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These systems are also known as sealed or unvented systems. The boiler provides central heating and hot water; it heats water which is then pumped round your radiators or the coil inside your hot water cylinder depending on the required heating at that time. The water which is heated by the boiler is a sealed amount under pressure so there is no need for a cold water header tank in the loft or elevated position.
The sealed water can heat the domestic hot water in two ways, in an unvented cylinder or a vented cylinder.
An unvented cylinder stores the hot water in a pressurised hot water cylinder somewhere in the house or garage. Unlike an open vented system the hot water is at mains pressure, this means that there is no need for a cold water header tank in the loft or elevated position, and the hot water storage tank can be placed anywhere in the house.
A large volume of hot water is available throughout the day
The hot water is at mains pressure
No need for any cold water storage tanks in elevated positions
The hot water tank can be located anywhere in the house
More than one shower or hot tap can be run at the same time
Ideal if you are looking at doing a loft conversion and removing the header tanks
The hot water is not instantaneous
Once the hot water that is stored in the cylinder runs out you have to wait for it to be reheated
The cylinder needs to be located somewhere in the property
These boilers are suited for larger homes with families and have multiple bathrooms and showers