Heated water expands — roughly 4% by volume between 10 °C and 90 °C — and in a closed, unvented system that expansion has nowhere to go. Left unabsorbed, it raises system pressure until something yields. Expansion control is therefore a designed safety function, not an accessory package bought at the last minute, and on unvented hot water it is a legal requirement in the UK under Building Regulations Approved Document G, Part G3.
**Expansion vessel.** A closed vessel with a built-in diaphragm, specified to BS EN 13831, that absorbs the water as it expands. The air-side pre-charge is what lets water enter without an immediate pressure rise; get it wrong and the vessel is effectively inert.
**Pressure reducing valve.** The inlet control device, typically factory-set around 3 bar, holding stable downstream pressure regardless of mains fluctuation. Balanced-spring, drop-tight designs stop the relief valves nuisance-weeping, and an integral strainer protects the seat.
**Expansion relief valve.** The device that relieves the small, everyday volumes of expanded water to drain before the safety valve acts; it often forms part of an expansion group, a product type covered by BS EN 1488.
**Combined temperature-and-pressure relief valve.** The final safety device, specified to BS EN 1490, covering valves from DN15 to DN40 in working pressure classes from 1 bar to 10 bar. It responds to excessive temperature through its sensing element, and to excessive pressure — the two failure modes that matter most.
In Australia and New Zealand the same duty is often served by an expansion control valve under AS/NZS 3500.4, set slightly below the main relief setting (commonly around 500 kPa) so it weeps and relieves the pressure rise first. The principle holds everywhere: the safety valve stays the last line of defence, not the everyday regulator.
BS EN 12897 covers indirectly heated unvented storage water heaters and is the framework in which components such as the T&P valve are specified. BS EN 1490 sets requirements and tests for combined temperature and pressure relief valves; BS EN 1489 covers pressure safety valves; BS EN 1488 covers expansion groups; and BS EN 13831 covers closed expansion vessels with a built-in diaphragm. BS 7074 provides application, selection and installation guidance for expansion vessels in sealed systems, and BS EN 806 with its companion guide BS 8558 covers the design and installation of water services inside buildings.
The calculation is short, and worth doing properly rather than picking a vessel by cylinder size:
**Vessel volume = (e × system volume) ÷ acceptance factor**
**Acceptance factor = 1 − (Pi + 1) ÷ (Pf + 1)**
Here *e* is the expansion coefficient of water at the maximum stored temperature — about 2.9% at 80 °C, closer to 3.5–4% at 90 °C. *Pi* is the vessel pre-charge in bar, which should match the setting of the pressure reducing valve. *Pf* is the maximum system pressure in bar, taken as the relief valve setting with a margin. The system volume must be the volume of water actually being heated, including any secondary or buffer volume, not just the nominal cylinder capacity.
Two consequences follow. A vessel sized on a low pre-charge becomes undersized on site once the pre-charge is raised to match a 3 bar reducing valve, because the acceptance factor changes and the required volume goes up. And verifying the pre-charge with an accurate gauge — vessel isolated, air side depressurised — is a commissioning step in its own right. A vessel shipped at 1.5 bar against a 3 bar inlet swallows much of its capacity before doing useful work.
The sequence in a correctly assembled unvented package is consistent: pressure reducing valve, check valve, expansion vessel connection, expansion relief valve, and finally the combined temperature-and-pressure relief valve mounted on the cylinder itself.
Two rules are non-negotiable in a G3 installation: nothing may be fitted between the T&P relief valve and the cylinder that could isolate it, and the valve's rating must match the cylinder's rated heat input. Typical UK packages run a 3 bar reducing valve, an expansion relief valve set around 6 bar and a T&P valve sensing at approximately 90 °C, but the figures are fixed by the cylinder manufacturer's schedule and the cylinder's pressure rating — confirm against the kit documentation rather than assuming a national default.
A relief valve is only as good as its discharge route. Approved Document G3 requires discharge to terminate safely and visibly, conventionally through a tundish. The D1 pipe between valve and tundish is restricted in length and diameter — a maximum of 9 m of 15 mm metal pipe is the usual benchmark, with longer runs needing a larger diameter — and D2 downstream of the tundish must be one size larger. Both should be metal. Wrongly sized, plastic or blocked discharge pipework is among the most frequently cited defects on unvented installations, and it is a defect of installation, not of product.
One maintenance point is often missed: a T&P relief valve that has discharged may not be reusable. Many designs are intended for replacement after operation, and a valve weeping continuously points to a control problem upstream — usually a failed pre-charge, a passing reducing valve or a missing check valve — rather than a faulty safety valve. Diagnose the cause, replace the valve, then re-verify the whole chain.
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**About the author — Zhejiang Xindong Sanitary Ware Co., Ltd.**
Zhejiang Xindong Sanitary Ware Co., Ltd. manufactures and exports brass valves, thermostatic mixers and sanitary fittings from Yuhuan, Zhejiang, where it has operated since 2006. Its range includes pressure reducing valves, relief valves and valve bodies for domestic hot water and building-services applications, supplied across the UK, Europe, North America and Japan. A number of products hold WRAS approval, and the company supports OEM and ODM programmes with technical documentation and third-party test reports.
Enquiries: [email protected] | +86-0576-87499008 | https://www.cn-xindong.com
*Engineering guidance only. Sizing, setting and discharge arrangements must be verified against the applicable building regulations, the relevant standards and the manufacturer's instructions.*
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The company, founded in 1999 and located in Yuhuan,specializes in taps, thermostatic shower valves and shower kits. With over 30,000 sqm of space and 380+ employees, we hold numerous certifications and 65+ patents.
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