Going to be changing my central heating pump soon. I'm hoping to get away with just changing the pump head but if I end up changing the whole thing is it best to use a fibre or rubber washer for the joints between the pump and gate valves? Some people have told me to use a rubber washer with no jointing compound, but others have told me to use a fibre washer with some boss white on the male threads and smeared thinly across the washer. Those in favour of fibre have said that the rubber washers can swell and leak after a while or can distort when the 2" nut between the pump and gate valve is tightened up. I was going to use rubber, but I'm unsure now.