Consider the HW (white) EV first. A typical example would be a 250L HW cylinder with a 20L EV.
If the EV is prcharged with air to 3bar and mounted and then the mains PRV is set to and maintains 3bar pressure then there is no water in the EV, if the 250L of water is heated to 60C (from say 15C) then the water will expand by 4.05L and will (have to) enter the EV, you will now have 15.95L air at one side of the diaphragm and 4.05L of water (in the EV) on the other side but the force of the expanding water will have increased the EV (+ cylinder) pressure to 4.02bar.,
Consider the Rad, CH system. If you had a 100L system volume with a 18L EV, you generally would precharge the EV to 1.0bar, you would then open a filling valve to a pressure of 1.5 bar, this gives you 3.6L of (reserve) water + 14.4L of air in the EV, when the water heats to say 60C, 1.62L of expanded water must enter the EV so you now have , 3.6+1.62), 5.22L of water and 12,78L of air in the EV bur the EV pressure is now 1.82bar. the reason that you precharge to 1.0bar and fill to 1.5bar is that you have 3.6L of reserve water to make up for any tiny leaks or collapsing air bubbles and don't have to top up the system more frequently, one to 2 years could be normal, ai 1bar there is still enough pressure to feed upstairs rads etc.
