The Zilmet Hydro-Pro 2 is a 2-litre potable expansion vessel designed for domestic hot water systems, unvented cylinders, water heaters, pump sets, and anti-water-hammer applications.

The vessel uses a fixed Zilan DW diaphragm membrane — not a bladder — which isolates system water from the pre-charged air chamber. The membrane's seamless shape has no corners or creases where stagnant water or sediment can collect, actively inhibiting bacteriological growth and keeping the system hygienic. The carbon steel shell is finished internally with an exclusive epoxy powder coating and externally with an epoxy-polyester paint — both providing long-term corrosion resistance.

MPN: 11A0000200 WRAS Approved NSF / IAPMO / ACS 10 Bar Max

Technical Specifications

Parameter	Value
MPN / Product code	11A0000200
Vessel volume	2 litres
Connection	½" BSP male (G½")
Max operating pressure	10 Bar
Operating temperature range	-10°C to +99°C
Factory pre-charge pressure	3.5 Bar
Shell material	Carbon steel — deep-drawn, MIG welded
Internal coating	Exclusive epoxy powder coating — corrosion resistant
External finish	Epoxy-polyester paint — rust resistant — blue/grey
Membrane type	Fixed Zilan DW diaphragm — food-grade, potable water approved
Connection fitting material	Carbon steel
Diameter (ØD)	142mm
Height (H)	196mm
Drawdown @ Pmax 10 Bar	1.2 litres (heating) / 1.1 litres @ 0.8 L/min (pump duty)
Certifications	PED 2014/68/EU · WRAS · ACS · IAPMO · NSF
Manufacturer	Zilmet SpA, Limena, Padova, Italy

Key Features

WRAS-approved potable water use

Holds international approvals for use with drinking water including WRAS (UK Water Regulations Advisory Scheme), ACS (France), IAPMO, and NSF. The Zilan DW fixed diaphragm membrane and internal epoxy coating are food-grade rated — suitable for unvented hot water cylinder installations and mains pressure systems.

Fixed Zilan DW diaphragm — no stagnation

The fixed diaphragm design has no creases, corners, or dead zones where water can stagnate. This seamless geometry actively inhibits bacteriological growth — important in any potable water application. Water and air are completely separated at all times; no waterlogging or pressure loss through air absorption.

Exclusive MIG welded construction

Zilmet's exclusive deep-drawn MIG welding process eliminates all interior sharp edges, rough spots, and weld profiles. This protects the diaphragm from damage over the life of the vessel and ensures the internal epoxy coating bonds uniformly across the entire shell — preventing localised corrosion.

Factory pre-charged and tested

Every Hydro-Pro 2 leaves the Zilmet factory pre-charged to 3.5 Bar and individually pressure-tested. The pre-charge valve is fitted with a protective cap. Pre-charge can be adjusted on site with a standard Schrader valve pump if the system static pressure requires a different setting.

Compact — 142mm diameter × 196mm height

At 142mm diameter and 196mm tall, the Hydro-Pro 2 fits easily in the airing cupboard next to an unvented cylinder or in a tight boiler cupboard. The ½" BSP male connection is standard across virtually all unvented cylinder expansion vessel connections and cold water expansion fittings.

Anti-water-hammer protection

In addition to thermal expansion duty, the Hydro-Pro 2 absorbs the pressure spikes caused by rapid valve closure and pump stop/start cycles — protecting downstream components, fittings, and appliances from shock damage. Suitable for installation on pump sets, boosters, and solenoid-valve-controlled circuits.

Applications & Sizing Guidance

The Hydro-Pro 2 is sized for small domestic applications where a compact vessel is required. Typical uses include thermal expansion relief on unvented hot water cylinders up to approximately 150 litres (at typical UK mains pressures), cold water expansion on combination boiler systems, and anti-water-hammer damping on pump sets and motorised valve circuits.

The correct vessel size for a given installation depends on the system volume, maximum working pressure, minimum static pressure, and temperature range. The pre-charge pressure should be set equal to the cold static fill pressure of the system. Zilmet publish a full drawdown and sizing calculator at their website. Vessel sizing must be verified by a competent installer or engineer.

Pre-charge and drawdown data for Hydro-Pro 2: Factory pre-charge 3.5 Bar. Drawdown at Pmax 4–5 Bar: 0.4 litres (pump) / 0.5 litres (heating). Drawdown at Pmax 6–7 Bar: 0.8 litres (pump) / 0.9 litres (heating). Drawdown at Pmax 8–9 Bar: 1.0 litres (pump) / 1.1 litres (heating). Drawdown at Pmax 10 Bar: 1.1 litres (pump) / 1.2 litres (heating).

Installation Notes

The Hydro-Pro 2 can be installed in any orientation — vertically (connection up or down) or horizontally — although vertical connection-up is preferred as it keeps the diaphragm correctly positioned over time. Install as close as possible to the appliance it is serving, with a suitable isolation valve and check valve upstream to prevent system pressure from affecting the pre-charge. For unvented cylinder duty, the vessel must comply with the G3 Building Regulation requirements for the installation; the vessel pressure rating must meet or exceed the maximum system pressure.

Important: Always check and if necessary adjust the pre-charge pressure before filling the system. The pre-charge must equal the cold static fill pressure of the system — typically 1.0–3.5 Bar for domestic potable water. Do not pressurise beyond the vessel's 10 Bar rating. Expansion vessel sizing and installation on unvented systems must be carried out by a competent person under Part G of the Building Regulations.

Frequently Asked Questions

Is the Hydro-Pro 2 suitable for an unvented hot water cylinder?

Yes — the Hydro-Pro 2 is WRAS approved for potable water use and rated to 10 Bar and 99°C, covering the operating conditions of domestic unvented cylinders. Confirm that the vessel capacity is correctly sized for the cylinder volume and system pressure range; for cylinders over approximately 150 litres a larger vessel will normally be required.

What is the factory pre-charge and can I adjust it?

The vessel leaves the factory pre-charged to 3.5 Bar. This is suitable for systems with a cold fill pressure up to 3.5 Bar. If your system cold fill pressure differs, adjust the pre-charge using a standard Schrader valve pump with the vessel isolated from the system and fully depressurised on the water side. Always set pre-charge equal to the cold static fill pressure before reconnecting.

What is the difference between a diaphragm vessel and a bladder vessel?

In a diaphragm vessel (like the Hydro-Pro), the membrane is fixed to the inside of the shell and divides the vessel into two chambers — water one side, air the other. In a bladder vessel the membrane is a free-floating bag. The Hydro-Pro's fixed diaphragm design means the membrane never creases or bunches, giving a longer service life and the seamless anti-stagnation geometry that makes it suitable for potable water duty.

How do I know when the expansion vessel needs replacing?

Common signs of a failed expansion vessel include the pressure relief valve on the cylinder discharging regularly, rapid pressure fluctuations when hot water is drawn, or the vessel feeling completely hard with no give when tapped. Check the pre-charge pressure annually — if the vessel holds no pre-charge even after re-pressurising, the diaphragm has failed and the vessel should be replaced.

Can the Hydro-Pro 2 also be used for anti-water-hammer duty on a pump set?

Yes — the Hydro-Pro range is explicitly rated and sized for anti-water-hammer applications on electric pump sets. The pre-charged air chamber absorbs the pressure spike created by rapid valve closure or pump stop. At Pmax 10 Bar, the Hydro-Pro 2 provides a drawdown of 1.1 litres at up to 0.8 L/min pump flow rate — sufficient for small domestic booster and pump applications.

Potable Expansion Vessels