ideally u need as 22mm circuits flow and return running from boiler as far as possible and then 15mm feeding the radiators, system do work work on samaller pipework feeding rads such as 10mm & 8mm however u should not be running 15mm directley from boiler, basically the smaller the pipework the the less flow u have and u are more likely 2 suffer from blockages and blockages are harder 2 remove in small bore pipe
Blaiross, December 2011
