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Unvented hot water: layered safety controls (diagram)

How the layered safety controls on an unvented hot-water cylinder protect against overheat, and where a discharge ends up.

Schematic of an unvented hot-water cylinder with a defence-in-depth ladder of three controls: control thermostat, then high-limit cut-out, then temperature and pressure relief valve. The cylinder is shown with expansion provision (vessel or air gap), and the relief valve discharges through a tundish air break to a safe, visible discharge point.
Unvented hot water (G3): layered safety controls

The diagram shows the unvented hot-water cylinder in the centre, storing hot water at mains pressure, with the controls that keep it safe drawn as a defence-in-depth ladder on the left.

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