Gas and oil are the most popular fuels for central heating and hot water systems in the UK.  When choosing the hot water storage system that is most relevant to this type of system most specifiers, installers and householders will opt for INDIRECT cylinders.  Indirect cylinders usually have a coil fitted to transfer energy from the PRIMARY (central heating) circuit although they can utilise tank in tank construction to transfer heat or a plate heat exchanger. These can be mains pressure or tank fed. 

Dealing with mains pressure first, these can be UNVENTED where a store of water is heated for use in the property connected directly to the mains with appropriate controls, or THERMAL STORE where the energy for generation of hot water is drawn from a store of water using a heat exchanger.  This heat exchanger can be a coil or a plate heat exchanger.

Tank fed systems are referred to as OPEN VENTED.  This is where the more traditional copper cylinders are found.  These hot water storage systems usually generate hot water with an integral coil connected to the central heating system.  Hot water is then GRAVITY fed, using the height of the cold water cistern to drive water to the taps in the house.