The message
from Fred contains these words:

Sorry I haven't replied to this thread for a while, I shall try to do
so today or tomorrow. In the meantime can I ask one quick question:

Do I need 28mm pipe for central heating/hot water if the system is
pumped? Is it that 28mm is only for gravity fed systems? If there is a
pump in place, does 28mm offer any advantage over 22mm?

The larger the bore the more water can be pumped though and further but
in a normal pumped system you may not need 28 mm even if you have one
circuit serving the whole house.

My information is grossly out of date but the figures I have (taken from
Ravensbourne Beginners Guide to Central Heating) a

Length limits at maximum flow rates (3 feet/sec)

Tube size (mm) Max flow (gallons/hr Max length (feet)

15 105 80
22 240 120
28 390 175

And at those values the heat capacity and head loss a

BTUs/hr inches water gauge/foot run

15 21,000 0.90
22 48,000 0.60
28 78,000 0.41

--
Roger Chapman