In the United Kingdom, unvented hot water cylinders must by law be installed by a qualified engineer who holds a current Unvented Hot Water (G3) qualification. This is a legal requirement under Part G of the Building regulations (Approved Document G).
Why a G3 qualification is required:
Unvented cylinders operate at mains pressure and store water at temperatures up to 60–70°C. If incorrectly installed or if safety components fail, the consequences can be severe. The G3 qualification ensures that the installing engineer understands the safety controls and can commission and test them correctly.
Who is qualified to install an unvented cylinder in the UK:
- Gas Safe registered engineers who also hold the G3 Unvented Hot Water Systems
qualification. - OFTEC registered engineers (for oil-fired systems) with G3 qualification.
- Qualified plumbers and heating engineers who hold a current G3 certificate, obtained
through a BPEC, City & Guilds, or equivalent accredited training programme. - OSO Approved Certified Installers — engineers who have completed OSO’s own training
programme and are approved to install, service, and warrant OSO cylinders.
What the installation must include:
Building Regulations Part G requires that the installation of an unvented cylinder is notified to the relevant local authority building control unless the installing engineer is registered with a Competent Person Scheme (such as NAPIT or APHC). Most G3-qualified engineers are registered with such a scheme and can self-certify the installation, removing the need for a separate building control application.
Upon completion, the installer should provide a commissioning checklist and hand over all documentation including the manufacturer’s installation manual, warranty registration details, and a record of the safety valve settings.