Should I use fibre or rubber washer?

1 Jan 2016
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United Kingdom
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.
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Don't use paste on the washers, fibre or otherwise. It sticks. If you do, and need to remove at a later date, you will wish you hadn't.

As said, use rubber, but don't over tighten. If using existing valves, make sure faces are clean. If using new valves, stick to gate type NOT slotted 1/4 turn.
If you have movement in the pipes go rubber, if you are trying to slide the washer into a slot the same width then go fibre so the don't fold up. No sealant and no ptfe, just clean surfaces
That's great, rubber it is then, thanks for the replies.
I put PTFE on the threads on the pump valve to make removal a bit easier in the future...

...ok guys, start slagging.... NOW...
Thanks for the advice, we now have heat again and the missus has stopped the earache! The old pump was a doddle to remove and I'd bunged the outlet and vent pipes at the header tank so there was very little water loss. Replacement pump was easy to fit and, with rubber washers and no paste used as suggested above, hand-tightening the 2" nuts between the pump and gate valve and then nipping up at most a quarter of a turn seems to have done the trick. The one irritation is that I still have a very slight leak (no more than about one drip per day) from the gate valve body where the spindle goes in (not from the gland nut). I thought I'd stopped this and it only started again when everything was up and running, but I've sealed one of these 'live' before with ptfe and hope to do so again.

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