Why an expansion vessel?

Pressure expansion vessels are designed to compensate for fluid volume changes caused by temperature changes and to keep the system pressure within the necessary limits. They are used in heating systems, solar thermal systems and drinking water circuits when hot water heaters are installed in the system.

The vessels are made of deep drawn welded steel sheet with anti-corrosion surface treatment. The steel vessel is fitted with a non-permeable highly elastic bladder, resistant to high temperatures. The bladder can be replaced in vessels of 50l volume and bigger.

One section of the vessel is filled with gas, the other one with liquid. When cold, the expansion vessel is almost completely filled with gas. As the system temperature rises, the fluid volume and pressure increases, the fluid enters the expansion vessel and compresses gas at the other side. When the temperature drops, the fluid is pushed back from the expansion vessel by gas.

All the expansion vessels are 100% tested and certified to the PED Directive (97/23 EC)..

HS Expansion vessels are designed to be installed in heating systems or in sealed cooling circuits, permitting to absorb heating fluid volume fluctuations caused by its temperature changes.

SL Expansion vessels are designed to be installed in solar thermal systems.

HW Expansion vessels are designed to be installed in systems with DHW heating. Besides the volume fluctuations they also absorb water hammers, extending the service life and increasing the reliability of both hot water tanks and the entire system.


Since January 1, 2000, Pressure Vessel Passports are not enclosed to expansion vessels.

January 1, 2000, Government Regulation 182/1999 Coll. came to force that was replaced by Government Regulation 26/2003 Coll. on May 1, 2004, that sets the technical requirements on pressure vessels. The Directive 97/23/EC of the European Parliament and of the Council on the approximation of the laws of the Member States concerning pressure equipment has been implemented into the above mentioned legal norms that state that expansion vessels shall not be accompanied by Passports because pressure vessels are products subject to conformity assessment.

This fact is obvious from Decree 18/1979 Coll. (amended) in which dedicated pressure equipment is specified and some conditions set to ensure its safety, where § 1.2 says unambiguously that the Decree does not cover products subject to conformity assessment by a special law2a (Act 22/1997 Coll.) and Government Regulation2b (Government Regulation 182/1999 Coll.) prior to introducing them to the market, nor technical equipment subject to the supervision of authorities listed in § 3.2. b of the law.

The standpoint of Czech Technical Inspection dated May 9, 2015 (in Czech language) confirms this fact.


The procedure for expansion vessel volume calculation can be found here:

expansion vessel
volume calculation