ABSTRACT
In these systems the water is in a closed circuit and is subjected to pressure, by either steam or gas, so that its temperature may be raised above 100 °C. As the temperature of the water rises, the pressure in the system must also be raised to ensure that pressure is always above the evaporation pressure, otherwise the water would flash into steam and the systems would not function as high-temperature hot-water heating systems. The pressure on the water can be produced by steam maintained in the boiler, or a steam space inside an external drum. When a gas is used for pressurising, it is maintained in a pressure vessel and the boilers are completely filled with water. One of the simplest methods of pressurising a system is by means of a high-level head tank. This method however is limited, since few buildings are of suitable design. It would require a minimum height of 30 m above the highest main pipe in order to produce a pressure of 300 kPa.
