"The Natural Philosopher" wrote in message
...
My peak hot water requirements are easily met with a mains pressure
cylinder allowing me to use a mere 12kw boiler.
You need around 50kW to get a decent hot water supply.

The important thing is that your cylinder is fed by mains cold water rather
than a header tank. Our present house had a cylinder and a conventional
(non-combi, non-condensing) gas boiler. It took ages to heat up a tank of
water if you wanted more than one bath, and it also took about 5 minutes for
the water to run warm due to the length of pipe runs and the very low flow
rate, determined by a) 15 mm instead of 22 mm pipe for hot water, b) header
tank so only about 3 metres head.

When the cylinder developed a leak, we were originally going to have it
replaced, but the heating engineer found that the boiler was badly corroded
and wouldn't last much longer. So we had a combi/condensing boiler fitted,
giving mains-pressure hot water which runs hot much more quickly and is much
easier to mix with cold water in a shower or sink tap: the old one was so
low pressure that the cold tap could only be slightly open, otherwise the
combined water was virtually cold.

Our previous house had a cylinder (so the same time-to-reheat problem) but
at least it was mains-fed.