Snorvey wrote:Quite unusual last night as the pressure had gone up to 2.4 bar (I had set it to around 1.5 bar a couple of days prior to that). Today though, it's dropped to 0.5 bar (but still running). Something weird going on here.
Aaaaaah!
That sounds like your expansion vessel is knackered (or just needs recharging).
When the water heats up, it expands. Because the system is sealed, the extra volume needs to go somewhere.
So there is an expansion vessel, something like this
https://www.dhsspares.co.uk/product/ide ... 8l-1179334(I don't think that's the right model for your boiler, just an example)
In between the two domed halves is a rubber diaphragm. On one side is pressurised air (which can compress further to absorb the expansion of the water) and the other side is full of water connected to your heating circuit.
If the air leaks out, then the diaphragm will move completely to one side, filling the whole space with C Heating water. Then, when the water expands, it has no where to go. Pressure build and builds, until some water discharges out of the pressure relief valve, because it can't compress like air can.
When the system cools down again, there is too little water left in the system (because some has been discharged outside) so the pressure is low.
That sounds a lot like your symptoms. High pressure when hot, low pressure when cold.
Sometimes the vessel can be recharged - it has a car tyre type valve on it and you can literally pump it up with a car tyre pump.
But often the diaphragm has become perished or perforated, so recharging is only a temporary solution, but replacement is not expensive, depending on the design of the boiler. Sometimes the boiler needs taking off the wall to change the damn thing. Others are more accessible and it is a quick and simple job.