Design and economics.
Having had to go without cooling last July for three days whilst a main bearing was replaced on my single air to air unit, I will next time have several smaller outdoor units feeding one or two wall/ceiling cassettes… 
Ah, these are air/air units that are like air conditioners, in reverse, and can often be used as air conditioners in the summer. They heat/cool air rather than act as a boiler to heat radiators and water for a tank.
I’d go for these but my wife is too sensitive to the fan noise.
Ours are silent
Then you have an option of a ducted system with a large indoor unit inside an attic.
We don’t have an attic! We do have lots of radiators, though.
In fact all four possibilities - air/water, air/air, water/water and water/air exist.
Depends on your definition of “better” I suppose - certainly they have a more reliable source of heat in the winter months but, as you observe, are more expensive because of the groundwork, and not every site is suitable. I believe they usually bury the pipework about 1m down for horizontal installations, though vertical boreholes are also an option.
I’m also not sure if water source heat pumps can be reversed to provide cooling.
How do air/water heat pumps perform when the outsude temperature falls to -10 or below.
Could you please explain further?
Lots of myths about heat pumps, but that’s because there are so many ways to fit them. The easiest and cheapest way is air-sourced feeding internal fan units. We have two external units, one each side of the house, and one feeds 4 internal units, the other feeds 3. You only use the internal unit that you need to use and it works like a fan heater, or indeed a fan cooler in the summer. They can heat a good sized room and while of course better insulation means they cost less to run, we have a stone house on which the walls are uninsulated but it is plenty warm enough. We use the cooling only in extremis, on those handful of nights where it is impossible to sleep even with a normal fan running.
People with central heating already fitted think automatically of replacing their main boiler with a heat pump and, while this works, as many have pointed out, it is not as good a system when heating water and pumping through the house - you lose a lot of the efficiency and most of the flexibility.
When temps get very cold they can be a bit feeble, but we have electric radiators and a woodburner for those occasions, which were we are (17) are thankfully very rare - a few years now since we had less than -5°
Yes, you can do this, although I’ve never been sure if it was a good idea to have a cold floor.
In English, the two main types are called ‘air source heat pumps’ and ‘ground source heat pumps’. We have a GSHP although its a very early one and only two of the three circuits are working. It will probably need replacing at some point. Modern heat pumps are so good now that we may actually replace it with an ASHP which would be much cheaper.
Modern ones are very good. Heat pumps are very popular in Scandinavia where these sort of temps are common. Of course, the efficiency drops as the outside temp reduces, so a GSHP would always be much better in such a cold climate.
There are heat pumps now that can match gas boilers, as has been mentioned. We would be replacing our electric hot water tank (which will need replacing sooner or later as it’s fairly old) as well and this would be an electricity consumption saving. We don’t run our radiators from our existing gas boiler that hot but it makes a difference, which is why I am confident that our radiators are up to the job.
I saw a couple of questions about the effectiveness of heat piumps when it’s below zero outside. Put it this way, one of the biggest adopters of heat pumps in Europe is Norway. While we think of freezing conditions as ‘cold’ - there is still thermal energy that can be harvested by a heat pump. I regard it as a heat concentrator. It’s true that heat pumps are more efficient when the environment is not so cold, but when it does get very cold the heat pumps can still warm your house!
Certainly in UK heatpumps are bulky, often noisy and only work in very modern, very well insulated, properties.
I would get some unbiased expert advice before committing!
presumably 'cos they find the heat down in the earth… (which is impossible in my area…solid rock)
That’s ground source heat pumps much dearer than the most common air source
I really would like to understand how (or if) an air-source heat pump can heat and keep our large house to the 23C we require in winter months… when it’s bitterly cold outside and often -5C/-10C… brrrrrr
You need the CoC input and output curves.
Sadly that level of detail is not always easily available.
The temperature itself is not so much the issue - per unit volume if you have air at -10°C and you make it -20°C you get the same amount of energy as taking air at 30° and dropping it to 20° In fact as the cold air is denser there’s slightly more energy available.
However the differential is what makes the conversion less efficient - there are two stage pumps which can overcome this, or some pumps just have an auxiliary heater (doesn’t do much for efficiency but can get the output to a useable temperature).
The system we have isn’t great below 0° and positively unimpressive below -10!
Ours is a:
15KW Saunier Duval Genia Set Max Haute Température
Puissance Calorifique : 15 KW ETAS 125 COP 3.8 ETAS
ECS intégrée
Unité extérieure “Grand Froid” Silencieuse (I have been impressed at how quiet the double fans are)
It has coped just fine so far (touch wood) with the Auvergne winters and a room temperature setting of 21°C (which is still too warm for me, so I just wander around shorts and T-shirt).
You can download the bumpf here.
Tbh I cannot have any faith in air source maybe ground source, via bore hole, in the right area,
but pricey!
But why?