I have a traditional 2 bed terraced house built in 1911. I have seen in a few places comments that the layout is not well suited to air to air heating. I'm beginning to think that's back to front. Maybe my problem is there's not currently any equipment suited to a house like mine?

Given there are nearly 7 million terraced houses in the UK maybe this is a missed opportunity.

With high ceilings and a layout which doesn't encourage sharing heat or cooling between rooms my ideal set up is a five port mini-split system. That gives indoor units in every room, i.e. the living room, kitchen, both bedrooms and even the bathroom. I'm lucky with the pipe runs and I've had a quote for such a system. The main downside was cost. To be fair I had an air to water quote that was higher, and that was after the £7,500 BUS grant.

I think in a way that's because the equipment available is just too powerful for my modest and well insulated house. My heat loss is calculated at about 5kW with the living room having the highest demand, roughly 1kW for 21C when it's -3C outside. However the best available system was a 9kW outdoor unit, indoor units would be ~3kW in the living room and ~2.4kW for all the other rooms.

I would certainly be able to let room cool when not in use and bring them up to temperature very quickly. Especially as with internal brick walls mean things actually cool very slowly. I wonder if there would be any problems associated with running the equipment so much under capacity? 

I think the minimum output was about 3kW so I expect the outdoor unit would have to stop and start a lot, particularly when demand is low i.e. October, November, possibly December, March and April. Does anyone know if that would shorten its lifespan?

I think what I need is about a 5kW outdoor unit with indoor units in the 1-2kW range. I suspect the only reasons they aren't on the market are historical ones where most of the demand is commercial. I'd be happy to learn differently.

One of my main objectives for a long time has been low running costs. Hence the insulation. Plus heating only when and where I need to or, in the jargon, zoning and set-backs. My yearly gas consumption for heating is about 3,000kWh. Which suggests with the actual temperatures I've been living comfortably with my heat loss is more like 2kW. 

Last November I ran my gas combi heating like an air to water system to see what that's like. While it was indeed comfortable my total energy consumption was roughly 50% higher than my usual 'extreme thrift' strategy. I doubt any gain in SCOP can offset that.

Which leads to another question. Would running an air to air system so much under capacity adversely affect the SCOP?

Actually I imagine not. I'm very impressed with the inherent efficiency of air to air systems. Its pure heat pumping, using phase changes and latent heat to full advantage. In part it's my physics background that rebels against air to water systems. What does a water heating circuit bring to the party? It's as if back in the 1950s when everyone kept food cool in a pantry someone said, "There's this great new thing called a refrigerator. Just stand this box in the kitchen and you can run a water circuit from it to cool down your pantry." Or you could put the food straight into the box. Not entirely fair I know.

Still one of the attractions of an air to air system for me is being able to rip out all the radiators.