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Expansion Vessels

An expansion tank or expansion vessel is a small tank divided in two by a rubber diaphragm and is installed on sealed systems to allow for the 4% expansion in water as the system heats up and therefore protect from excessive pressure. The tank is partially filled with air which absorbs shock caused by water hammer and absorbs excess water pressure caused by thermal expansion.

The rubber diaphragm in modern expansion tank prevents this undesired transfer of air, and helps maintain low levels of oxygen, reducing corrosion in the system. One side is connected to the pipe work of the heating system and therefore contains water. The other, the dry side, contains air under pressure, and normally a Schrader valve (car-tyre type valve stem) for checking pressures and adding air. When the heating system is empty or at the low end of the normal range of working pressure, the diaphragm is pushed against the water inlet; as the water pressure increases, the diaphragm moves, compressing the air on its other side.

When expansion tanks are used in domestic hot water (DHW) systems, the tank and the diaphragm must conform to drinking water regulations and must be capable of accommodating the required volume of water. In Europe the design and the construction of expansion tanks are ruled by EN 13831 according to Pressure Equipment Directive (PED) 97/23/EC.

Used to offer continuing protection to sealed hot water systems from excess pressure, expansion tank are essential and must be regularly checked to ensure they are fit for purpose.

Difference between heating expansion vessels and potable expansion vessels

There are two main types of Expansion Vessel dependent on what application you have. You will normally find a Red Vessel (Heating Vessels) to be used on a Sealed System Heating application whilst the most common use for a Blue Vessel or White Vessel (Potable Vessels) would be in a potable application such as an Unvented Water Heater or a Pumped System.

Potable expansion tank are built from different materials than Heating expansion tank to ensure that the water is contaminant free and safe for human consumption.

Heating Expansion Vessels

The purpose of an expansion tank is to compensate for the variation in the volume of water caused by a change in temperature within the heating system. For example, water heating up from 0C to 100C increases its volume by about 4.5% - and the expansion tank provides space to accommodate this increase. Complete Pump Supplies has a full range of tank sizes and heating expansion tank in stock.

Potable Expansion Tank

These cold expansion tank provide potable water at a pre-defined pressure, regardless of boost pressure, in order to limit the pump insertions due to the pressurised air between the membrane and the internal surface of the tank. When the pressure increases, the air cushion compresses and allows water to fill the tank. Complete Pump Supplies has a full range of expansion tank in stock now.

Our Expansion Vessel range includes vessels for heating systems and potable water and varities in size from 2 litre for use in instantaneous water heater applications, all the way up to 1000 litres or more for use on commercial applications for heating systems, or for potable water applications.

  • Up to 10 bar version with removable diaphragm suitable for heating, chilled water and potable water.
  • Up to 6 bar version with fixed diaphragm suitable for heating and chilled water.

Complete Pump Supplies supply expansion tank from carefully-chosen companies -  including Flamco, Reflex, Lowara and Grundfos - to ensure our customers have the best, most efficient and most reliable models to choose from, such hot water expansion vessels and potable water expansion vessels, whatever their intended application. We have ensured that all expansion tank supplied by us are manufactured in strict accordance with such standards as EN13831 and in compliance with relevant organisations such as the EC Pressure Equipment Directive (PED) and UK Water Regulations Advisory Scheme (WRAS).


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