a continuous, pressure-tight (brazed/soldered?), pipe run, rather than an assortment of self-seal joints which will eventually fail. It doesn't really complicate the install very much (if the installer has the kit, which they should) and makes for a more robust, longer lasting, job.

Theo

The pre-gassed ones were designed for DIY installation or by general builders, with no requirement for special kit or knowledge.

You had better tell the plumbers and building regs that. O-ring self sealed joints have been standard for over 20 years on plastic pipes under pressure