[harleyman]

Been asked about a back flow preventer ain't got a clue i think it's some thing to stop water passing back in to water suply???

[nathankaye]

Im guessing its a one way valve.
What is it for, as in why have you been asked?

Ones may use one, if they have a flat tank as they are prone to air traps and this would prevent it. From tank to pump.

[Don Kee]

None return valve, it just makes sure the water doesn’t go in the wrong direction.

In the case Nath mentions, it stops the water travelling backwards and being replaced with air

[CleanClear]

Been asked about a back flow preventer ain't got a clue i think it's some thing to stop water passing back in to water suply???

Non return valve :
https://www.suctionhose.co.uk/products/brass-non-return-valve-swing-check?

Some information here on why they are needed :
https://www.aquacure.co.uk/buyers-guides/mains-water-installation-kits