Gravity Fed /Fully Vented System: This type of system the water is usually boiler heated then dispersed via a single 3 port valve or two 2 port valves to the boiler coil of the hot water cylinder to produce hot water and/or to the radiators to provide household heating. This system is called 'fully vented' because no part of the system is pressurised. In the loft there are, as a rule, two water tanks.
The larger of these is the cold water tank (or cistern) which supplies water by the force of gravity to the hot water cylinder. Both the cylinder and the domestic hot water system vent into this tank. The smaller tank also facilitates gravity in its operation; this 'feed and expansion' tank vents the radiator circuit and ensures it is topped up. The two tanks are normally both fitted with a float valve so that if they drop below a set level they are refilled from a mains supply.
This type of system is inherently safe as there is no chance of a dangerous pressure build up but there are some draw backs. The cold water is fed to your hot water cylinder by the force of gravity which means the water pressure is comparatively low. As the height difference between the outlet and tank changes the pressure throughout a gravity system will differ between outlets.
The tank size or amount of water in it will make no difference to this. Additionally this system takes up a lot of space because of the need for loft tanks and a hot water cylinder.
Below is a simple digram of how a open vented system works in the home: