In closed heating systems, water expands when heated. The pressure expansion vessel absorbs this excess volume, preventing unacceptable pressure increases. It is divided into a water chamber and a gas chamber separated by a flexible rubber diaphragm, with the gas chamber pre-charged with nitrogen. As water temperature drops, water contracts and gas pressure pushes water back into the circuit. Sizing depends on system volume, temperature differential, and maximum allowable pressure. An undersized vessel causes frequent activation of the safety relief valve, leading to water loss and potential system damage.