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 have not been enclosed to expansion vessels in accordance with applicable generally binding legal regulations.

Stationary pressure vessels (hereinafter referred to as "SPV") can be manufactured or renovated either according to an older set of Czech technical standards (in this case ČSN 69 0010) or according to a newer European technical regulation, which is Directive 2014/68/EU of the European Parliament and of the Council, on the harmonisation of the laws of the Member States relating to the making available on the market of pressure equipment, which was implemented into the Czech legal system by Government Regulation No. 219/2016 Coll., on the assessment of conformity of pressure equipment when they are supplied to the market (hereinafter referred to as "PED"). For older SPVs manufactured according to ČSN 69 0010, it is mandatory to have a pressure vessel passport, which is a document describing the production of the SPV, identifying its manufacturer, technical characteristics and parameters, data on safety valves and safety equipment, drawing of the vessel, strength calculations of welds and material, wall thickness of the vessel and material of which it is made, instructions for the maintenance of the vessel, a schedule of inspections and revisions and many other data, listed in Annex A of the ČSN 69 0010-7.2.

The manufacturer's obligation to supply a SPV passport is based only on ČSN 69 0010, while European legislation (i.e. PED, or Government Regulation No. 219/2016 Coll., and EN 13445, EN 13831 and other European standards) does not impose this obligation. The expansion vessels distributed by us are manufactured exclusively in accordance with European legislation, therefore the manufacturer is obliged to supply with the vessel only an EU Declaration of Conformity, an instruction manual, or a certificate issued by a Notified Body.


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

expansion vessel
volume calculation