I have a bit of an issue with my Vaillant TurboMAX Plus 837 combi boiler.
The system keeps dropping pressure. There is no leak, the water is being ejected via the 3 bar pressure relief valve but only when the water is hot. I have topped the boiler up at night (to 2 Bar) after it cools and in the morning the pressure has remained and no water has been ejected but when the boiler is on, the water pressure doesn't get above 3 bar but water is ejected even when the pressure only rises to 2 bar. There is air pressure in the expansion vessel.
The PRV is the prime suspect, but just wanted to ask if anyone had any other thoughts, just because it only passes water when the water is hot.
From the symptoms you descibe, I would not consider the prv as a prime suspect. Generally, prv's fail by constantly dribbling redardless of whether the boiler is on or not.
The Vaiilant pressure should kept at around 1to 1.5 bar. Anything greater is not necessary and simply puts a greater strain on the rest of the system.
With the system cold, and it must be cold, pressurise to one bar then fire the heating up. Watch the gauge constantly as the system warms. If the pressure rises above 2 bar, the pressure vessel could be the culprit, and needs testing.