Hot water cylinder

1 Vented and unvented

A vented cylinder has an open vent pipe discharging over a cold water cistern, the tank that feeds it³. An unvented one runs without that open vent, so Approved Document G looks for at least two separate safety devices on top of the thermostat³. It gives an example pair: a non self-resetting energy cut-out, plus a temperature relief valve or combined temperature and pressure relief valve³.

Requirement G3 aims to keep stored water from ever exceeding 100°C, and to carry any discharge somewhere visible and safe³. So a pipe you might see running from a cylinder through a tundish, a small visible air gap, is part of its safety system and not a leak to be plugged³.

2 Temperature and bacteria

HSE's page for landlords recommends setting a cylinder so that water is stored at 60°C, and says tenants should not change that setting². It also says the risks from hot and cold systems in most homes are generally considered low, with temperature control the most reliable measure². Stored at that heat, water can scald, which is where the bath limit comes in³.

Requirement G3(4) limits hot water delivered to a fixed bath to 48°C, and applies to dwellings when erected or formed by a material change of use³. Approved Document G names an in-line blending valve or similar with a maximum temperature stop, which building users should not be able to alter easily³. It does not, on that wording, reach an existing bathroom left as it is.

3 Sizing and the airing cupboard

One maker's rule of thumb is under system boiler. Vaillant's slimline range is marketed to fit a utility room, loft or garage, so a cylinder need not sit in an airing cupboard⁴.

Older lagging is another matter. HSE lists lagging on pipes and boilers among places asbestos may be found in buildings built or refurbished before 2000⁵. Leave it alone if in good condition, and tell any contractor before work starts⁵.

Viessmann's user manual for its Vitodens 050-W asks that the safety valve on a connected cylinder is checked every six months, and warns never to close it⁶. That is one maker's note for its own products; ask your installer what applies to yours. A separate valve protects the heating side, covered under pressure relief valve.