I think the main reason a2w is more popular in the uk is the grant. Long term, I think air to air will be more common, when the grant is removed for a2w I think the market could collapse for this unless the spark gap is reduced. A2a is more viable financially even without a grant and the air con element is only going to get more important in future.

If the same price I would go for a2w at the moment as I have not really experienced a2a heating, so I would be reluctant to remove radiators etc then find out it is not as comfortable.

I would also be unsure how many units I would need for A2a and whether I could get by with 2 units or need them in each room. 

With existing rads it seems easier to just install A2W. I've seen some larger houses with multiple internal A2A units have pipework running all over the outside of the building which didn't look great to me. The two enclosed 28mm pipes plus the overpressure pipe on the rear of my house are already quite obvious.

We have both.

The advantage of A2A is a very rapid response and the ability to cool in the summer - our A2A is for the south facing rear of the house largely for summer cooling.

Our house is very well insulated and has a massive slab with UFH - we use the ASHP to warm the slab overnight using Economy 7, with a winter CoP of around 4 our heating and hot water is more-or-less free.

The selection of a particular technology ought to depend on the lifestyle of the occupants and the characteristics of building; what suits for one combination might not be cost effective for another.

Posted by: @editor

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So the big question for homeowners is, if both were subsidised, which would you install: A2W or A2A?

I have A2W.  It works well.  However ...

If I had a modern, open plan low loss house then maybe A2A... except that such a house would probably already have UFH in place - in which case A2W without a doubt.

If I were doing a modern newbuild then it seems to me that the Romans had the right idea, UFH is the ultimate go to solution if its feasible (can do cooling also) and so it would be this with A2W.

With a not so modern, not so low loss house with lots of subdivisions and an infrastructure already in place for distributing water to those subdivisions, then it seems to me A2W is the natural choice.

With a smaller house that had creaking water distribution infrastructure then certainly I would consider A2A.  A single A2A in an open plan downstairs might well work well and is a simple and cheap install.  

Also in thinking about any of the above I wouldn't rule out A2A for the majority of the area with a bit of underfloor electric or fan heaters in less used rooms.  I have done this in a couple of community centres where the vast majority of the largely open space is well suited to A2A but there were a couple of areas which just needed a bit of background heating that didn't justify another A2A outlet.  The same, it seems to me, could well apply to domestic properties.

In summary it depends on whats already there and the shape and nature of the house

We have both. The A2A works well in our kitchen where we dont have enough easily accessable wall for radiators. 

Here in Australia we have very few A2W installations. Usually only in older wealthier suburbs in Melbourne, none up North and almost non existent for new builds.

Reasons of course is that we need cooling, but also there are very few experienced installers, and installed costs are at least double  that of an A2A installation.

As an example, I visited a new build a couple of weeks ago in a so called passive solar house in Central Victoria.

My rough guess is that heat loss would be less than 10Kw. 

But an 18Kw A2W heat pump had been installed with a 100L buffer to UFH.

So the new homeowners are using 60Kwh per day in winter, most of this to run the heat pump, which is short cycleing. On an off every 8 minutes 🙁

Ill be directing the owners to this forum!

@simonoz, welcome to the forums. A bit of an ignorant question, but how cold to winters get in Melbourne?

We currently have an A2A Midea unit with 3 indoor units and it works well, we put it in for cooling ( we have Solar) and gas for heating currently.

Interesting over time we using it more for heating at the beginning and end of heating session !

works well but like any heating system without any mass its doesn't warm up the fabric of the building.

Also it is suppose to be service each year..... £ 250 for a 45 min look over... not good value in my mind but it was only £3k to be installed.

I will still be replacing the 30 yr old gas boiler with a ASHP an long side it shortly...

@rob-heatpump-bob, £3k for three units?

A2A is really good. Simple. Cheap. Great success in other countries. Use it 80% of the year then kick on your alternative source. Even a single cassette can heat large open plan areas due to convection.

In Japan it is very popular to install a split unit and add cassettes to rooms as you can afford / want it. With single outdoor unit. 

@editor Thanks Mars 😊. Metro Melbourne rarely gets down to zero but some outer areas in the hills sometimes a bit below zero.

Further south in Tasmania a bit colder!

I exchanged emails with the owner of that 18Kw unit who’s installer said they really didn’t know how to improve the efficiency of the install😳 - there really are very few A2W experienced installers over here!