Where should checkvalves be installed?

[Woody3]

Where should checkvalves (single & double) be installed?

I'm just about to fit out our en-suites, which consist of:

1 x Bar mixer shower (built in check valve)

1 x Mixer tap on basin - should this be on the cold or hot feed or both?

1 x WC - I can't imagine how backflow could happen here, but some suggest that a checkvalve should be installed on the feed?

I've googled, read forums, but everyone seems to contradict one another!

[Tom_C76]

Check valves (single) on both sides of the mixer tap. Service valve on the WC.

[Woody3]

Thank you Tom!