Requirement G3 of the Building Regulations sets out how a hot water system with a storage vessel must be designed and installed in England, and it treats unvented cylinders as notifiable work unless the installer belongs to a competent person scheme.

What Requirement G3 says

Approved Document G, the government guidance on Part G of the Building Regulations 2010, applies to work in England (GOV.UK also says it applies to building work on excepted energy buildings in Wales, so if you live elsewhere, check the requirements that apply in your nation). Requirement G3(2) says a hot water system, including any cistern or vessel that supplies or receives expansion water from it, must be designed, constructed and installed to resist the effects of temperature and pressure that may occur in normal use or in reasonably anticipated malfunctions, and be adequately supported.

Requirement G3(3) says a system with a hot water storage vessel must include precautions to prevent the stored water exceeding 100°C and to ensure any discharge from safety devices is conveyed to where it is visible but will not endanger anyone in or around the building. G3(4), a 48°C limit on water delivered to a fixed bath, applies only when a dwelling is newly erected or formed by a material change of use.

Safety devices

Approved Document G says unvented storage systems should incorporate at least two independent safety devices, in addition to the thermostat that controls the stored water temperature. An acceptable approach it describes is a non-self-resetting energy cut-out that disconnects the heat supply if the system overheats, plus a temperature relief valve or a combined temperature and pressure relief valve to discharge water safely in serious overheating; alternatives are acceptable if they give equivalent safety. Where the indirect heat supply comes from a boiler, the energy cut-out may be on the boiler.

Packaged units and labelling

For unvented systems of up to 500 litres and under 45kW, the guidance says the system should be a proprietary unit or package that meets an appropriate standard, giving BS EN 12897:2006 as an example. The unit should be permanently marked with the manufacturer’s details, model reference, rated capacity, operating pressures and data on each safety device, and carry a warning to the user not to remove or adjust components and to switch the heater off and contact the installer if, for example, hot water flows from the discharge pipe. A warning to the installer that the installation is subject to the Building Regulations, with the installer’s name, address and completion date, is also shown.

Discharge pipework: tundish and pipe runs

Safety devices should discharge, directly or through a manifold, via a short length of metal pipe to a tundish. The guidance says the tundish should be vertical, in the same space as the cylinder, fitted as close as possible to and lower than the safety device, with no more than 600mm of pipe between them, and any discharge should be visible at the tundish. The discharge pipe from the tundish should end in a safe place where there is no risk to people nearby and should be at least one pipe size larger than the safety device outlet, with larger sizes needed for long or complex runs. It warns that discharge can be high-temperature water and steam, which may damage asphalt, roofing felt and non-metallic rainwater goods.

Notification, competent persons and certificates

Installing an unvented system is notifiable building work that must be notified to the building control body before work starts. However, the guidance says that if the installer is registered with a competent person scheme for unvented hot water systems, advance notification is not needed: the installer self-certifies that the work complies, and the owner or occupier is given a certificate of compliance, usually issued by the scheme operator. It also says good workmanship is essential and that where commissioning is carried out it must follow a procedure approved by the Secretary of State, which for dwellings is set out in the Domestic Heating Compliance Guide.

FAQs

Does this apply if I have a combi boiler and no cylinder?

Not in the same way. Requirement G3(3) and the unvented storage guidance apply to systems with a hot water storage vessel, so ask your installer whether your proposed system includes one.

What should I ask my installer?

Ask whether they are registered with a competent person scheme for unvented systems, whether the cylinder is a labelled proprietary package, where the tundish and discharge pipe will be, and how you will receive the certificate of compliance.

Is the same guidance used in Scotland, Wales and Northern Ireland?

GOV.UK says Approved Document G takes effect for use in England and also applies to work on excepted energy buildings in Wales. If you live elsewhere in the UK, check the building requirements that apply in your nation.

Bottom line

An unvented cylinder can be installed safely and legally, but only if it has suitable independent safety devices, properly arranged discharge pipework and the right paperwork. In England, expect either building control notification or a competent person certificate. This is general information; always confirm details with your installer and building control body.

Sources

This guide draws on the following primary sources, current as of 20 September 2026:

• Ministry of Housing, Communities & Local Government via GOV.UK, “Approved Document G: Sanitation, hot water safety and water efficiency” (PDF)
• GOV.UK, “Sanitation, hot water safety and water efficiency: Approved Document G”

Related guides

Best Boiler Deals is an independent guide. We may earn a fee from some links; this never affects what we write. This article is general information, not regulated advice on any specific installation — always confirm the details with a Gas Safe registered engineer.