What is G3, and who can work on unvented cylinders?

An unvented cylinder stores hot water at mains pressure, which is why showers fed from one feel the way they do, and why the law takes a particular interest in who works on it. Stored hot water under pressure is safe because of a set of safety controls working in layers, and those controls are the reason a specific competency, universally known as G3, exists. This guide covers what G3 actually is, how installation and servicing differ, why G3 and Gas Safe registration get confused, and one claim you will read elsewhere that deserves correcting.

What does G3 actually refer to?

It is a Building Regulations reference, not a trade body or a register. Requirement G3 sits in Approved Document G, the part of the Building Regulations guidance for England covering sanitation, hot water safety and water efficiency, and its hot water safety provisions include unvented hot water storage systems. When an engineer says they are G3 qualified, they mean they hold the recognised competency for that class of work, gained through a training and assessment course run by bodies such as BPEC or City & Guilds. The certificate is the industry's evidence of competence for unvented work, and training schemes typically expect it to be renewed every five years, in the same spirit as the ACS cycle on the gas side.

Why does unvented work need its own competency?

Because the failure mode is unlike anything else in domestic plumbing. A vented cylinder is open to the atmosphere through its tank and overflow; an unvented cylinder is a sealed vessel holding hot water under mains pressure, kept safe by controls that limit temperature and pressure and by discharge arrangements that give an overheating or over-pressurised system a safe route out. Fitted and maintained correctly, the layers back each other up and the cylinder is an unremarkable white tank in a cupboard. Fitted wrongly, the safety case is compromised in ways an untrained eye cannot see. The specifics, which valves, what settings, what pipe runs, belong to Approved Document G and the manufacturer's instructions, and this guide deliberately reproduces none of them.

Who can install an unvented cylinder?

Installing an unvented hot water storage system is controlled building work. That means it must either be notified to the local authority's building control or be self-certified by an installer registered with a competent person scheme, and the approved document expects the work to be carried out by someone with the specific competency for it, which is what the G3 certificate evidences. In practice, a compliant install is done by a G3-qualified engineer whose scheme registration lets them certify their own work, and the homeowner receives a compliance certificate afterwards. An installation with no notification and no certificate is a problem that surfaces later, typically at conveyancing, when a buyer's solicitor asks for paperwork that does not exist.

Who can service one, and what does a service involve?

Servicing is not notifiable the way installation is, but the same competency logic applies: the annual service is substantially a check of the safety controls, and manufacturers' instructions direct that it be done by a competent person, which for unvented work means someone G3 qualified. A service typically involves inspecting and testing the safety devices, checking the expansion arrangements, and confirming the discharge route is intact and unobstructed, all against the manufacturer's instructions for that cylinder. It is precisely because the service is a safety-controls check that cutting corners on it is false economy: the controls are the reason the cylinder is safe, and the service is the only time anyone looks at them.

Is G3 the same as Gas Safe registration?

No, and the confusion runs in both directions. Gas Safe registration is the legal register for gas work; G3 is a Building Regulations competency for unvented hot water storage. They are different schemes covering different risks, and holding one says nothing about the other. A Gas Safe registered engineer is not automatically qualified to install or service an unvented cylinder, and a G3-qualified plumber cannot legally touch gas. The overlap in practice is that many heating engineers hold both, because the common real-world job, a system boiler feeding an unvented cylinder, spans the two: the boiler and its gas supply are Gas Safe territory, the cylinder is G3 territory, and a single engineer with both tickets can sign off the whole system.

Is an annual unvented cylinder service required by law?

No, and this is the correction worth making, because plenty of pages ranking for this question say otherwise. No statute requires an annual service of an unvented cylinder in a private home. What makes annual servicing near-universal advice is the manufacturer: the instructions call for it, the warranty on the cylinder is conditional on it, and the safety controls it checks are the ones doing the quiet work every day. Landlords sit slightly differently, since general duties to keep a rented home safe and in repair make maintaining the hot water system part of the job, but the annual-service obligation as such comes from the manufacturer's terms, not from a servicing law. Note the boundary with gas, too: an unvented cylinder is not a gas appliance, so it does not appear on the CP12, even when a gas boiler heats it. Get it serviced annually because it is genuinely good advice, not because the law says so.

How do engineers keep unvented work organised?

Unvented work is record-shaped in the same way gas work is: an install that must be evidenced, an annual service the warranty depends on, and a customer who will not remember either date. Engineers who add cylinder work to a gas round do best treating the cylinder like another appliance on the property's history. Manifold keeps jobs, certificates and service records against the property, so the install paperwork, each year's service and the gas records sit in one place, and the annual visit gets chased automatically. For the customer that is one engineer and one reminder covering the boiler service, the safety check and the cylinder; for the engineer it is the same visit sold three ways.