Pipes have to be sized to ensure that they allow sufficient water to flow through the pipes to heat the radiators and domestic hot water cylinder.
Below is outlined a very simple approach to pipe sizing for a domestic heating system. The chart indicates approximately how much heat a pumped copper pipe will convey. As you plan your system each radiator will be feed with a flow and return pipe this all connect back in parallel and as each radiator is added to the circuit you need to total the heat required to determine the size of the pipe.
