Do NRV's reduce flow on gravity and/or need minimum pressure to work?

[WaterTight]

Fitted a mixer tap and the mains forces it's way back up gravity hot. But the hot pressure is already very poor due to it being a flat with a combination cylinder on the same floor level as the tap. Carefully chose one that looked fairly modern (as customer wanted) but had good flow on low pressure (from previous experience having installed it before) and so am worried fitting a non-return valve may make currently reasonable flow very poor or possibly that it might not even have sufficient pressure to get through the non-return. I'd estimate there is about 0.06-0.07 bar at most on the hot (just over half a meter between tap and tank)

I suppose other option would be to source a tap where the hot and cold don't mix in the body/spout, that also has great flow on low pressure. Wouldn't know where to start

Thanks

 

[ShaunCorbs]

It maybe ok for now but I’ve had them stick shut you could try a swing check valve

 

[WaterTight]

Never even heard of them, always something new to discover, cheers Shaun.

 

[Last Plumber]

It won't work I wouldn't expect.
NRV's normally have a minimum pressure required. Most have the figures available. I think between 0.1 and 0.3 bar depending on make/type.
You need to separate the two as you said somehow. Two taps, independent flow mixer or tank feed the cold and have them both as poor as each other.