I have replaced an old Ariston undersink 10 litre 2kw water heater with the current model: https://www.screwfix.com/p/ariston-undersink-water-heater-2kw-10ltr/3248g The old one seemed to produce a lot more hot water and the current one produces about 3.5-4 litres of hot water before becoming luke warm. The only thing I changed was to add in the diastatic valves between the old brass fittings and the water heater. There is an expansion vessel that I have removed, tested the valve and re-pressurised to 3.5bar. I phoned Ariston technical last week and the guy I spoke to said that if the expansion vessel is ok then it is likely a faulty heating element. However, when I removed the unit to take back and exchange at Screwfix, it was full of hot water so I assume this is not the case. There is no pressure reducing valve fitted on the cold feed but the old model ran fine without that. I do not have a mains pressure tester. Just looking for advice from people that have some experience with these as I asked a mate and he said he has only put one in and installed the pressure reducing valve with it and wasn’t particularly impressed with the volume of hot water produced. Does it sound faulty, like the Ariston technical bloke said? I could get a mains pressure tester but I’d be reluctant as I can’t see it getting much use but will do if that’s the logical next step although just buying the valve isn’t going to cost much more. Are they just a bit rubbish? The reviews on the website would indicate they do a good job and the older model produced a fair bit of very hot water. All I want it for is to be able to have a sink of hot water and at the moment I can get half a sink of warm water.