@editor Yes BUT that was 3 years ago installed in January when they were offering deals, as no one want AC fitter in Winter  right....

The problem I see with A2A (and I'm interested if there's a good solution to this) is that many homes have multiple small rooms which need their own heat source to keep them at a reasonable temperature - en-suites, cloakrooms, utility rooms, bathrooms etc.

With a gas boiler / A2W, there already a small radiator in each room, but I'm not sure it's feasible to have a A2A  cassette/indoor unit in every one of those rooms - is it possible to have refrigerant pipework going to every room, find suitable wall-space, feasible to run in condensate drains etc.

But I do see a strong benefit from A2A and it's ability to provide cooling, which is increasingly necessary given some of the heat waves we're now getting and solar gain in homes. With conventional A2W you can't have that, and would need to run two systems, A2W for the existing central heating system, with A2A in certain key rooms.

I was thinking about a hybrid approach - get an A2W unit which can also provide cooling (many A2W units can, but not all those offered under BUS are able, or installed to make this usable) and plumb in a few fan-coil units in key rooms, on a separate pipework loop controlled by a diverter valves, so that one A2W unit can provide heat via the existing radiator system, DHW and then in the summer, switch to cooling mode, providing chilled water to the fan coils to cool the key rooms in the house.

I have voted for air to air on the basis of a future 'hypothetical' new house largely open plan living spaces, with multi-split systems including bedrooms. A2A has the advantage of being very quick and responsive to heat or cool the air within the room, with the option to quickly cool rooms in summer. Overheating is an increasing issue, especially in a well insulated newer properties without MVHR or mechanical ventilation. For small wet room heating load I would probably go with direct electric UFH on a timer to coincide with morning and evening usage. This is common in Scandinavia, where A2A heat pumps are the norm and nearly all home heating is electric + the obligatory log burner.

For my current home [1992 EPC C], A2W is definitely the right choice as a retrofit, utilising all the existing secondary pipework, and the addition of new primaries. Many radiators were changed, more than needed, but the old ones were heavily sludged, dated, and quite shabby looking. For what little extra it cost, replacing an extra 6 or 7 was money well spent.

From a professional standpoint, I would always advocate convective heating with some radiant heat as best comfort. Convective heat warms air within a building but radiators also provide some radiant heat. A2A heating or cooling systems are at a disadvantage, they are quick to heat or cool the air within the room but purely by convective means, so are often perceived as less comfortable, especially when surfaces and fabric within the room lag in their thermal response. 

To be blunt, I wouldn't choose.

If we were to move tomorrow and have to fit a new heating and hot water system into our new home, I'd put our needs out to tender and then let the various installers put proposals in front of me. I'd then choose which of the installers I felt I could trust out of those who came back to us, and then look at what the compromises were of each proposal in that subset. If the winner was A2W, so be it. If it was A2A, fine. In other words, just as I've done before, I'd choose the installer and then let them do their job.

In the home we had before our current one, the big limitation was that it had a combi boiler as a result of no room for a hot water cylinder. Even a heat store like @toodles' thermino would've been a push meaning going full A2W would've been a non-starter. Keeping the combi (or even changing to an electric combi) for DHW whilst going A2A for heating might've been possible. In our current home, there was room for everything and the existence of underfloor heating meant A2W was a natural good fit. I don't care about the technology; merely how well a solution works within the constraints of the home.

With BUS, I believe a2w will be a better solution.

I would install the a2a and a heat battery but the installation of a2w will probably use the full 7.5k grant... 

Posted by: @temperature_gradient

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I was thinking about a hybrid approach - get an A2W unit which can also provide cooling (many A2W units can, but not all those offered under BUS are able, or installed to make this usable) and plumb in a few fan-coil units in key rooms, on a separate pipework loop controlled by a diverter valves, so that one A2W unit can provide heat via the existing radiator system, DHW and then in the summer, switch to cooling mode, providing chilled water to the fan coils to cool the key rooms in the house.

This would really be ideal and fans could also add to the normal heating function of a radiator.

A lot more work as it involves electrics around the house...

I wonder whether cold water could have an impact on pipes and junctions designed for heat and some replacement is needed 

Posted by: @batpred

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I wonder whether cold water could have an impact on pipes and junctions designed for heat and some replacement is needed 

From what I've read, with chilled water, as you'd expect, condensation can form on the pipework so the pipes need thoroughly insulating and insulation joints taping, using water vapour-proof pipe insulation to prevent condensation problems. 

But I would expect the existing/original pipework, as it is connected to the existing radiators which are used only for heating, would only ever see hot water, being down-stream of the diverter valve. I think the chilled water would be restricted to newly installed pipework - the pipework to/from the heat-pump, the diverter valve and the dedicated pipework to the fan-coils.

Looking through the installation manuals for some A2W heat pumps, they can support this type of operation as these kinds of installations are shown in the manual. What I haven't been able to find, is one that can support/describes how to have the flexibility of having radiators (for heating only) combined with fan-coils for both heating and cooling purposes - that would be pretty cool, being able to use the fan coils to provide a form of boost/rapid heating of certain rooms.

Posted by: @temperature_gradient

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What I haven't been able to find, is one that can support/describes how to have the flexibility of having radiators (for heating only) combined with fan-coils for both heating and cooling purposes - that would be pretty cool, being able to use the fan coils to provide a form of boost/rapid heating of certain rooms

I have one fancoil in an otherwise all radiator system.  I am working on adding diy fan assistance to another radiator.  The latter is for cooling only.

The combination works well for heating and yes the fancoil will give a rapidish boost.

I haven't tried it in ernest for cooling but initial impression from a quick trial is that the fan coil does usefully cool with the ft set above dew point.  Standard radiators do nothing for cooling, I am hoping the fan assisted one will but won't know until summer.   There are positive reports of DIY fan assisted radiators for cooling on open energy monitor.  It's not Aircon but hopefully takes the edge off on a hot day.