I'm thinking of getting a high-spec system boiler with a "tank-in-tank", which I'm led to believe is an optimal way of ensuring lots of high pressure hot water to multiple showers/taps at once. Is this true?
I'm also thinking of getting under floor heating. For the lowest space heating costs, would you just heat the water for that with the same system boiler, or have a completely separate ASHP to heat the UFH water? Is it even possible to have 2 separate heating systems like this in the same house?
If you also spent time trying to insulate and make air-tight (this is for a detached retrofit) and use an MVHR system, could you get away with no heating on the 1st floor, relying on ground floor UFH to heat the whole house?