G3 and Unvented Hot Water Cylinders Explained

An unvented hot water cylinder is a sealed pressurised vessel — it holds hot water at mains pressure rather than being open-vented to a loft tank. That gives great showers, but if its safety controls failed it could rupture violently. So Building Regulations Approved Document G3 layers up three independent protections, requires a discharge route via a tundish, and bans anyone who isn't a competent person from installing one.

G3 is the single most-tested area of UK domestic plumbing. Get it wrong and the consequences are serious. Get it right — qualified installer, layered controls, annual service — and it's an excellent, low-maintenance way to deliver hot water, especially for an electric boiler install where mains-pressure showers are part of the appeal.

What "unvented" actually means

A traditional vented cylinder is fed from a cold water storage tank in the loft and is open to the atmosphere via a vent pipe rising up over that tank. Pressure at the taps is limited by the height of the loft tank — typically about 0.1-0.3 bar. An unvented cylinder is fed directly from the cold mains, runs at mains pressure (around 1.5-3 bar in most homes), and is sealed. There is no open vent.

That sealed, pressurised design is the upside (great flow rates at every tap, no loft tank needed) and the downside (without safety controls, an overheating sealed vessel would generate steam at pressure — a serious hazard). G3 exists to manage that downside.

The G3 safety hierarchy

Approved Document G3 sets out a three-level hierarchy of protection. Each level is independent — a failure of one must not disable the next.

Expansion: vessel or bubble-top

When water heats from 10°C to 60°C it expands by roughly 1.7%. A 200 litre cylinder gains over 3 litres of volume. In a sealed system that has to go somewhere or pressure shoots up. G3 systems handle it with one of two methods:

• External expansion vessel — a steel pressure vessel with a rubber diaphragm and pre-charged air space. The standard solution on most installs.
• Internal air bubble (bubble-top) — a trapped air space at the top of the cylinder itself, recharged automatically or manually. Older Megaflo-style designs use this.

Either way, if expansion fails the safety chain catches it: pressure climbs, the expansion relief valve weeps water down the tundish, you see a visible discharge, you call us out. We cover the failure modes in detail in expansion vessels and system pressure.

The tundish and discharge pipe (D1/D2)

Every G3 cylinder has a discharge route from its safety valves to a safe place outside the building. The standard arrangement, set out in G3 paragraph 3.55 onwards, is:

• D1 — the short, fall-graded copper or stainless pipe from the T&PRV down to the tundish. Maximum 600 mm.
• Tundish — an air-break funnel, located visibly within 500 mm of the valve, providing a visible warning and a category-protected air gap.
• D2 — the longer discharge pipe from the tundish to the outside termination point. Must be one pipe size larger than D1 (or as per the G3 sizing tables for length and bends).
• Termination — to a safe visible location: low-level outside wall away from people, a hopper, or a discharge into a soil stack via an approved trap, never simply into a basin or directly under a path.

The competent person rule

You cannot lawfully fit an unvented cylinder in the UK without being a competent person. The qualifications are:

• BPEC Unvented Hot Water Storage Systems or City & Guilds 6131-01 — the standard G3 ticket.
• Registration on a competent person scheme — schemes operated by APHC, BESCA, CIPHE, NAPIT or others. The scheme handles Building Control notification and issues the compliance certificate.

Pipe Assassin holds current G3 certification, which is why we can carry out unvented installs and annual services across London, Hertfordshire and Essex. If you're planning a new cylinder or swapping out a tired one, see our electric boiler and cylinder installation and Hertfordshire water hygiene pages.

The annual service — what we actually do

1. 1

   Isolate, drain to the vessel, check the pre-charge

   Expansion vessel pre-charge naturally drops over time. We isolate the cylinder, drain just enough to depressurise, and reset the vessel to the cold mains pressure (usually 3.0-3.5 bar).

2. 2

   Operate the temperature and pressure relief valves

   Lift each safety valve manually to confirm it operates and reseats cleanly. Replace if it sticks or weeps after testing.

3. 3

   Clean the inlet strainer

   The combined inlet group has a debris strainer that scales up, especially in hard-water areas like much of Hertfordshire and Essex.

4. 4

   Check the tundish and discharge route

   Confirm the tundish is dry (no ongoing weep), visible and accessible. Walk the full D2 length to verify the outside discharge is unobstructed and still safely terminated.

5. 5

   Verify thermostats and temperature

   Measure stored temperature; confirm it's at 60-65°C. Check the high-limit reset is intact and not tripped.

6. 6

   Record and certify

   We complete the manufacturer's service log in the bench book on the cylinder side. This is what keeps your warranty and home insurance valid.

Where to go next

For the wider regulatory picture, read the UK water regulations pillar and Building Regs Part G overview. For the hot-water-temperature side, see safe hot water temperatures.

Frequently Asked Questions

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