Question 1

What is an unvented cylinder?

Accepted Answer

An unvented hot water cylinder (commonly called by the brand name Megaflo, though this is just one manufacturer) is a sealed hot water storage vessel connected directly to the mains cold water supply. Because the cylinder is sealed and connected to mains pressure, it delivers hot water at mains flow rate - providing much better shower and tap pressure than a traditional vented (gravity-fed) system with a cold water cistern in the loft. The cylinder is heated either by an indirect coil connected to the boiler, by an immersion heater, or by both. To accommodate the expansion of water when heated, either an internal air bubble or an external expansion vessel absorbs the volume increase.

Question 2

What safety devices does an unvented cylinder require?

Accepted Answer

An unvented cylinder must have a minimum of three safety devices as required by Building Regulations Part G (G3): a pressure reducing valve (PRV) on the cold supply to limit incoming pressure (typically set to 3 bar); a pressure relief valve (PRV) that opens if pressure in the cylinder exceeds the set limit (typically 6-8 bar), discharging to a tundish and then to a safe, visible discharge point outside the building; and a non-self-resetting thermal cut-out that isolates the heat source if water temperature exceeds 90 degrees C (for immersion heaters) or a temperature relief valve (also called a combined temperature and pressure relief valve, TPR valve) that discharges if water temperature reaches 95 degrees C or pressure exceeds the set limit, regardless of the heat source. These devices must be installed in a specific configuration and discharged safely.

Question 3

Who can install an unvented cylinder?

Accepted Answer

Installation and commissioning of an unvented hot water system is notifiable work under Building Regulations Part G (Schedule 2A). It must be carried out by a competent person who is registered with an approved scheme for G3 (Unvented Hot Water Systems) work - such as those operated by CIPHE, APHC, or through Gas Safe Register for combination boiler/unvented cylinder packages. A competent person scheme registration allows self-certification of the work, bypassing the requirement to notify the local authority Building Control. If not registered under a competent person scheme, the work must be notified to Building Control and inspected by them. Using an unregistered installer is illegal under the Building Regulations and will affect building insurance and complicate future property sales.

Question 4

What is the difference between a vented and an unvented cylinder?

Accepted Answer

A vented cylinder (traditional hot water cylinder) is open to atmosphere via a vent pipe rising above the cylinder and terminating safely over the cold water cistern in the loft. The system operates at low pressure (determined by the head of water above the cylinder - the height difference between the cold water cistern and the hot water outlets). Flow rates are low at high draw-off rates. An unvented cylinder is sealed, connected directly to mains cold water, and operates at mains pressure - giving much better flow rates and pressure throughout the hot water system. The trade-off is that the sealed system must have multiple safety devices, requires annual servicing (checking safety valves and expansion vessel), and must be installed by a G3-registered competent person.

Unvented Cylinder

Just so you know...