I have added a small bathroom and get knocking for a couple of secs when the taps are closed (lever action mixer on bath and sink). The hot water although low pressure and low flow is the worse.
I have connected in to the existing copper pipes with Polypipe.
approx 4m under the floor boards, 600mm vertical and 1200mm horizontal with Ts to sink and bath on this leg. Although clipped to the wall I can see the pipe move due to the knocking with the bend at the end of this leg moving approx 1mm on the pipe.
I cannot stop this movement even when I hang on the the bends at each end of the pipe. I have a plastic isolating ball valve in both the 15mm plastic pipes in this horizontal length that is knocking.
any ideas ?