Water when heated expands so where does the extra volume go?
It goes into the expansion vessel unless something is stopping it.
The expansion vessel has a rubber diaphragm running down or across the middle. When purchased one side is inflated to about 1 bar pressure so when fitted the vessel is full of air which equals the volume of the vessel.
After fitting the vessel, the water side is filled with water using the flexible filling loop until the water pressure also equals 1 bar pressure.
So with the pressure being equal, the diaphragm does not move, but when expansion takes place volume of water goes up and the volume of air goes down because it's being compressed and causes an increase in pressure on both water and air.
The pressure rise is only about 1 bar when the heating is on, that is providing the EV was full with air before expansion took place.
If the valve on the vessel leaks like it does sometimes on a tyre, the volume of air gets reduced while the volume of water in vessel increases.
This results in a much smaller amount of air being compressed and the pressure would increase sufficient to burst pipes, so the PRV set to 3 bar stops this from happening.
What may have happened is:
Air leaks from EV
Water pressure eventually reaches 3 bar
PRV opens
Pressure drops to 0.5 bar
PRV closes
Pressure is reset to 1 bar and the same thing happens.
After curing the problem with EV, the 3 bar pressure is not reached
But pressure is still lost but over a longer period of time.
This is most likely because the PRV has not seated due to sediment on the valve sealing face.
If hot water is leaking from the discharge pipe, it may well have evaparated before it reaches the out let, so you dont see it.
There is also the possablity of the diaphragm being split, or the pipe from EV to system being blocked.
Other leaks can't be ruled out but many problems seem to be expansion vessel related.
