Unvented cylinders are mains pressure hot water storage systems that provide a highly efficient way of storing and distributing domestic hot water at mains pressure throughout your home.
These cylinders are not open to the atmosphere, unlike open vented cylinders which are open to the atmosphere via the cylinder’s vent pipe and the cylinder’s cold feed pipe from the cold water storage cistern/tank. These mains pressure cylinders provide quicker heat recovery than the low pressure open vented cylinder, i.e, it takes less time to heat up.
Unvented hot water cylinders provide an excellent supply of hot water, powered by the pressure of the water mains. They have good re-heat times and high-quality insulation. All unvented cylinders need servicing (best done on the same visit as the boiler service). The stitch-in-time effect is subtle with an unvented cylinder: as the air-gap or expansion vessel charge depletes over time the cylinder will weep a little water through the safety valve when the cylinder heats up & the water expands. Initially, this just wastes water (up to 3½ litres for a 185 litre cylinder). As time passes the valve can begin to release water more easily, and can eventually dribble continuously. Then it will need to be replaced.