Pressure holding

A system for maintaining a constant static pressure is used in heating and cooling systems to order to ensure uninterrupted operation.

Constant static pressure in heating or cooling systems can be achieved in more ways than one. 

In smaller boiler-supplied heating systems, constant static pressure can be achieved with an open expansion vessel. This vessel is placed at the highest point in the system. A static pressure corresponding to the physical height is thus created.

In smaller closed systems, constant static pressure can be achieved with a pre-pressurised expansion vessel. Using a pre-pressurised expansion vessel in larger systems would be inconvenient due to large size necessary. 

Therefore, pressure holding systems are used in larger systems. These can contain a pressure expansion vessel, with an attached compressor or a pressure holding pump. The compressor or pump automatically ensures that the correct static pressure is maintained in the system by regulating vessel pressure.

In systems where water is added frequently (for example district heating systems), the pressure holding system is combined with a water feeding system based on pressure holding pumps. Speed controlled pumps are preferred to avoid pressure surges that can occur when pressure holding pumps are switched on and off.