Expansion tank

An older style of expansion tank was larger, oriented horizontally, and had no rubber diaphragm separating the water from the air pocket.

The rubber diaphragm in modern expansion tanks prevents this undesired transfer of air, and helps maintain low levels of dissolved oxygen within the pipes, reducing corrosion in the system.

Although such systems were remarkably trouble free, there are concerns about the potability of water from roof tanks due to the possibility of contamination.

[3] In Europe the design and the construction of expansion tanks are ruled by EN 13831 according to Pressure Equipment Directive (PED) 97/23/EC.

Similar devices are used in large-scale pumping stations, where they may be called an expansion chamber[4] or a hydrophore, to maintain an even pressure and to reduce the effects of water hammer.

Expansion tank in a central heating system
Expansion tank in a 1987 Saab 90 (brown plastic tank with white lid at top of picture)