Who can install an unvented hot water cylinder in England?

What does Building Regulation G3 require of the person installing an unvented cylinder?

Requirement G3 of Schedule 1 to the Building Regulations 2010 says an unvented hot water storage system may only be installed by a person competent to do so. G3 applies in England. It also requires safeguards that stop the stored water exceeding 100°C, and a route that discharges escaping hot water safely.

Approved Document G is the official guidance on meeting G3. For systems up to 500 litres and 45 kW it expects a tested prefabricated unit or package. On top of the thermostat, it expects at least two independent safety devices. Above 500 litres, the design should come from a suitably qualified engineer.

Approved Document G also sets out how the discharge works. The relief valve or combined temperature and pressure relief valve discharges through a tundish, which is an open funnel you can see water run into. The pipe between valve and tundish should be no longer than 600 mm, with the tundish below the valve and as close to it as possible.

How does an installer prove G3 competence for unvented hot water work?

G3 competence is evidenced by a training certificate for unvented hot water storage systems and a registered operative identity card. Baxi's G3 course description states that candidates who pass are qualified to install, commission and service unvented hot water systems, and that entry normally requires NVQ Level 2 or equivalent. Heatrae Sadia, which manufactures Megaflo cylinders, states that installers must be G3 trained to service its cylinders. It also describes G3 certification as the route into a competent person scheme and building control registration.

You can ask to see the evidence before anyone touches the cylinder. Four things are worth checking on any unvented hot water job:

• A current G3 certificate for unvented hot water storage systems
• A registered operative identity card, which names the body that issued it
• Competent person scheme registration, if the installer intends to self-certify the work
• Gas Safe registration as well, where a gas boiler heats the cylinder

Who notifies an unvented cylinder installation — a competent person scheme or building control?

An unvented cylinder installation in England is notified by one of two routes. An installer registered with a competent person scheme can self-certify the work, instead of seeking building regulations approval beforehand. Schedule 3 to the Building Regulations 2010 lists the work that may be self-certified and the bodies that register installers for it. Regulation 20 covers the certificate issued to the building owner and the notice given to the local authority.

If the installer is not in a relevant scheme, the work must be notified to a building control body before it starts, as a building notice or a full plans application. Where the work provides a hot water storage system, a statement has to accompany that notice. The statement gives the name, make, model and type of system, the body that certified its performance, and the body that issued the installer's registered operative identity card. Settle who is doing the notifying before work is booked, not afterwards.

The same rules bite when you swap an old vented tank for a pressurised one, which is a bigger change than it looks. Our guide to converting a vented system to an unvented cylinder covers what that conversion involves.

Is Gas Safe registration enough to work on an unvented cylinder?

Gas Safe registration on its own does not cover unvented hot water storage. G3 competence covers the cylinder, its controls and its safety devices. Gas registration covers work on gas fittings, which includes the boiler that heats an indirect cylinder. Where a gas boiler is part of the job, both apply.

Regulation 3 of the Gas Safety (Installation and Use) Regulations 1998 as amended says nobody may carry out work on a gas fitting unless competent to do so. It also requires that such work in the course of business is done only by a member of a class of persons recognised by HSE for that purpose. HSE explains this as registration with the Gas Safe Register.

A direct unvented cylinder heated by immersion elements involves no gas appliance, so the gas duties do not come into play. G3 competence under the Building Regulations 2010 still applies to that installation in England.

Can you replace or service an unvented cylinder yourself?

Replacing or servicing an unvented cylinder is not householder work in England. G3 reserves installation of an unvented hot water storage system to a person competent to do so. Servicing touches the relief valves and the expansion arrangement, which are the parts that keep a pressurised store safe. Heatrae Sadia says unvented systems carry an ongoing servicing requirement, and that relief valves should be checked and opened manually to rule out blockages in the valve or the discharge pipe.

Plenty is still safe for you to look at when hot water goes wrong:

• The boiler pressure gauge, so you know whether pressure has dropped
• Thermostat and programmer settings, including hot water timings after a power cut
• The reset button on the boiler, used once as the manufacturer's instructions describe
• Bleeding radiators, if they are cold at the top

Anything involving gas, the flue, or the inside of an appliance casing is work for a Gas Safe registered engineer. Cylinder safety devices and the expansion vessel are work for a G3-qualified engineer. Our guide on whether unvented cylinders and Megaflo systems need an annual service explains what that visit covers.

Do the water regulations apply to an unvented cylinder as well as G3?

The Water Supply (Water Fittings) Regulations 1999 apply alongside G3. Schedule 2 requires every unvented water heater, other than an instantaneous heater of up to 15 litres, to have a vent pipe, a temperature control device, a temperature relief device and a combined temperature and pressure relief valve — or to accommodate expansion within the secondary system. Once stored volume reaches 15 litres, a temperature relief valve or combined valve is required.

Some work on water fittings needs the water supplier's consent before it is carried out, and certain fittings must be notified to the supplier. Ask at the quotation stage who is handling that for your job. Sizing sits alongside these rules, since a cylinder that is too small will not meet the household's demand: see what size hot water cylinder you need.

What happens if an unvented cylinder was fitted without notification?

Contravening the Building Regulations is a criminal offence under section 35 of the Building Act 1984, as substituted by the Building Safety Act 2022. The penalty can include a fine and imprisonment, with further daily amounts for a continuing contravention. Section 36 lets the building control body serve a notice requiring non-compliant work to be removed or altered. In England that section 36 period is 10 years from completion of the work, following the Building Safety Act 2022 change in force since 1 October 2023.

If you have inherited a cylinder with no paperwork, the sensible first step is a look at the system by a G3-qualified engineer. The safety devices, the discharge route and the expansion arrangement all show whether the installation follows Approved Document G.

If you are planning a new or replacement cylinder, our Megaflo and unvented cylinder page sets out the free home survey and the written fixed price you receive before any work starts.

Sources

• Building Regulations 2010, Schedule 1 (Requirement G3)
• Approved Document G with 2024 amendments (PDF)
• Building Regulations 2010, Schedule 3 (self-certification)
• Competent person schemes (GOV.UK)
• Water Supply (Water Fittings) Regulations 1999, Schedule 2 – Hot water services
• Gas Safety (Installation and Use) Regulations 1998, regulation 3
• HSE – Gas Safe Register
• Building Act 1984, section 36
• Baxi – G3 Unvented Hot Water Storage Systems course
• Heatrae Sadia – Importance of servicing your cylinder