Heatrae Sadia Megaflo Eco direct cylinder 170ltr

An unvented cylinder stores hot water at mains pressure. It is fed straight from the cold water main, with no open vent pipe, and supplies taps and showers from that pressurised store. This category covers indirect and direct unvented cylinders, mostly in stainless steel, in standard, slimline and horizontal formats from about 90 to 300 litres.

Indirect, direct and solar

• Indirect: heated by a coil connected to a boiler or other heat source, with an immersion heater fitted as back-up.
• Direct: heated inside the cylinder itself, usually by immersion heaters.
• Solar and twin coil: a second coil takes heat from solar panels or another heat source.

Sizing and siting

Choose the capacity in litres to suit the expected hot water demand. Check the incoming mains, because manufacturers set a minimum working pressure and flow rate, and often specify a 22mm cold supply. Compare the height and diameter with the cupboard, leave room to reach the valves and immersion heaters for servicing, and plan a route for the discharge pipe from the tundish to a safe, visible termination point.

Safety and regulations

An unvented cylinder is supplied as a package with its safety and control devices, typically a temperature and pressure relief valve, an energy cut-out, an expansion vessel and an inlet control group with a pressure reducing valve. In England, unvented hot water storage over 15 litres must be installed by a competent person under Building Regulations Part G (G3). The installation is notifiable building work unless the installer is registered with a competent person scheme, and fittings must be suitable under the Water Supply (Water Fittings) Regulations 1999. Manufacturers require servicing every year by a competent installer.

For other hot water options, see cylinders and water heaters, or browse valves.