About ten years ago, I installed a central heating for two one-bed
flats in my house. The boiler is in the loft and the flats are
respectively in the ground floor and 1st floor. The only 22mm pipe I
used was the feed and return from the boiler to each of the central
heating circuits. The pipework within each flat's circuit is all 15mm.

I later learned that I should (according to the conventions) have used
22mm pipe for the 'rings' of each circuit too, with 15mm pipe used
only for the short spurs going to and from each rad. Nevertheless, the
system has worked perfectly for ten years.

I now want to give the Ground floor flat its own combi boiler, and
isolate that circuit from the rest of the house. In view of the fact
that the entire system has worked fine for ten years, is there any
point in adding any 22mm pipe to the ground floor circuit when I add
the new boiler? Can I simply use 15mm for the feed and return from the
new boiler? The furthest radiator will be just under twelve meters'
pipe-length away from the new boiler.

Thank you,

Duke D