Do check valves restrict flow or pressure?

[gavioli]

I'm about to fit a mixer shower and of course I'm told because of regs check valves must be fitted. Now I've just looked at these valves and the area that lets the water through seems tiny! What my concern is that the flow or pressure then to the shower just won't be as good if they were not fitted.

Maybe I'm worrying over nothing and the flow is the same ?

Any knowledgable person for and answer to this ?

Thanks.
Nick

PS. (before someone comes back with the obvious - I do know they stop back flow and that's why they're needed)

 

[ChrisR]

IF the hose can dangle into the bath water or shower tray then you need single or double CV. If you buy a stpid bit of plastic to fix to the bottom end of a riser rail and restrain the head then you don't need the check valve on the hose.

If you don't restrain the hose, it must not be possible to get the head into a wc bowl. If the head can dangle into the bath then;

if the mixer has a self-resetting diverter such that turning the water off makes the flow go to the bath and not the shower when its turned on again, you need a single check valve onthe hose. Easy to buy, small chrome thing, whereas

if the mixer does NOT have a self-resetting diverter such that turning the water off makes the flow go to the bath and not the shower when its turned on again, you need a double check valve in the hose. Harder to buy, but MIRA et al do them. If you don'y want to do that, you can put double check valves in each pipe serving the mixer. They can of course be bigger.

You shouldn't need any on the mixer inputs otherwise, because the supplies should be from the same source so one pressure shouldn't be higher than the other.

If you have gravity supplies serving the mixer, its from a cistern so as long as that has a float operated valve to BS1212 part 2,3 or4, you're OK - no check valves.

If you're mains supplied, the resistance of a dcv wouldn't matter too much.

OK?