I am running a closed loop hot water system @ 3barg and 80degC. At the moment my expansion vessels are pre-charged to 1.5barg. When we turn off the boilers at the weekend, obviously the water in the loop will cool and contract => system pressure will drop =>Pressurisation pumps turn on in order to maintain 3 barg in the system. Then when the boilers start running after the weekend and the water heats up , the system pressure will shoot up to 4.5bar (relief valve setting)
Question is: What pre-charge should be in the expansion vessels to rid me of this problem? I think 3barg but others disagree.
The purpose of the expansion vessel is to take up the extra water as the water heats. If you increase the vessel initial pressure, the water pressure will immediately jump up to the vessel setting due to hydraulic pressure with no place for the water to go until the system pressure exceeds the vessel setting. so your thinking needs be reversed.
Set the vessel pressure at the pressure the system must have to stay above the highest radiation or coil (plus 4 psig for venting.) The water will immediately enter the vessel as the pressure exceeds this minimum cold fill pressure and the tank can then take the extra water until the maximum water temperature and a pressure 5 psig below the relief valve pressure.