Thought I had a problem with an Ariston 30L water heater in a village hall whereby the hot water flow has steadily reduced to almost nothing over a period of a year. When you open the hot water tap you get a burst of water then it slows and stops after only a small amount has come out. Suspected blocked filter or outlet but all OK. Next suspect is what looks like a Zip pressure reducing valve (nominal 3Bar) with filter. Checked filter, clear and OK. Have not done a static pressure test yet on incoming cold but it may be less than 3 bar in the day. Does a 3 bar pressure reducing valve need more than 3 bar to flow or should it pass water and gradually shut off as the pressure increases to 3 bar. I can only get this one to flow by increasing the pressure setting well above the nominal 3 bar factory set point and that seems counter-intuitive.