Expansion vessel

An expansion vessel is a vessel used in a sealed hot water or heating system to accommodate the increase in water volume as the water is heated. In home energy documents it can appear with unvented hot water cylinders, heat pump hydraulic schematics and installation checklists, sometimes with notes about vessel size, system volume or pre-charge pressure. It is not a buffer vessel, hot water cylinder, safety valve, pressure relief valve, guarantee of correct system pressure or proof that the system has been designed, installed or commissioned correctly.