The Runarsson expansion system is mainly used in large combined heat and power plants and district heating plants. It is used to control boiler water pressure in the heating system due to water thermal expansion caused by temperature changes. This system consists of a expansion tank, pump control and working pressure maintain system. The operating pressure is maintained at a fixed level and is independent of the water level in the expansion tank. The flexible membrane minimizes the oxidation of boiler water, thus preventing corrosion and extending the service life of the entire system. Proper tank selection ensures efficient use of tank’s volume, and the pressure control system is used to accommodate water which volume has increased, which prevents the entire system from exceeding the set working pressure level.