Open vented boiler systems
Conventional boilers are open vented boiler systems. In an open vented boiler system, the system utilises a feed and expansion tank to supply the central heating circuit with water and accommodate the expansion of water when it is heated by the boiler.
The feed and expansion tank is usually situated in the loft, where it forms the highest point of the central heating system and feeds into the boiler using gravity.
It is for this reason that the cold water tank is installed in the loft as high as possible to increase the water pressure. It is also the reason it is sometimes called a header tank.
An open vented boiler system works by the domestic water tank in the loft being filled with cold water from the mains supply and this water then circulating through the boiler several times where it is heated until it reaches the required temperature and then being stored in the hot water cylinder until you use a shower or a hot tap.
Water in the heating circuit passes through the boiler where it is heated. This water circulates through the radiators heating them up which then transfers the heat energy into the air warming the property.
Whilst most feed and expansion tanks are made from plastic today, it should be noted that in the past they were often made from asbestos and galvanised steel. Typically, these tanks have a capacity of 18 litres.