Unvented Cylinder Regulations: Part G and What You Should Hold

Which regulations apply to an unvented cylinder?

The main one is Part G of Schedule 1 to the Building Regulations 2010, which covers sanitation, hot water safety and water efficiency. The official statutory guidance is Approved Document G, published on GOV.UK by the Ministry of Housing, Communities and Local Government. The current edition is the 2015 edition with the 2016 and 2024 amendments.

Within Part G there is a specific requirement for hot water storage systems that hold water above atmospheric pressure. The aim is simple: a cylinder holding hot water under pressure must not be able to burst, and any discharge from it must go somewhere visible and safe. The Approved Document describes what an acceptable solution looks like, including the safety devices and the discharge arrangement, and an installer works from it.

The Water Supply (Water Fittings) Regulations 1999 sit alongside it. They are about protecting the public water supply, so they affect how the cold inlet is arranged, including backflow protection. They do not replace Part G, and the water supplier, not building control, enforces them.

Wales has its own version of Approved Document G, and Scotland and Northern Ireland have their own building standards. This page describes England. If you are elsewhere, read the equivalent document for your nation.

Do I need building regs for an unvented cylinder?

Yes. Installing an unvented hot water storage system is notifiable building work, so one of two things has to happen. Either the installer belongs to a competent person scheme, which lets them self-certify the work and have a certificate issued to the occupier afterwards, or the building control body is told before work starts and may check the job.

The route used does not change what has to be installed. Both routes expect the same safety devices and the same discharge arrangement. What differs is who confirms it, and what you are given at the end.

A like-for-like replacement is a new installation. A new cylinder in the same cupboard brings a new set of safety devices and a new discharge connection, and the old installation does not carry its approval across. Owners most often ask about paperwork at this point, and the guidance treats installing an unvented system as notifiable. Ask the installer how the notification will be made.

Can I fit my own unvented cylinder?

The guidance says building owners and occupiers should take care to choose installers who have the necessary skills, and gives registration with a competent person scheme for unvented systems as one way of showing them. In practice that means someone with the training to install the safety devices, set them up correctly and commission the cylinder. An owner without that background should not fit one.

There is also a practical reason. An unvented cylinder holds a large volume of water at mains pressure, so a fault in the discharge arrangement or an incorrect relief valve setting has consequences that a vented cylinder does not. Even the discharge pipe sizing and the termination point are set out in the guidance.

You can legally carry out some peripheral work, like decorating the cupboard or arranging access. You should not touch the safety devices, the valve group or the electrical connections. Nothing in a sealed relief valve or an electrical enclosure is a householder job.

If you are weighing up a DIY replacement of any kind, the guide on replacing a hot water cylinder yourself covers what is realistic and where the line sits.

What does the installation have to include?

The Approved Document lists the safety devices an unvented system must have, and the cylinder manual specifies the exact parts and settings. In outline, the installation needs:

• A control thermostat, plus at least two independent safety devices in addition to it. An approach the guidance gives as acceptable is a non-self-resetting energy cut-out together with a temperature relief valve or a combined temperature and pressure relief valve.
• An expansion relief valve on the cold inlet group, as the manufacturer specifies.
• A means of absorbing expansion, which is an expansion vessel or an internal air gap.
• A discharge pipe that carries any water from the relief valves to a safe place, through a visible tundish with an air gap, with a vertical drop of at least 300 mm below the tundish before any bend and a continuous fall of at least 1 in 200 after it, in a pipe sized for the valve outlet.
• Pipework and fittings suitable for the pressure and the temperature.
• Commissioning of the whole system, including the setting of the pressure reducing valve and the checking of each device.

What documents should an owner hold?

This is the part most owners never check, and it is what a buyer, a surveyor or a landlord inspection will ask about. Ask for these at the end of the job and keep them with the property papers.

• The certificate showing the installation was notified: where the installer belongs to a competent person scheme, the occupier should be given one within 30 days of completion, and otherwise the building control completion certificate.
• The commissioning record, which lists the pressure settings, the checks made and the date. Many manufacturers supply a checklist for the installer to complete.
• The installation and user manual for your cylinder, with the data plate model and serial number matching the cylinder.
• The service record. Manufacturers expect regular checks, and a gap in the record can affect any warranty.
• Notes on where the tundish and the discharge pipe terminate, so that nobody later boxes them in or alters them.

How do the rules affect servicing and repairs?

Part G governs the installation, but the safety devices keep mattering afterwards. Replacing a relief valve, a combination valve or a thermal cut-out puts a new safety device into the system, and the guidance expects such parts to match the cylinder and be fitted by someone with the skills. Whether a given part replacement is itself notifiable depends on the exemptions in the Building Regulations, so ask the installer.

That is why work on an unvented cylinder is approached in order: confirm the fault first, then replace the part, then recheck the system. A discharge fault is not worth the risk of a guess. The checks involved are laid out in unvented cylinder servicing, and the work on installation itself on the unvented cylinder fitting page.

Landlords are expected to keep these systems in safe working order for tenants, which is why the paperwork matters beyond the day of installation. The landlord cylinder checks guide covers that side.