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Hot water: open-vented vs unvented cylinders

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How vented and unvented cylinders manage pressure, expansion and safety.

Comparison schematic of hot water cylinders. The open-vented cylinder is cistern-fed with an open vent pipe at atmospheric pressure; the unvented cylinder is sealed and mains-fed with an inlet control set and a temperature and pressure relief valve discharging through a tundish to a safe outside point. — Hot water vented vs unvented schematic

Both systems store hot water, but they manage pressure and expansion in opposite ways.

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