G3 Unvented Cylinder Regulations Explained 2026

Building Regulations Approved Document G3 is the document that turned the unvented hot water cylinder from a curiosity into the default hot water solution in London. Before G3 came into force, an unvented system was a specialist install with bespoke safety arrangements. Since the current regime, a properly specified Megaflo, Telford Tempest or Joule Cyclone is the answer for almost any London property on a decent cold mains — and the rules that govern that install are concentrated in fifteen or so pages of the Approved Document. Most landlords and homeowners have never read them. This guide unpacks what G3 actually requires in 2026, who is allowed to do the work, and why the annual service every cylinder manufacturer demands is not a marketing upsell but a hard warranty condition.

Two things drive almost every G3 question we field on the phone. The first is whether a particular engineer is qualified to do the work. The second is what an annual service is for, given the cylinder "looks fine". The short answers are that only a G3-ticketed engineer can lawfully install or commission an unvented vessel over 15 litres, and that the annual service exists because the three safety devices protecting your family from a 90-degree-Celsius scalding event have moving parts that fail silently. The long answers are below.

What Approved Document G3 Actually Is

G3 is one part — paragraph G3 — of Approved Document G to the Building Regulations 2010 (as amended). The full title is "Sanitation, hot water safety and water efficiency". G3 specifically covers the hot water safety element, and it applies in England and Wales. Scotland uses an equivalent provision in the Scottish Technical Handbook; Northern Ireland uses its own Building Regulations. The standard sits alongside WRAS approval under the Water Supply (Water Fittings) Regulations 1999 for wetted components, BS 7593:2019 for the treatment of water in central heating systems where the cylinder is fed by a boiler coil, BS EN 806 (the replacement for the legacy BS 6700) for pipework design, and the Benchmark commissioning scheme operated by the Heating and Hotwater Industry Council.

G3 only applies where the stored water exceeds 15 litres. A 10-litre under-sink point-of-use heater is outside scope; a 120-litre flat cylinder is inside scope; every standard domestic unvented cylinder is inside scope. The Approved Document gives the technical solutions the Secretary of State considers sufficient to meet the functional requirement — alternative engineered solutions are theoretically possible but never used in domestic practice because the standard solutions are well understood and accepted by every Building Control body in the country.

The G3 Ticket — Who Can Legally Install

The G3 ticket is the common-trade name for one of two recognised qualifications:

• BPEC HWSS — Hot Water Systems Supplementary, awarded by BPEC Certification. The most common route. A two-day course plus assessment, renewed by re-assessment every five years.
• City and Guilds 6189 — Less common in modern practice but recognised as equivalent. Same five-year renewal cycle.

Either ticket allows the holder to install, commission, modify, service and decommission an unvented hot water vessel under G3. The ticket is personal to the engineer, not transferable, and the engineer must produce it on request. Most cylinder manufacturers — Heatrae Sadia (Megaflo), Telford, Joule, Range, Gledhill, Worcester Greenstore — require the installer's ticket number on the Benchmark logbook as a condition of the 25-year warranty. No ticket number on the logbook means no warranty, regardless of how good the install actually was.

Our G3 unvented cylinder engineer service for London dispatches a ticketed engineer to your door for new installs, conversions, services and emergency repairs. Every engineer on the team holds a current BPEC HWSS ticket and a Gas Safe registration so the boiler-and-cylinder pairing is signed off on the same visit.

The Three Mandatory Safety Devices

The whole engineering purpose of G3 is to prevent a stored-water explosion. Unlike a vented cylinder, an unvented vessel has no open atmospheric vent — pressure cannot relieve itself naturally. Three independent devices are therefore required, each one a backstop for the previous one's failure:

• Cylinder thermostat — The primary control. A non-self-resetting thermostat set typically at 60–65 degrees Celsius that cuts the heat source (boiler coil or immersion) when the stored water reaches setpoint. This is operational control, not a safety device, but it is the first line.
• Energy cut-out — A separate, non-self-resetting thermostat typically set at 80–85 degrees Celsius. If the cylinder thermostat fails on, the energy cut-out trips the heat source manually and requires a competent person to reset. This is safety device one.
• Temperature and pressure relief valve (T and P) — A combined relief valve set to open at 90 degrees Celsius or 7 bar, whichever the water reaches first. Mounted on the cylinder body, factory-fitted on every modern unvented unit. If both thermostats fail and the water boils, the T and P discharges into the tundish and out to safe termination. This is safety device two — the ultimate backstop.
• Expansion valve — A pressure-only relief valve set at around 6 bar, mounted on the cold inlet group. Protects against an over-pressurised cold supply or a failed expansion vessel. This is the working safety valve — it lifts in normal nuisance scenarios before the T and P ever needs to.

