2 Multi boiler setup, 1 for DHW the other for CH.

[gpbeck]

17 litres at 45deg is one big shower. Two at most. Mains pressure, flow? 3000m2 house with one bathroom???

 

[Dr John Doolittle]

17 litres at 45deg is one big shower. Two at most. Mains pressure, flow? 3000m2 house with one bathroom???

Not 45 c, that would be dangerous. @ 35 c the flow is 20l. Mains pressure is reading 4.6 bar currently. 2 bathrooms but family growing up so 2 showers operational at once will be normal. Rinnai :: Rinnai Sensei Next Generation Commercial water heaters - https://www.rinnai-uk.co.uk/products/commercial/rinnai-sensei-next-generation-commercial-water-heaters/

 

[Timmy D]

Have a look at plate heat exchangers.

2 x system boilers.
Fed into a low loss header.
Which feeds a phe could provide your hot water on demand and heating.

To fully utilise your gas meter at most distances, 35mm pipe will likely be needed.