Knocking on central heating when it starts up each morning

pipework trapped in joists when expanding makes knocking noise.

Rapid expansion of the timber when a heating pipe is pressed against it. To rectify put some insulating materiał between the pipe and the timber surface. This could required some work if it is under a floorboard or in an area difficult to access

Two option: 1: pipe not supported/clipped to joists. 2. Air in the heating system- need to check pressure and check radiators .

The space for the pipes at the joist is tight so when heating is on the pipe expands because there in mo room for expansion it creates noise The gap should be bigger

Possibly air in the system...