The expansion of heated water itself is absorbed by an expansion vessel — internal to the cylinder on a Megaflo Eco air-bubble design, or external on a Telford, Joule and most others. The vessel pre-charge is typically 3.0 to 3.5 bar against a known-zero water-side pressure, and a waterlogged or under-pressurised vessel is the single most common failure mode we attend on. We cover the symptoms and the fix in our expansion vessel replacement service.

Each of these devices must be WRAS-approved under the Water Supply (Water Fittings) Regulations 1999. The factory cold inlet group supplied with a Megaflo, Telford Tempest or Joule Cyclone already contains the expansion valve, check valve, pressure reducing valve and isolating valve in one assembly, pre-tested to WRAS standard.

Discharge Pipework — D1, D2, Tundish and Termination

If the T and P valve ever opens, the discharge is near-boiling water at mains pressure. Where that water ends up is the difference between an annoying maintenance call and a scalding injury or a flooded ceiling. Approved Document G3 prescribes a two-stage discharge route in some detail:

• D1 — The short pipe from the T and P valve outlet down to the tundish. Same internal diameter as the valve outlet. Material: copper to BS EN 1057 (the British and European standard for copper tube for water installations). Length typically 300 mm or less. Vertical, with no isolation valves anywhere on the run.
• Tundish — An open air-break visible to the occupant, fitted within 500 mm of the T and P valve. Allows visual diagnosis (water in the tundish means a relief device has lifted) and provides an air-gap to prevent any back-flow contamination of the stored water.
• D2 — The pipe from the tundish down to the safe visible termination. One pipe size larger than D1 to begin with, plus a further size up for every 14 metres of run or every more-than-9 elbows. Minimum gradient 1 in 200 to drain. Material: copper or solvent-weld plastic rated for the temperature.

The Approved Document G Table is the working reference. A typical 22 mm T and P valve on a domestic Megaflo gives a 22 mm D1 to the tundish, a 28 mm D2 leaving the tundish, stepping up to 35 mm if the run exceeds 14 metres or has more than nine bends. The termination must be visible, in a position where the occupant can see a discharge has occurred, and safe — typically a low-level external wall above an open gully. Where an external route is not practical (basement flat, internal core stairwell), the modern solution is a Hotun air-gap visible indicator that drains into the property's internal soil stack with a tundish-equivalent air break.

A discharge route hidden behind a kitchen unit or terminating into a sealed drain is one of the commonest non-compliances we find on a Building Control inspection. It is also straightforward to put right at install — designing the route is a five-minute job before the cylinder is sited, and a forty-minute job once it is in place.

Building Control Notification and the Compliance Certificate

Every new unvented cylinder install and every vented-to-unvented conversion is notifiable work under the Building Regulations. There are two routes:

• Full Building Control submission — Lodge a building notice or full plans with the local authority, pay the fee, arrange inspection. Slow, expensive, used only when the rest of the work on the property already involves Building Control (an extension, a loft conversion).
• Competent Persons Scheme self-certification — A G3-qualified engineer registered with an approved CPS notifies the work to the scheme, which in turn notifies Building Control on the engineer's behalf and issues a Building Regulations Compliance Certificate direct to the homeowner. The approved schemes for unvented hot water are WaterSafe (operated by the WRAS body) and BESCA.

The CPS route is the default for domestic work. The homeowner receives the certificate by post within 30 days, the install is logged on the national register, and the certificate becomes part of the property's permanent compliance record. Solicitors ask for it on every future sale of the property — the absence of a certificate is a routine pre-completion enquiry that delays exchange. Every ERL install includes notification and the certificate in the fitted price; we do not charge extra for the paperwork.

The Benchmark commissioning logbook is the parallel document. Benchmark is the Heating and Hotwater Industry Council's industry scheme for commissioning and servicing of central heating appliances and stored hot water. The engineer completes the install section at handover, the homeowner countersigns, and every subsequent annual service is entered in the rolling service log. The Benchmark book lives with the cylinder; cylinder manufacturers require it for warranty claims and a missing book is, in warranty terms, equivalent to no install having ever taken place.

