Unvented Hot Water Systems: G3 Qualification and Certification Requirements

Unvented hot water systems are now the default choice for mains-pressure domestic hot water in the UK, but they come with a set of legal obligations that catch people out every year. Get the qualification wrong, skip a safety device, or run the discharge pipe incorrectly and you're looking at a failed inspection at best, and something far worse at worst. Here's everything you need to know, written by someone who's dealt with the consequences of corners being cut on these jobs.

What Is an Unvented System and Why Does G3 Exist?

An unvented hot water storage vessel stores domestic hot water at mains pressure, without the open vent and cold water storage cistern that a traditional vented system uses. The benefits are obvious: powerful showers without a pump, consistent pressure throughout the property, no cold water tank in the loft, and faster recovery times on modern cylinders like the Megaflo, Heatrae Sadia Megaflo, OSO, or Ariston.

The risk is equally obvious. You have a large volume of water stored at high temperature and pressure. If the temperature rises above 100°C or the pressure exceeds the vessel's design limit, you get a BLEVE (boiling liquid expanding vapour explosion). This is not a theoretical risk. Unvented cylinders without proper safety devices have killed people. That's why Building Regulations Approved Document G3 and BS EN 12897 set out mandatory requirements, and why there is a specific qualification requirement for anyone who installs them.

The 15-Litre Threshold

The G3 qualification requirement applies to any unvented hot water storage vessel over 15 litres capacity. In practice, every domestic cylinder you'll ever install will be well above this threshold. A standard single-property cylinder is typically 150 to 300 litres. The 15-litre figure is there to exclude point-of-use water heaters under sinks, not to create a loophole for larger vessels.

G3 Qualification: What It Is and Who Needs It

To legally install an unvented hot water storage vessel over 15 litres in the UK, you must hold a current G3 qualification. This is not optional, it is a legal requirement under Building Regulations, and it is an individual qualification, not a company one. If you're employed by a plumbing firm, your qualification doesn't cover your apprentice. They need their own.

Routes to G3 Qualification

Competent Person Schemes and Self-Certification

If you are registered with a Competent Person Scheme (CPS) that covers unvented hot water, such as APHC, CIPHE, or SNIPEF, you can self-certify your installations under Part G of the Building Regulations without submitting a Building Notice to the local authority. This is the way most professional plumbers operate because it removes the delay of waiting for a building control inspection and puts you in control of the certification process.

If you are not registered with a CPS, every unvented cylinder installation must be notified to the local authority either as a Building Notice before work starts, or via a Full Plans submission. The local authority will then arrange a building control inspection. Not notifying is a criminal offence under the Building Act 1984.

Safety Devices: The Mandatory Kit

The whole point of G3 is the safety device specification. Every unvented cylinder installation must have all of the following correctly installed and tested before commissioning. Missing or incorrectly installed safety devices are the most common reason for failed inspections.

Temperature and Pressure Relief Valve (T&P Valve)

The T&P valve is the last line of defence. It is factory-fitted to the cylinder at a set temperature (typically 90-95°C) and a set pressure. If either the temperature or pressure exceeds the set point, the valve opens and discharges hot water through the discharge pipework. The valve must never be removed, capped, or replaced with anything other than an identical specification valve. Test it during commissioning by manually lifting the test lever — water should discharge freely. If it doesn't move, or it fails to reseat, the valve is defective and must be replaced before the system is commissioned.

Tundish

The tundish is the visible air gap in the discharge pipework, positioned within 500mm of the T&P valve. It serves two purposes: it creates a visual indicator that the valve has discharged (you can see water flowing through it), and it provides an air break to prevent back-siphonage. The tundish must be visible and accessible, not buried in a cabinet or boxed in behind a panel. The space below the tundish funnel must be open to air. Blocking it defeats the purpose entirely.

Discharge Pipe Sizing and Routing

This is where most problems occur. The discharge pipe from the tundish to the final outlet must be correctly sized and routed. The fundamental rule is that the discharge pipe must terminate in a visible, safe, and accessible location, and must not present a scalding risk to anyone who might be near it when the valve operates.

The correct approach per Approved Document G3 is: the D1 section (from the T&P valve to the tundish) is typically 22mm and no more than 500mm long. The D2 section (from the tundish to the final outlet) must be at least one pipe size larger than D1, so minimum 28mm copper or equivalent. The D2 pipe must terminate in a visible location — typically discharging over a trapped gully at low level, terminating no higher than 100mm above the floor of the gully to prevent splash, and in a location where a member of the public cannot be scalded if the valve operates at 2am without warning.

