Pipe Sizing

[Max-1stSaxon]

Hello i am helping to complete a full install soon and would just like to clear some things up, i will be installing a Valiant Ecotec 637HE System Boiler,the install will consist of 15 radiators and an unvented cylinder, the property is a 4 storey house with the boiler in the basement, my question is what size primaries both main and heating will be needed,the main flow and returns to the cylinder cupboard i am intending on running 28mm (will be about a 8 metre run)

then my querie is whether i can reduce down after my 2 port into 22mm and run the heating circuit of all 22mm and only reducing to 15mm for the tails? or whether it is still the best idea to run 28mm heating flow and returns and then branch off into 22 and feed in 15mm?

Max

[fuzzy]

only reduce down if the pipe splits or it will be pointless running the larger pipe at all

[Piper]

It wouldn't be "pointless", as it would reduce the fictional loss, but it may not be necessary. What's the heating load? 15 radiators doesn't tell us much.