M
morrismini
You may want to take the advice of morrismini with a few grains of salt, he is not a pro,
Oh I'm a full pro OK, that is obvious by what I write. You are another waste of space which clutters this forum.
Unvented cylinders? With most having a pressure reducing valve to 3 bar? Nah. They restrict flow. A thermal store, or large combi, using a plate heat exchanger, can take up to 9-10 bar and no pressure reducing valves to go wrong and reduce flow. If you have over 3 bar an unvented cylinder can reduce flow/pressure.
If you have 5-6 bar main and want to us that for a high pressure power shower? Don't get an unvented cylinder.