A mains pressure hot water cylinder holds water at some pressure above atmospheric. It is normally supplied direct from the mains and the system is sealed to the atmosphere. This type of system therefore does not require a cold water storage header tank in roof space, thus saving space. Also due to working at higher pressures the cylinders are designed to higher standards.
Modern unvented cylinders have many benefits over traditional hot water storage systems:
Indirectly Heated Cylinders are designed to run in conjunction with an external heat source. They are often supplied with an immersion heater as back-up.
Directly Heated Cylinders are generally heated by electric immersion heater supplied with two or more immersion heaters and can take advantage of "economy tariff" electricity.
Cylinder Size
Both direct and indirect mains pressure cylinders are available in various sizes for most applications, from 100 litres to 500 litres.
Controls
Unvented systems have a number of controls built into their design to prevent an uncontrolled supply of heat to the stored water and are generally sold as a package with the controls required.