Got a customer who has had problems with heat getting to all his radiators. 16 in total, on a 22kw Vaillant system boiler doing both his heating and hot water primaries. Boiler has now melted and requires changing. Have suggested bigger boiler, upgrading initial flow and return from boiler for first six metres from 22mm to 28mm, and serious powerflush of the system. Thought about changing from a system to open vented boiler with a pressure vessel and keeping it sealed. Any thoughts or helpful suggestions? is a 15/60 the biggest domestic pump out there?