A precisely dimensioned expansion installation not only determines the proper operation of the heating system, but also ensures a lot of safety and durability, since the expansion system prevents both too high and too low prepress in the installation.
At too high pressure there is a risk of leakage of heating hose or accidental opening of overpressure protection, while in case of a (too) low pressure damage to pumps may result from cavitation or worse, the entire system may fail.
Expansion systems are subdivided into different groups, so there are closed expansion vessels which keep the pressure in the installation within certain standards, which are usually used for smaller plants.
For larger installations, expansion machines are used which regulate the pressure within the installation by means of a compressor or a pump.
If an installation is fitted with a vertical storage tank, the installation pressure is determined by the height of the water contained in this tank, but it is sotex advisable to use a so-called nitrogen generator, these generators separate pure nitrogen from the outside air by means of Carbon 
filters and use this nitrogen as air cushions above the water surface in the storage tank, thereby preventing oxygen corrosion in the upper layer of the storage tank.