Close the stopcock before going to bed and see what happens, that way your narrowing the issue down to high overnight inlet presure or a fault with your system.
Already done so. Problem doesn't occur.
I am glad that he is not a shipright ! If a system is losing pressure then it has a leak, no need to do any testing at this point because it is obvious you have one. A leak needs to be fixed, just topping up to maintain pressure is not the way to go. Say you need to add 250ml every four weeks, by the end of the year you have lost 3.25 litres somewhere and I would be very worried having seen the longer term consequences of such leaks to a property. Also just topping up without adding inhibitor is going to cause further long term issues with corrosion in the system so if your plumber cannot find the leak then get one who can.
Have you contacted your local water company to see what they say regards there supply pressure ? The fact you don't get the issue when not connected to the mains does hint towards high pressure on the inlet but then your system pressure regulator valve is there to reduce the incoming pressure to an acceptable limit. The spec for the Joule cylinder is 25 Bar, way above what any supply pressure could be so fitting a
would not help. What you might need to do is replace the actual pressure valve on your cylinder which are not that expensive unless there is still detritus in the system preventing the valve from working, ie letting through.
So with no external input from the mains then we can assume that the pressure within the tank is at lets say Xc, now the only way that the pressure can now increase to release via the valve into the tundish is by raising the temperature by heating the water which should not happen with no demand from the controller and no immersion. The mystery is where is that heat coming from or is the release valve letting go because of detritus. What you need is to measure the tank pressure at the point it releases. To solve your issues just requires logical thought and basic physic's because we may think of gremlins in the system but I have never seen one yet. Is the water really hot and steaming when it releases into the tundish ? That would show that it is being heated somehow, if it is just hot water then the release is not due to expansion due to heating.
I've been looking at various smart heating/thermostat programmers and notice that some of them have inbuilt short night time boiler run cycles, no idea why. I wonder if "off means off" on your system or is something cycling without user input?
Effectively bypassing the pressure regulator. Making a simple sketch is often a good approach to solving problems as you can just shown a mass of pipework as a basic line diagram, I have got full diagrams of my heating system, both pipework and electrical in case I ever need to get someone else to work on it and won't have to try and explain things as not getting any younger.
