Definitely air to air (and we did)

We have a big solid stone 1860s victorian pile with 18" thick solid walls. It's grade II listed so a lot of insulation is simply not possible and would wreck the building.

Why do you think it impossible?

Because both the inside surfaces and outside surfaces of the walls are of historic value. We looked at insulation. There are things that can and have been done like magnetic secondary glazing but the walls, floors and some other elements of the structure are not really improvable in a way that would get LBC.

You can't put EWI on without changing the aesthetics of an un rendered building, but can a rendered one. So one is Listed building friendly, the other not.

IWI and floor insulation on the other hand doesn't change the way a building looks so can be done. Granted it's messy, expensive and risky*, but you haven't sited those as reasons.

* You do need a good system and someone who really knows what they're doing to fit it. Like with ASHPs, not a common thing.

I tend to have a problem with absolutes, @deltona.

Yes, there are situations where EWI can be added without changing the aesthetics of a building. That doesn't mean it is listed building friendly, it means it may be.

Similarly, IWI and floor insulation do not, as you say, change the external look of a building, so they may be allowable but it's not a guarantee.

@etchedpixels specifically said that both internal and external faces of the walls in this case are of historic value, which implies his assertion that insulation is not possible is being made from a position of knowledge rather than assumption. I would personally not have a problem with firing "have you considered this?" questions towards @etchedpixels but I think you're probably on dodgy ground to be stating that he's wrong and that something can be done without anything more than what's been written on this thread to go on.

Definitely air to air (and we did)

We have a big solid stone 1860s victorian pile with 18" thick solid walls. It's grade II listed so a lot of insulation is simply not possible and would wreck the building.

Why do you think it impossible?

Because both the inside surfaces and outside surfaces of the walls are of historic value. We looked at insulation. There are things that can and have been done like magnetic secondary glazing but the walls, floors and some other elements of the structure are not really improvable in a way that would get LBC.

You can't put EWI on without changing the aesthetics of an un rendered building, but can a rendered one. So one is Listed building friendly, the other not.

IWI and floor insulation on the other hand doesn't change the way a building looks so can be done. Granted it's messy, expensive and risky*, but you haven't sited those as reasons.

* You do need a good system and someone who really knows what they're doing to fit it. Like with ASHPs, not a common thing.

I realise what's holding me back is that without experience I have no feel for how warm air heating will spread through a house. 

There's a saying I picked up when studying physics. An ounce of experiment is worth a pound of theory. It was a long time ago. So if it was the winter I'd play around with a couple of fan heaters to see what positions might work. Maybe I'll be better off waiting...

On the ground floor there's the living room and kitchen. Would heat from the living make it to the kitchen or would it escape up the stairs, which can't be closed off? 

Upstairs is probably OK for heating. Just one bedroom and the bathroom. I can see heat spreading there OK. Up to the spare bedroom in the attic as well.

Would the attic be cooled by a unit on the floor below though? I imagine not and naturally it overheats most (and gets used maybe 20% of the time).

So much of this depends on the building. A unit at the bottom of the stairs will slowly warm the upstairs in my experience and heat will usually spread through a building nicely. How fast and where though depends on what the walls are made of (eg it's much slower to heat/cool with real internal walls than bits of plasterboard and stud). On old old houses with big solid internal walls you really don't get much through the walls unless you are maintaining a temperature steadily for a very long time.

Cold air falls, so it's not going to affect the attic much if you are not feeding cold air into the attic. If the attic is being cooked by the sun through the windows and the roof then it's not going to help. If it's being cooked by heat rising from the rest of the house then maybe.

Best thing I know for cooling an attic room is to put solar panels over it ;)

Fan heater experiment sounds a great idea.

Definitely air to air (and we did)

We have a big solid stone 1860s victorian pile with 18" thick solid walls. It's grade II listed so a lot of insulation is simply not possible and would wreck the building.

Why do you think it impossible?

Because both the inside surfaces and outside surfaces of the walls are of historic value. We looked at insulation. There are things that can and have been done like magnetic secondary glazing but the walls, floors and some other elements of the structure are not really improvable in a way that would get LBC.

Love that! 
I also value real world feedback from the most demanding users. 
A thought: how large could an airbnb with a simple a2a heating system be and still survive a real world rating from its customers? Also a great testbed for the controls..

I realise what's holding me back is that without experience I have no feel for how warm air heating will spread through a house. 

There's a saying I picked up when studying physics. An ounce of experiment is worth a pound of theory. It was a long time ago. So if it was the winter I'd play around with a couple of fan heaters to see what positions might work. Maybe I'll be better off waiting...

On the ground floor there's the living room and kitchen. Would heat from the living make it to the kitchen or would it escape up the stairs, which can't be closed off? 

Upstairs is probably OK for heating. Just one bedroom and the bathroom. I can see heat spreading there OK. Up to the spare bedroom in the attic as well.

Would the attic be cooled by a unit on the floor below though? I imagine not and naturally it overheats most (and gets used maybe 20% of the time).

A2A would only work in smaller properties (and I think there's a consultation running to eventually allow more people in flats to have them).

That's possibly the most bogus heatpump comment I've heard this week 8)

The context of my comment is retrofit cost effective installs for heating, looking to leverage BUS. As far as I am aware, the main goal of the government push for A2A heating is to decarbonise at a large scale. The announcement indicated it could be most cost effective than what A2W is delivering for flats and small houses. 

I agree there are other cases where A2A can be user to remove a fossil fuel boiler. But how many millions of dwellings could be in scope? 

We have a big solid stone 1860s victorian pile with 18" thick solid walls. It's grade II listed so a lot of insulation is simply not possible and would wreck the building.

Why do you think it impossible?

The question then is the one you raised, will need lots of indoor units or not? I have a modest 2 bed terrace built in 1911 with high ceilings and one of the bedrooms is in the attic. I can't tell whether the heat would circulate or not. 

If you leave the doors open very likely not.  Rooms share heat anyway, with open doors more so.  One unit on each floor may well do the job.  Electric mats or small radiant heaters are good for small closed off spaces (eg a WC)

Good point about hot water. I have an EV charger and cheap overnight electricity so a simple immersion heated tank will be my most economic choice.  Plus it means I'm not limited to the Daikin Multi+.

The question then is the one you raised, will need lots of indoor units or not? I have a modest 2 bed terrace built in 1911 with high ceilings and one of the bedrooms is in the attic. I can't tell whether the heat would circulate or not. 

Also how much I'd really use cooling.

I suppose it's also about why I want to make the change. Getting rid of gas seems a good move to me. First I'm so well insulated that the standing charge is over 35% of my gas bill. Second is about emissions, not just the CO2 but the methane in production and distribution. I'm fortunate to be able to make the change now. As a bonus getting the boiler off the kitchen wall will add space where I need it most. 

I suppose I'll just have to talk to some installers for answers.