There are two types of unvented hot water storage cylinders, both are manufactured to BS EN 12897:2006 Specification for indirectly heated unvented (closed) storage water heaters, and available as direct fired/heated or indirectly heated vessels:

​​The types of unvented hot water storage cylinders

​unvented hot water storage cylinders incorporating an internal expansion air gap.

unvented hot water storage cylinders using an external expansion vessel

​The direct system uses a direct type unvented hot water cylinder that does not contain any form of heat exchanger.

There are two very different types:

​Direct storage systems

​Electrically heated – this type of cylinder does not contain a heat exchanger.

​Gas or oil fired – the design of these water heaters originated in North America.

They consist of a hot water storage vessel with a flue pipe that passes through the centre.

​Direct unvented under sink storage heaters
Unvented under sink hot water storage heaters are connected direct to the mains cold water supply and deliver hot water at near mains cold water pressure. Because they have less than 15 litres of storage, they are not subject to the stringent regulations that surround the installation of larger unvented hot water storage units.