Why the Annual Service Is Mandatory (Not Optional)

The annual G3 service is not a statutory legal requirement in the same sense as a gas-boiler safety check on rented property. It is, however, a hard condition of every major UK cylinder manufacturer's warranty. Heatrae Sadia (Megaflo), Telford, Joule, Range, Gledhill, Worcester Greenstore and Vaillant uniSTOR all require an annual service by a G3-qualified engineer, with the visit entered in the Benchmark service log. Miss a year and the 25-year cylinder warranty falls away. Miss two and most insurers will refuse a water-damage claim on the basis that the property was not maintained to the manufacturer's instructions.

The technical reason is that the three safety devices have failure modes that are not visible from outside. The T and P valve seat can corrode and weep, or scale and seize closed. The expansion vessel diaphragm can perish and waterlog. The energy cut-out can drift out of calibration. None of these failures produces a noisy symptom until something else has already gone wrong — at which point the failure has already happened. The annual service is the only mechanism that catches a degrading safety device before it matters.

A compliant annual service from ERL covers the six checks summarised in the FAQ schema at the foot of this page, takes around sixty minutes, and is £155 fixed across London. The visit ends with a Benchmark service entry, a written report of any consumables replaced, and a quoted price for any follow-on parts work. Our unvented cylinder service for London covers every brand on the UK market. If you also have a boiler feeding the cylinder, ask for a combined boiler-and-cylinder service visit — our boiler service and repair team work alongside the G3 engineers so both certificates are signed off on the same call.

The Five Most Common G3 Failures We See on Service

Across roughly 800 unvented cylinder services and repairs ERL attends each year in London, five failure modes dominate the first-attendance findings:

1. Waterlogged expansion vessel — Diaphragm has perished, the air side has filled with water, the vessel can no longer accommodate the thermal expansion of the heated water, the expansion valve nuisance-lifts and the tundish drips on every hot water cycle. By far the commonest fault on cylinders over five years old. Fix: replace the external expansion vessel (from £265 fitted) or, on a Megaflo Eco, re-pressurise the internal air-bubble per manufacturer procedure.
2. Scaled T and P valve seat — In London's hard water, the valve seat scales and the valve drips or, worse, seizes closed and cannot function in an emergency. A functional test on the annual service catches this. Fix: T and P valve replacement at £195 fitted.
3. Failed immersion heater element — Either the element has corroded through (loss of hot water on immersion-only operation) or the immersion thermostat has drifted (over- or under-heating). Fix: immersion heater replacement from £225 fitted including a new gasket and any required descale of the boss.
4. Drifted cylinder thermostat — The cylinder thermostat fails open or closed, the cylinder either never heats or boils up to the energy cut-out trip point. Fix: cylinder thermostat replacement from £155 fitted including recalibration.
5. Discharge route non-compliance — D2 undersized, hidden behind cabinetry, terminating into a closed drain, or missing the tundish entirely on a pre-1998 install. The most common failure on a Building Control retro-inspection. Fix: reroute the D2 to the nearest external wall or fit a Hotun air-gap — typical price £195 to £350 depending on route.

Items 1 and 2 alone account for around half of the call-outs. Both are entirely preventable with an annual service. The annual visit is not, in our experience, an upsell — it is the cheapest single thing a cylinder owner can do to protect a £600 to £1,200 piece of equipment with a 25-year design life.

FAQs

The FAQ schema at the foot of this page covers the six questions we field most often on the G3 line: what the G3 ticket is and why your engineer needs one, whether annual servicing is legally required, the difference between the D1 and D2 discharge pipes, Building Control notification and the Compliance Certificate, what an annual service actually includes, and whether a homeowner can install or replace an unvented cylinder themselves.

For a wider view of how the unvented cylinder sits alongside the rest of the hot water and heating system in a London property — boiler interface, central heating water treatment to BS 7593, smart-cylinder upgrades — our central heating service hub for London and our Mixergy smart cylinder install page are the next reads. If you are considering a brand-new install or replacement, the Megaflo installation and Telford Tempest installation pages set out the fitted prices in London by size.

Save the number now — 0207 046 1363 or WhatsApp 07456 975436. Whether you need a brand-new G3 install, a vented-to-unvented conversion, an overdue annual service or an emergency call-out on a tundish that will not stop dripping, a G3-ticketed engineer is on the road in Central London within two hours.

John Alexander N. — Director, Emergency Repairs London