Expansion Vessel

As water heats up, it expands. In a sealed system there has to be somewhere for that expanded volume to go. The expansion vessel is a sealed chamber with a pre-charged air side separated from the water by a flexible membrane. As the water expands, it compresses the air charge. The vessel must be sized correctly for the cylinder volume — undersized vessels cause the T&P valve to drip regularly as the pressure relief point is exceeded during normal heating cycles. This is a design error, not a valve fault. The pre-charge pressure of the vessel must match the cold working pressure of the system, typically 3 bar.

Pressure Reducing Valve (PRV)

Mains water pressure varies considerably across the UK. An unvented cylinder has a maximum working pressure, typically 6 to 10 bar depending on the model. If the incoming mains exceeds the cylinder's working pressure, either directly or through pressure surges, the cylinder will be overstressed. The PRV sets a fixed maximum incoming pressure, usually 3 bar for domestic installations. It must be set and tested during commissioning with a pressure gauge — not just tightened up and assumed to be correct.

Line Strainer

The line strainer protects the PRV and the cylinder's internal components from debris in the incoming mains supply. It must be fitted upstream of the PRV and should be checked and cleaned during any service visit. Blocked strainers cause flow restriction and pressure fluctuations. It's a minor component but a mandatory one under G3.

Commissioning and the G3 Certificate

Once the installation is complete and all safety devices are in place, the system must be formally commissioned and the commissioning record completed. This is not optional paperwork — it is the documentary evidence that the installation was carried out by a qualified person, that all safety devices were tested, and that the system was operating within its designed parameters at the point of handover.

Commissioning Checks

A proper commissioning sequence includes: verifying incoming mains pressure before the PRV and confirming the set pressure after; checking the expansion vessel pre-charge pressure; testing the T&P valve operation; confirming the discharge pipework route and termination; setting the thermostat to a minimum of 60°C (to prevent Legionella growth) and verifying the thermostat cut-off temperature; testing the high-limit thermostat (the second temperature control, typically set at 80-85°C); checking for leaks at all connections under working pressure; and running the system through a full heat cycle to verify temperature, pressure, and recovery performance.

What Goes on the Certificate

The unvented hot water storage vessel commissioning record must capture the cylinder make, model, serial number, and capacity; the incoming mains pressure and PRV set pressure; the expansion vessel size and pre-charge pressure; confirmation that the T&P valve, tundish, and discharge pipework have been installed and tested; the thermostat set temperature; and the installer's G3 qualification details. Building control officers use this certificate to confirm compliance with Approved Document G3 — a certificate with missing fields or incorrect data is not useful evidence of a compliant installation.

Notifying Building Control

If you are self-certifying through a Competent Person Scheme, you must notify the scheme of the installation within the timescale specified by the scheme (typically 30 days of completion). The scheme then notifies the local authority on your behalf and issues the homeowner with a certificate of compliance. If you are not in a CPS, you must have the building control inspection arranged before the cylinder is put into service. Don't commission the system and then notify — get the inspection arranged first.

Common Mistakes That Fail Inspections

After years of seeing G3 inspections, the same errors come up repeatedly. Discharge pipe terminating into a soil pipe stack rather than a safe visible location is probably the most common. Expansion vessel pre-charge pressure not matching system working pressure is a close second — many installers check the vessel is physically connected and move on without checking the pre-charge. Thermostats set below 60°C to satisfy a customer who wants "instant hot water" is another frequent problem, as is a T&P valve test lever that won't move because the valve is seized after years without testing on a previous installation that was never properly commissioned.

A pressure test certificate for the cold water pipework feeding the cylinder is also worth having on record, particularly on new builds or where pipework has been extensively modified. Building control officers on new build inspections often ask for this alongside the G3 commissioning record.

Servicing and Ongoing Compliance

G3 compliance doesn't end at commissioning. Manufacturers recommend annual servicing of unvented cylinders to check the T&P valve, expansion vessel pre-charge, and anode rod condition (on stainless steel cylinders where applicable). The T&P valve must be replaced every five years or at every service if it shows any sign of weeping or failure to reseat. Expansion vessel membranes have a finite life and should be checked at every service.

For landlords and letting agents, unvented cylinders form part of the overall water safety obligations under the Legionella ACoP L8. The minimum 60°C thermostat setting is not just a G3 requirement — it is also the primary control measure for Legionella in a domestic hot water system. Any service engineer who adjusts the thermostat below 60°C on a rented property is creating a Legionella risk that the landlord is responsible for managing.

Keeping a complete service record from commissioning through every subsequent visit is the professional standard. If a cylinder fails catastrophically and there's a legal inquiry, the paper trail from the original G3 commissioning record through every service is your evidence that the system was properly maintained.

Published February 2026. This article is for general guidance only and does not constitute legal or professional advice. Always refer to the relevant standards and consult qualified professionals for definitive requirements.