"Ed Sirett" wrote in message
...
On Fri, 13 Jul 2007 16:00:13 +0000, olo wrote:

Also hot water takes longer to come through a 22mm pipe - the volume of
the
pipe is proportional to the square of the radius (or diameter), the flow
rate to some lesser power. This means it's brill once the hot water has
arrived, as the flow is larger, but there's a longer period flushing out
the
cold.

Fair point.

Considering all opinions so far - how about 22mm dedicated cold supply
from stopcock to boiler and then 15mm from boiler to shower/taps. This
will minimise pressure drop on the cold side and give shortest waiting
time for the hot water

That would be my choice if starting from scratch, except that I would not
bother with a 'dedicated' supply if doing it in 22mm. I would try to make
sure the boiler's cold supply is upstream of most/all other usage points.

Which is near impossible in most cases.