For homes with 2 or more baths. System boilers are pressurized with mains water and are accompanied with either a separate open vented or an unvented cylinder. Rather than trying to heat a lot of water instantly they can be set to heat a larger volume of water indirectly. Vented cylinders have a tank above the cylinder in the airing cupboard or loft and rely on gravity to supply the hot water. Whereas un-vented cylinders are mains cold water pressure fed. The size of the tank is usually designed around how many bath and shower rooms the property has.