Gas

Unvented Cylinder (G3) Requirements

An unvented cylinder stores hot water under mains pressure, so Building Regulations Part G3 requires a layered set of safety controls and a properly sized discharge route to deal safely with over-temperature and over-pressure.

Updated 20 Aug 20268 min read

An unvented hot water storage system holds hot water at mains pressure with no open vent, which brings excellent flow but means an uncontrolled failure could release scalding water or steam. Building Regulations Part G3 (schedule 1, England and Wales) sets out the controls and the safe discharge arrangements that prevent that, and requires the work to be carried out and commissioned competently.

This guide explains the layered safety controls, the tundish and the D1/D2 discharge pipework rules, who is allowed to install an unvented cylinder, and what to check and record at commissioning. It describes the requirements and the governing documents rather than quoting every dimension — always work to the cylinder manufacturer's instructions and Part G3.

Key takeaways

Unvented hot water systems are covered by Building Regulations Part G3, which requires layered safety controls and a safe discharge route.
Controls are layered: the operating thermostat, then a non-self-resetting high-limit (energy cut-out) thermostat, then a temperature and pressure relief valve as the ultimate safeguard.
There is also an expansion valve (pressure relief) and expansion accommodation — a potable-rated expansion vessel or an internal air gap/bubble.
Relief valve discharge passes through a tundish, then via the D2 discharge pipe to a safe, visible termination that cannot scald anyone.
Installing and commissioning an unvented system requires the relevant competence (the G3 qualification); it is notifiable Building work.

Why unvented systems are regulated

Because the water is stored under mains pressure with no open vent, an unvented cylinder can store energy that, if temperature control failed, could flash to steam and burst the vessel. Part G3 of the Building Regulations exists to prevent that by requiring independent layers of protection and a discharge route that safely gets rid of any relieved water.

The requirements apply to any unvented hot water storage vessel over the small threshold set in the regulations (very small storage volumes are exempt). For everything above it, the full G3 control package and discharge arrangement are mandatory.

The layered safety controls

G3 requires temperature control to be provided in independent layers so that no single failure can lead to a dangerous over-temperature. The first layer is the operating (control) thermostat, which switches the heat source at the normal storage temperature. The second is a non-self-resetting energy cut-out (high-limit thermostat) that isolates the heat source if the control thermostat fails and does not reset itself.

The final, ultimate safeguard against over-temperature is a temperature and pressure relief valve, typically set to relieve at around 90–95°C / a defined pressure, which dumps hot water if both thermostats fail. Independently of temperature, an expansion (pressure) relief valve protects against over-pressure, and expansion of the heated water is accommodated either by a potable-rated expansion vessel or by an internal air gap/bubble, depending on the cylinder design.

Three lines of defence

Control thermostat, then non-self-resetting high-limit cut-out, then the temperature and pressure relief valve. Each is independent so that one failing does not defeat the next — this layering is the heart of G3.

The tundish and D1/D2 discharge pipework

The relief valves discharge into a tundish — an open air break, fitted close to the valves and visible, that shows when a valve is passing and prevents any backflow. The short section from the safety valve to the tundish is the D1 pipe; from the tundish onward is the D2 pipe.

The D2 discharge pipe must be at least one size larger than the D1 nominal outlet, be laid to a continuous fall, be kept as short and straight as practicable (with allowance made for each bend and for length, resizing up where these are exceeded), and be in a material that can take boiling water. It must terminate somewhere safe and visible where the discharge cannot scald anyone or cause damage — low level near a gully, or otherwise to a suitable point — so that a relieving valve is both harmless and noticeable.

Size and route the D2 to the standard

The exact minimum bore, maximum length and the deduction per bend are given in Part G3 and the cylinder manufacturer's instructions. Size the D2 to those figures — an undersized or overlong discharge pipe defeats the whole safety arrangement.

Who can install one, and notification

Installing and commissioning an unvented hot water system is competent-person work: the operative must hold the relevant unvented hot water qualification (the G3 ticket). It is notifiable Building work, so the installation must be either notified to Building Control or self-certified by an installer registered on a competent-person scheme.

If gas is the heat source, the gas side of the work must additionally be carried out by a Gas Safe registered engineer — the G3 qualification covers the unvented water system, not the gas appliance.

Commissioning and what to record

At commissioning you set and confirm the storage temperature, check the cold water inlet arrangement (pressure-reducing valve where fitted, expansion valve, check valve and the expansion accommodation), verify the temperature and pressure relief valve and expansion valve operate and discharge correctly through the tundish, and confirm the D2 pipe is correctly sized, routed and terminated. You then demonstrate the system and controls to the user.

Record all of it on an unvented commissioning certificate (the G3/Benchmark cylinder record), with the settings, the safety-device checks and the discharge arrangement noted. This is the evidence the system met G3 and supports the cylinder warranty — and, as with a boiler, it should be handed to the customer and kept against the job.

Frequently asked questions

Do I need a G3 qualification to install an unvented cylinder?

Yes. Installing and commissioning an unvented hot water storage system is competent-person work requiring the relevant unvented hot water (G3) qualification. If the heat source is gas, that part of the work also needs a Gas Safe registered engineer.

What is the tundish for?

The tundish is a visible air break between the safety valve outlet (D1) and the discharge pipe (D2). It shows when a relief valve is passing water and prevents backflow into the system. It must be fitted close to the valves and remain visible.

How is the discharge pipe sized?

The D2 discharge pipe is at least one size larger than the D1 valve outlet, run to a continuous fall, kept short and straight, and enlarged where length or bends exceed the limits set in Part G3 and the manufacturer's instructions. It must terminate safely and visibly where the discharge cannot scald anyone.

Is an unvented cylinder installation notifiable?

Yes. It is notifiable Building work under Part G3, so it must be notified to Building Control or self-certified by an installer on a competent-person scheme, and commissioned and recorded properly.

From guidance to action

Unvented hot water commissioning (G3)Record the safety-device checks and discharge arrangement on a signed G3 certificate.Cylinder reheat calculatorEstimate reheat time and heat input for the stored volume.