Expansion Vessels Explained: Heating System and Unvented Cylinder Vessels for UK Plumbers

Expansion Vessels Explained: Heating System and Unvented Cylinder Vessels for UK Plumbers

An expansion vessel is a sealed pressure vessel containing a rubber diaphragm (or bladder) with nitrogen gas pre-charge on one side and system water on the other. As water temperature rises and the water expands, it compresses into the vessel against the gas charge — absorbing the pressure increase that would otherwise build up in the system. Without a functioning expansion vessel, a sealed central heating system or unvented hot water cylinder will repeatedly lift the pressure relief valve, causing water loss and eventually system failure. This guide covers the two main types, how to size them, and how to diagnose a failed vessel.

Two Types: Heating System vs Potable Water

There are two entirely different expansion vessel categories, and they are not interchangeable:

Heating System Expansion Vessels (Red)

Used in sealed (pressurised) central heating systems — where there is no open-vented feed and expansion cistern in the loft. The vessel absorbs the expansion of heating circuit water as it heats from cold (~10°C) to operating temperature (typically 70–80°C for a radiator system, lower for UFH).

Heating vessels are typically red and use a diaphragm or bladder suitable for inhibitor-treated heating water. They are not suitable for potable (drinking) water — the materials are not WRAS-approved for contact with drinking water.

Heating expansion vessels are mounted on the return side of the circuit (coolest point) to reduce temperature stress on the diaphragm, and connected via a short length of pipe with an isolation valve and flexible hose connector (to prevent vibration transmission from the pump). The Schrader valve on the gas side allows pre-charge pressure to be checked and adjusted with a tyre pressure gauge.

Potable Water Expansion Vessels (Blue or White)

Used in unvented (pressurised) hot water cylinder systems — Megaflow, Vaillant, Gledhill, and equivalents. As cold mains water in the cylinder heats, it expands — since the system is sealed (no open vent to cistern), this expansion must be accommodated. The potable expansion vessel on the cold inlet side absorbs this expansion.

Potable water vessels are blue or white, and constructed with materials that are WRAS-approved for contact with drinking water. They are not interchangeable with heating vessels. Connecting a red heating vessel to a potable water system (or vice versa) is a Water Regulations contravention and a potential health risk.

Sizing an Expansion Vessel

Expansion vessels must be correctly sized for the system volume and temperature range. An undersized vessel runs out of absorption capacity — the system pressure rises too high and the PRV lifts repeatedly. An oversized vessel is wasted cost but causes no harm.

Quick sizing guide for heating systems (sealed, radiator-based):

• Calculate the total system water volume (heating circuit, cylinder coil, pipework) — typically 40–60 litres for a 3-bed house, 70–100 litres for larger properties
• Expansion vessel size = approximately 10% of total system volume for a standard 80°C max temperature system with a 1.0 bar static charge and a 3 bar maximum working pressure
• A 12-litre vessel covers a system volume up to approximately 80–100 litres at standard conditions; an 18-litre vessel covers up to 120–150 litres
• Use a manufacturer sizing calculator (Flamco, Reflex, or Zilmet all provide free online tools) for accurate sizing — especially for UFH systems at lower temperatures or systems with large water volumes

For unvented cylinder potable expansion vessels: The cylinder manufacturer specifies the vessel size in the installation manual. Do not substitute a different size without recalculating — an undersized vessel causes repeated tundish discharge and eventually cylinder PRV failure.

Pre-Charge Pressure

The nitrogen pre-charge pressure in the gas side of the vessel must match the static head pressure of the system at cold fill. If the pre-charge is too low, the vessel diaphragm is pushed back against the gas side before the system reaches operating temperature, reducing capacity. If the pre-charge is too high, the vessel contributes little to pressure control.

Standard settings:

• Heating system: Pre-charge pressure = cold fill pressure of the system. For most domestic systems filled to 1.0 bar, the vessel pre-charge should be 1.0 bar. Vessels are typically supplied pre-charged to 1.5 bar and need to be reduced with a tyre pressure gauge at the Schrader valve before installation.
• Unvented cylinder: Pre-charge pressure as specified by the cylinder manufacturer — typically 3.5 bar for a system supplied at 3.5 bar mains pressure, but always check the installation manual.

Always check and set pre-charge with the vessel disconnected from the system (water side depressurised). Checking pre-charge with water pressure on the diaphragm gives a false reading.

Diagnosing a Waterlogged (Failed) Expansion Vessel

When an expansion vessel fails — the diaphragm splits or the gas charge leaks out — the vessel becomes waterlogged (filled with water on both sides). Signs of a waterlogged vessel:

• System pressure rises rapidly when the heating fires and drops again when it cools (the vessel can no longer absorb expansion)
• PRV (pressure relief valve) discharging repeatedly — water dripping from the tundish discharge pipe
• Boiler lockout on high pressure
• When the Schrader valve on the vessel is pressed with the system depressurised, water (not air) comes out

A waterlogged expansion vessel must be replaced — it cannot be repaired. Before replacement, also check that the PRV itself has not been damaged by repeated opening (PRVs that have lifted repeatedly sometimes fail to re-seat fully and continue to weep).

Expansion Vessels at APM Plumbing & Electrical

APM stocks red heating system expansion vessels (8L, 12L, 18L, 24L, 35L) and blue potable water expansion vessels (2L, 5L, 8L, 12L, 18L) from leading brands including Flamco, Reflex, and Zilmet. All vessels are supplied pre-charged and include a Schrader valve for pressure checking and adjustment. We also carry expansion vessel brackets, flexible vessel connectors, and replacement vessel isolation valves.

Browse our expansion vessels or visit the Acton trade counter. Trade orders placed before 2pm ship same day across London and the South East.

Products Available from APM Plumbing & Electrical

Key expansion vessels and filling components available for trade collection or delivery:

Get It from APM Electricals

APM Electricals, 24 Western Avenue, Acton, London W3 7TZ. Call 020 8702 8080 or visit www.apmi.uk for same-day trade counter collection and next-day delivery across London and the UK.