"Christian McArdle" wrote in message
. net...
We will be fitting new 22mm pipes throughout, will this help to
alleviate this?

Quite the opposite. The throughput of a standard combi boiler is so low
that
no more than 15mm is needed unless your mains water is very low pressure.
However, the 22mm pipe has twice as much water in it per unit length, so
it
takes twice as long to clear out the cold, given that the flow rate is
limited to a fixed low value, around 11.5 lpm, which 15mm is very capable
of
supplying at mains pressure. Gravity systems needed larger pipes because
the
tank fed nature leads to very low pressures, so didn't have the "push" to
squeeze through smaller pipes.

Christian.


Quite so. Even without a fixed flow rate and with a gravity system you are
better off with the smaller pipe (as long as it doen't get too small!). The
volume of water in the pipe goes up as the square of the radius. The flow
rate through a pipe depends on the radius but at less then a power of two so
water at one end will be travel slower through a bigger pipe. Once the
water is hot at the outlet end, of course, it's an entirely different matter
as the big pipe will be delivering more.

This was the first question I had answered on this ng many years ago. I
fitted all 22mm hot pipe to my kitchen sink. This was a mistake as waiting
for the hot is more irritating than the speed the bowl fills when it is hot
and I made things worse rather than better. For a bath fed from a storage
cylinder, of course, you just turn the tap on and the fact it's cold for a
bit doesn't matter, you are interested in the flow rate over a longer
period, so big pipes are best - likewise old gravity CH systems.

Where you can't maximise flow rates (as with a combi boiler) you are better
off with smaller pipe.


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)