I have an old Firebird oil boiler (cant find the number) that works fine until the water pipes 'clonk' and then the hot water pressure drops. I switch the tap off and on again it works for a while until the 'clonk'. The cold water pressure is fairly high (normal) but when I reduce it to stop the 'clonk' the heating works but its pressure is not sufficient to have a good shower. The clonk comes from inside the boiler but cannot put my finger on exactly where. Is there a valve to control such events? We have always had high water pressure and its been running fine for years up till now.