Planning/hoping to make use of additional green borrowing on my mortgage to invest in a heat pump and home battery

I live in an end-terrace Edwardian-era home, circa 150 square metres internally. Double glazing, solid brick walls, some roof insultation in a loft extension but not loads. Its got high ceilings and its draughty, keeping it warm in winter with my gas combi boiler was expensive. I have quite large column (“designer”) radiators downstairs, and type 22s (but not massive ones) upstairs

From playing around with some online calculators, or heat geek estimates, or giving AI my floorplan and fabric details, I estimate the heat loss will be somewhere between 9 and 11kw.

With the recent heatwaves, the likelihood that summers will regularly be like this, and small children who in particular have been struggling to sleep in this heat, I’d like to get a system that can provide effective cooling as well.

As a result, I’m inclined to look towards an air-to-air heat pump as the answer.

From a reputable local company, I’ve been given a quote of c. £11k for a whole-house A2A system (2 external units and 6 internal), or c. £8k for just the bedrooms (1 external unit and 4 internal).

The internal units would be 2-3kw sized, except one larger 5kw for an open-plan room downstairs. I’ve been quoted for Daikin Comfora units.

The size of these seem like they would be more than enough to heat or cool the rooms that they are in, however the installer company themselves doesn’t recommend them as a whole house heating solution, suggesting that they’d just be a “top up” to our radiators. They say “for heating the whole house are really tricky with UK climate and relative poor insulation” – but is this just an outdated view? Or reflective of the fact that most people wouldn’t want to run their units continuously to provide the same kind of heat a radiator at high flow temps would achieve?

But one option would be to go with the whole house A2A, and then transition to a standalone heat pump cylinder as and when I wanted to.

Another option I’ve thought of would be to get the upstairs only A2A installed ahead of this winter, and see if it is sufficient to heat my upstairs rooms. If so, I could install the rest of the A2A units, or I could then look into getting a A2W heat pump to cover solely the downstairs rooms.

I understand that a heat pump installer would be able to provide a system that ignores the heat requirements upstairs if these are already supplied by a sustainable system and I’d still be eligible for grant? So we’d only need a 5 or 7kw unit for three downstairs rooms and a cylinder which would be a fair bit cheaper than a 10 or 12kw unit.

I’ve also seen that the Daikin X or Samsung Quint could do what I’m looking for in a single unit, but these aren’t currently available and likely to be a lot more expensive. And the guidance for the Daikin X says “Suitable for new-build or fully renovated homes up to 150 m” which isn’t right for me, and imagine the Samsung would be similar.

To me, once I’ve decided that I want cooling, the full house A2A system seems like the best option, even when accepting that I almost certainly won’t get any BUS grant. A2A units are able to modulate lower and generally have a higher SCOP than A2W units, so my potential savings would be even greater (some modelling I’ve done says I would save 50-70% on my heating costs)

I only feel reluctant because it seems like this isn’t a common option and even the installer seems not to recommend it.

Is there any reason I’m not considering that would make A2A less effective than boiler or A2W heating? (I would be getting a large battery with enough capacity to power my heating through a full day on all but the very coldest days, charged at cheap overnight EV rates).

Things I can think of:

• The noise – but I understand that more continuous running low and slow would be very quiet, more like background white noise overnight
• The “feel” of air blowing for warmth being different and less effective than radiant heat. This I can understand although can’t quite imagine – in any case, in our current setup due to the cost of running the central heating we have often just been cold in winter, and uncomfortably hot in summer, so I think this would be a trade-off we could put up with
• The idea that I’d be wasting the radiator system that’s already in place. This doesn’t concern me too much, and in the medium-longer term it would be great to be able to remove these for a bit of extra space back in the rooms
• Planning permission – for two external units in my end-terrace/semi, I’d need planning permission. This looks like it would cost c. £300 in my local authority.

Does anyone have any experience of whole home air-to-air heating and cooling, particularly in older, less insulated properties that could persuade me one way or another?