Why unvented work needs a competent installer, the two usual ways to show compliance in England, and what the householder is left with.
Why the work is controlled
A vented cylinder has an open vent to relieve overheating and expansion. An unvented cylinder has none: it stores hot water under pressure and relies on layered temperature, pressure and expansion controls. If those controls, the relief devices or the discharge pipework are wrong, the stored energy can be released violently as an explosion or scalding. That is why the work is restricted to competent installers and is subject to building control.
Two separate questions
- Is the installer competent? They need the skills and knowledge for unvented systems and their safety controls. Relevant unvented training is evidence of competence. Someone without that competence should decline the work.
- How is compliance shown? In England, installing an unvented storage vessel of more than 15 litres is notifiable. For eligible work, an installer registered with an authorised self-certification (competent person) scheme covering that work can certify it through the scheme. Otherwise, find out which building-control procedure applies before starting. Ordinary work can use the local authority or a registered building-control approver; higher-risk-building work has separate Building Safety Regulator requirements. Check the body and route that apply to the actual project.
Being competent does not give you the right to self-certify, and scheme membership does not make an installation sound. Check what the scheme actually covers: not every trade membership covers this work. If a customer asks for a 200-litre cylinder to be fitted quietly, decline: a customer disclaimer, or telling only the water supplier, does not replace the building-control route.
A vessel of 15 litres or less can be exempt from notification when any associated electrical work is also non-notifiable. That exemption covers notification only; the safety requirements still apply.
Guidance and instructions together
The Building Regulations set the requirements. Approved Document G gives statutory guidance on meeting Part G, though another way of complying is possible. Follow the cylinder manufacturer's instructions for device positions, settings and commissioning, and make sure the actual design meets the requirements. A replacement safety valve must match the rating in the cylinder's approval data. A valve can thread in perfectly and still relieve at the wrong point, or discharge constantly.
Leave it complete
Keep the tundish visible, so occupants and engineers notice when a safety device has operated. At handover, leave:
- the completed commissioning record
- the manufacturer's instructions
- the Building Regulations compliance documentation.
Explain operation, maintenance and what a discharge means. The paperwork records the checks and the compliance route; it cannot show that every control and the discharge arrangement are right. These routes apply in England; other UK nations have their own procedures.