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Unvented Mains Pressure System

Gravity hot water system

In an unvented system, a vessel is filled with cold water directly from the incoming water main. The vessel is kept under pressure from the incoming water supply and heated either by immersion heaters (a direct model) or by a heat source located outside the vessel (an indirect model). Examples of the latter might be a gas boiler, solar energy, and Aga etc. The 'vessel' to which we refer normally takes the shape of a cylinder. In an unvented mains pressure system the cylinder must also be regarded as a pressure vessel. This type of cylinder is referred to as 'an unvented cylinder' because it is not vented to atmosphere in the same manner as a normal copper cylinder be. Unvented means exactly that.

The unvented cylinder will have pipe work connected to it that will carry the heated contents around the property to various taps and outlets. As soon as an outlet is opened the pressure of the incoming cold water to the cylinder forces(displaces) the contents of the cylinder (hot water) out and towards the open outlet. Hence, hot water is delivered at the tap or outlet at a pressure equivalent to the force of the incoming cold water main to the cylinder.

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