An unvented sealed central heating system is a central heating system using a boiler that (as opposed to open or vented systems) feeds directly from the mains water supply and is supplied as a packaged unit comprising all the necessary controls and safety devices. A mains pressure unvented domestic hot water cylinder is used and so all tanks/pipework are eliminated from the loft. The whole heating system is completely sealed (i.e. unvented), which removes the risk of freezing pipes in the loft, does away with maintenance work in the loft, and reduces the risk of system leakage. Unvented sealed system boilers benefit the end user with increased flow rate on their domestic hot water to all outlets in the property as it is held at mains pressure.