A gravity hot water system is a traditional open-vented system with gravity fed domestic hot water heating (DHW) and central heating that is pumped around the system. In this design, the system is fed with water and kept under pressure via gravity-fed water from the main header tank in the loft. The hot water cylinder is heated simply via a gravity hot water circuit from the boiler, and central heating is provided via a separate pumped circuit from the boiler. A separate header/expansion tank is used to feed the DHW to prevent the risk of contamination with the primary circuit water.