Water is heated by your boiler in a gravity fed system. The water is then circulated through a pump to your radiators for central heating and/or your hot water storage cylinder through gravity, where the water stored is in turn heated.

A small feed/ expansion tank often situated in the loft  feeds the hotwater system the tank ensures the system is kept topped up and allows for any expansion or evaporation that may occur.

 A System Boiler has no need of a separate expansion tank. The hot water cylinder is supplied from another larger tank also located in the loft.  Water is supplied to the loft tank/s by the rising main,  through a ball cock valve. when levels drop the water is replemishe.

Because the tank/s in the loft are isolated from the main supply by a valve, they use only their height to force water down by means of gravity. The pressure at your hot tap is proportional to the distance of the tank above the outlet.