Also don't ignore the possibility of using one combi to heat a cylinder
of water as well as being able to provide hot water on demand itself.
(this can make sense where, one hot water outlet would be a long pipe
run from a main cylinder, and hence the combi can feed it directly, or
alternatively, when you have a gravity fed system that works well for
bath filling, but lacks pressure for showers.


That's a very neat idea which had never occurred to me: but that said,
after various problems with two different combis I'll never have another
if I have a choice.