No water from tap mystery

[Gromit]

I have an intermitent fault which has every plumber who's visited my home stumped.
Wall mounted vado mixer tap on a gravity system, every few days when I turn the hot side on no water comes out. Usually all I have to do is turn the tap on and off a few times and the water comes out. Occasionally no water on the hot side comes out for a few hours. The cold side never has a problem.

I have a gravity system with a Stuart Turner positive head 4 bar pump on the hot side and the cold is mains water (so I could have drinking water). Non return valves on both sides. The problem occurs whether the pump is switched on or off, so I know the pump isn't the problem. The cartridge in the tap has been replaced but to no avail.

One plumber said that it's probably a sticking non return valve and they are more trouble than they're worth. Vado specify check valves to comply with regs due to mains and gravity water mixing. Plumber says they are being pedantic.

Two 50 gallon tanks are 7 feet above the tap.

Does anyone out there have any theories? This is driving me up the wall! Thanking you in advance.

 

[Plumbstar Tom]

You have to have check valves by word of the WRAS regs

YOu could try taking the internal on from the product and fitting and inline NRV?

Other than that the mixer may be so restrictive for the pump

 

[Gromit]

Thanks Tom - the weird thing is that this happens even without the pump switched on. Are the check valves being pedantic and is there a real problem if they are not fitted? The issue is that if they can stick and cause a problem they are situated behind tiles so if it's just a rule for the sake of it maybe better off without them.

 

[Flanners]

dismantle tap, remove O ring on body, refit tap!

The fact that after you have turned it on and off a few times it works is why I am suggesting this.