The Zilmet Inox-Pro Mini Shock Arrestor (0.16 litre, ½" BSP) is a compact stainless steel potable expansion vessel serving two related functions: eliminating water hammer caused by the rapid shut-off of quarter-turn taps, solenoid valves, and washing machine inlets; and absorbing the thermal expansion of cold water in combi boiler installations where the incoming mains is closed by a water meter check valve or pressure-reducing valve. Constructed entirely from AISI 304 stainless steel with a food-safe butyl membrane, it is WRAS-compliant for connection to the drinking water supply. Factory pre-charged to 3.5 bar, it installs with a standard ½" BSP male fitting at only 72mm tall × 82mm diameter — small enough to fit behind a basin, inside a sink cupboard, or in-line on a 15mm or 22mm supply.
15 bar
Max working pressure
3.5 bar
Factory pre-charge
WRAS
Potable water approved
ℹ Two distinct uses for this vessel
(1) Water hammer arrestor: Fits as close as possible to the noisy outlet — a washing machine, dishwasher, solenoid valve, or quarter-turn basin tap — and absorbs the pressure shock when flow stops suddenly. Eliminates the bang.
(2) Combi boiler cold-feed expansion: Mains water expands when heated; if the incoming supply is isolated by a water meter or PRV check valve, this pressure has nowhere to go and can trip the boiler PRV or damage fittings. Installing this vessel on the cold mains before the boiler absorbs that expansion. Frequently required on combi boiler installations after Water Regulations changes.
Features & Benefits
✔ Full Stainless Steel Construction
Both the shell and flange are AISI 304 stainless steel — not galvanised steel or carbon steel with a stainless shroud. This gives the vessel an essentially unlimited service life in potable water systems and makes it suitable for coastal, hard water, and brackish water environments where corrosion of conventional steel vessels is a real-world problem.
✔ Potable Water — WRAS Compliant
The butyl membrane is food-grade and non-toxic, approved for contact with drinking water. The vessel complies with the Water Supply (Water Fittings) Regulations 1999 for installation on the cold water drinking supply — WRAS approval is a legal requirement for any fitting connected to the incoming mains. This is a genuine Zilmet product, not a cheaper compatible alternative.
✔ Ultra-Compact — Fits Almost Anywhere
At only 72mm tall and 82mm in diameter, this is one of the smallest expansion vessels in the Zilmet range. It fits behind a basin pedestal, inside a kitchen sink cupboard alongside the stop tap, in a combi boiler cupboard, or in-line on a 15mm or 22mm supply without requiring a dedicated space. The ½" BSP male connection attaches directly to a standard tee or manifold fitting.
✔ 15 Bar Working Pressure
Rated to 15 bar maximum working pressure — the highest rating in the Zilmet Inox-Pro range (larger vessels are rated to 10 bar). This makes the 0.16L vessel suitable for high-pressure mains supplies where the incoming pressure exceeds the 10-bar limit of larger expansion vessels, and provides a significant safety margin on typical UK mains pressures of 2–5 bar.
✔ Factory Pre-Charged to 3.5 Bar
The vessel is supplied pre-charged to 3.5 bar from the factory — matched to typical UK mains supply pressures for shock arrestor use. No pre-installation charging or pressure gauge is required in most standard installations. For expansion vessel use on a combi boiler cold feed, the installer should confirm the system's static mains pressure and adjust the pre-charge if needed.
✔ Eliminates Water Hammer Banging
When a solenoid valve, quarter-turn tap, or washing machine inlet closes instantly, the kinetic energy of the moving water column has nowhere to go — it impacts the closed fitting with a sharp pressure wave, causing the characteristic bang or hammer noise. Installed as close as possible to the offending outlet, this vessel's diaphragm flexes to absorb that pressure spike before it can propagate through the pipework. One unit typically resolves a single noise source; multiple outlets may require multiple arrestors.
Technical Specifications
| Manufacturer |
Zilmet (Italy) — supplied via Trade Tec |
| Series / Part No. |
Inox-Pro Mini / Z1-300016SSD (11B000AA02) |
| Vessel Capacity |
0.16 litres |
| Connection |
½" BSP male |
| Max Working Pressure |
15 bar |
| Factory Pre-Charge Pressure |
3.5 bar |
| Max Operating Temperature |
70°C |
| Shell Material |
AISI 304 stainless steel |
| Membrane Material |
Butyl (food-grade, non-toxic, potable water approved) |
| Flange Material |
Stainless steel |
| Height |
72 mm |
| Diameter |
82 mm |
| Applications |
Water hammer arrestor; combi boiler cold-feed expansion; coastal/brackish water installations |
| Compliance |
WRAS — Water Supply (Water Fittings) Regulations 1999 |
Frequently Asked Questions
Why is a shock arrestor needed when I fit a combi boiler?
Modern water meters and pressure-reducing valves include non-return (check) valves that prevent any backflow to the mains. This creates a closed system on the cold feed to the boiler. When the boiler heats water, the expansion of that water in the cold supply pipe has nowhere to go — pressure rises and can repeatedly trip the boiler's pressure relief valve or stress fittings and valves. A small expansion vessel fitted to the cold mains before the boiler absorbs this expansion. WRAS-approved Water Fittings Regulations guidance and many boiler manufacturers' installation requirements specify this vessel as a mandatory fitting on closed-system mains connections.
Where exactly should I install it to stop water hammer?
For water hammer, install the vessel as close as physically possible to the outlet causing the noise — typically within 300mm of the noisy tap, solenoid valve, or appliance inlet. Use a tee fitting on the 15mm supply pipe feeding that outlet and connect the vessel to the tee branch. The closer the vessel is to the noise source, the more effective it will be. A vessel fitted remotely on a long run of pipework provides much less benefit than one sited at the tap itself. If multiple outlets are causing noise, one vessel per outlet location is needed.
Can I use this on a hot water supply?
The Zilmet Inox-Pro Mini is rated to a maximum of 70°C, making it suitable for connection to warm or moderately hot supplies — such as the cold feed to a combi boiler where the water may be briefly warm. However, for installation directly on a hot water outlet or as a domestic hot water expansion vessel, a purpose-designed hot-side expansion vessel (rated for the required temperature and system pressure) should be used instead. This vessel is principally designed for cold water mains and cold-to-warm supply applications.
