I bought a solar heating system that included a 150 litre steel ('glass' lined) tank intended for unvented use. Because of its origin me and my tame Corgi plumber were dubious about using it unvented (it doesn't have all the CE / Kitemark stuff you'd get with a European one, so probably couldn't meet UK building regs even if installed entirely properly).
Now it's all up and running (but no solar input yet) it is producing ony 20 or so litres of 'hot' water. The top 10 litres or so are at 55Â° but it soon drops to less, and the probe thermocouple one-third down the tank (~40 litres from the top) takes over 2 hours to get from 19Â° inlet temp to 30Â°.
The only way I can get enough hot water for one shower and occasional washing-up is by setting the programmer to run 'all day'. The primary seems to be flowing fine and is set at around 65Â° (a bit hotter than used to be OK to fully heat my old cylinder). Primary circulation 'out' gets as hot as the 'in' after a very short period (minutes). Turning the pump up to '3' and closing the bypass had no effect, so I'm pretty certain it isn't a lack of heat input capacity. The supplier is as mystified as I am, but only bought a couple of these cylinders, so we have no history. He's prepared to swap it, but I need to rule out 'sillies' first.