As the name suggests, unvented cylinders are sealed to the atmosphere, whereas traditional vented cylinders are open to the atmosphere
The water contained within the cylinder is stored under relatively high pressure (typically 3 bar which is equivalent to 30 metres head), and this pressure is created by the inlet water supply (usually the incoming water main). Unvented cylinders do not generate or increase water pressure, they simply use the inlet pressure to supply the outlets. Like traditional vented systems, the heat source can be electric immersion heaters, a boiler, some other heat source or any combination of the aforementioned.
An important difference is that unvented cylinders must be installed by a competent and qualified G3 engineer; this is a legal requirement in the UK and all installations must be certified and registered with the local council.
Unvented systems can supply high pressure hot and cold water to all outlets, and the increased pressures make for better quality showers, without the need for pumps or booster sets. As both hot and cold services are fed directly from the mains supply, there is no requirement for a cold water storage tank, thereby keeping loft spaces clear, and pipework to a minimum.