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An unvented cylinder is kept under pressure from an incoming water supply and is heated by either immersion heaters (a direct unvented cylinder) or by a heat source located outside the vessel (an indirect unvented cylinder). Unvented systems are ‘sealed’, so they require an expansion vessel either next to or incorporated within the boiler or cylinder. An expansion vessel allows for the expansion of water in the system as it heats up, and accommodates the increase in pressure. Unvented cylinders are designed to withstand considerable internal forces and are extremely safe to use when fitted with all the correct pressure equipment. They are usually made from stainless steel due to the high tensile strength that can be achieved from a fairly thin sheet. For more information of unvented cylinders, click here.