Unvented cylinders are high pressure hot water storage systems that provide a highly efficient way of storing and distributing domestic hot water at mains pressure throughout your home. They are ideally suited to properties with more than one bathroom, where a combi boiler might have difficulty in keeping up with the hot water demand.
They are also an excellent way to upgrade an existing "tank fed" hot water system to give piping hot water at mains pressure and high flow rate, without, in many cases, the need to change your boiler.
These cylinders are not open to the atmosphere, unlike open vented cylinders which are open to the atmosphere via the cylinder's vent pipe and the cylinder's cold feed pipe from the cold water storage cistern/tank.
These high pressure cylinders provide quicker heat recovery than the low pressure open vented cylinder, i.e, it takes less time to heat up.
Because water is supplied from the mains supply and not from a cold water storage cistern/tank, the flow rate and water pressure from a unvented cylinder at the outlets (taps/shower heads) is much greater than from the storage fed open vented cylinder typically equivalent to mains pressure.
An unvented cylinder can be directly heated by electric immersion heaters, or a gas burner, or indirectly heated by a central heating boiler
There are a number of advantages in using an unvented hot water cylinder. For example:
-High water pressure - great for showers so no need for booster pumps
-Balanced water pressure at hot and cold taps/faucets
-Balanced shower pressure - allowing for safer temperature control
-Better high pressure terminal fittings - such as taps/faucets and valves
-No cold water storage cistern/tank - enabling loft conversion
-Improved cylinder location flexibility due to mains water pressure
-Quicker to install than an open vented cylinder and its associated fittings
The disadvantages of an unvented cylinder can be:
-With low mains water pressure and poor flow rate, the system may be unable to operate
-No water storage in the event of a water mains failure
-Extra controls occupy more space (unless using a composite/combination valve)
-Price - unvented cylinders usually cost more than open vented cylinders
The domestic/residential unvented cylinder will operate in a pressure range of 2 bar to 3.5 bar (200 - 350 Kpa). Before the installation of an unvented cylinder, you will need to establish whether your mains supply can provide adequate pressure and flow rate.
For typical domestic systems a minimum pressure of 1.5 bar (150 Kpa) and 20 litres/min water flow rate is required.