The Natural Philosopher wrote on 20/05/2017 :
work out how big a combi needs to be to say dump 400 litres a minute of water
at 60 degrees

then work out how much more expensive that is than a small boiler with a
mains pressure hot water tank

then do the sensible thng

For someone in at irregular times, where a timer cannot be set and who
doesn't mind waiting for the hot water to appear very slowly, they are
fine. For everyone else, who wants a bath filled rapidly, who might
need to run two taps at the same time - stored hot water.