Hot water cylinders and unvented systems

A vented cylinder is fed by gravity from a tank in the loft, so hot water arrives at whatever pressure the height of that tank provides, which is low. An unvented cylinder is fed straight from the mains, so every hot tap runs at mains pressure and a shower behaves like a shower. That is the whole appeal, and it is why most cylinder replacements now go unvented where the incoming main can support it.

The trade-off is that an unvented cylinder is a pressure vessel holding a large volume of water above 60 degrees, and it carries three layers of protection: the thermostat, a thermal cut-out, and a temperature and pressure relief valve discharging through a tundish to a safe visible point outside. Those have to be installed and discharged correctly, which is what the G3 qualification covers and why the work is notifiable. An unvented cylinder installed by somebody without it is both unlawful and genuinely dangerous.

Sizing follows use, not house size. Two bathrooms in simultaneous use want 210 litres or more; a single bathroom is comfortable on 150. Reheat time and the coil surface area matter as much as volume, and a cylinder sized for a heat pump needs a much larger coil than one sized for a boiler because it is heated with water at 50 degrees rather than 80.

Unvented cylinders need an annual service: the expansion vessel charge checked, the relief valves tested and the strainer cleaned. Skipping it is the commonest cause of a cylinder discharging water through the tundish for no apparent reason.