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Boss Expansion Vessels

BOSS Expansion Vessels for Heating & Chilled Water Systems

BOSS expansion vessels are high-performance, pre-charged pressure vessels designed for use in sealed heating (HTG) and chilled water (CHW) systems.

Manufactured by Flexcon, these vessels are engineered to safely absorb thermal expansion in closed-loop water systems, helping maintain stable system pressure and protecting boilers, pumps, valves, and pipework from damage.

Available in a wide range of sizes and pressure ratings, BOSS expansion vessels are suitable for both domestic heating systems and large commercial HVAC installations.

What Does an Expansion Vessel Do?

An expansion vessel is a critical safety and performance component in any sealed water system.

When water is heated, it expands. Without a vessel to absorb this extra volume, system pressure would rise to unsafe levels and could trigger boiler shutdowns or cause component failure.

The expansion vessel contains a flexible diaphragm that separates water from a pre-charged air cushion, allowing it to safely absorb pressure fluctuations.

Showing all 14 results

  • Sale! BOSS 8L Expansion Vessel 3 Bar for Sealed Heating & Chilled Water Systems

    BOSS 8L Expansion Vessel 3 Bar for Sealed Heating & Chilled Water Systems

    Original price was: £98.16£81.80.Current price is: £45.29£37.74. £45.29£37.74 inc VAT Incl. VATExcl. VAT

    BOSS Expansion Vessel 8L (3 Bar) – Sealed Heating & Chilled Water Systems The BOSS Expansion Vessel 8L (3 Bar) is designed for sealed heating systems and chilled water applications, providing reliable accommodation of thermal expansion within closed-loop systems. By maintaining stable system pressure, it helps protect boilers, pumps, valves, and other key components while…

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  • Sale! BOSS 12L Expansion Vessel 3 Bar for Sealed Heating & Chilled Water Systems

    BOSS 12L Expansion Vessel 3 Bar for Sealed Heating & Chilled Water Systems

    Original price was: £113.34£94.45.Current price is: £52.30£43.58. £52.30£43.58 inc VAT Incl. VATExcl. VAT

    BOSS Expansion Vessel 12L (3 Bar) – Sealed Heating & Chilled Water Systems The BOSS Expansion Vessel 12L (3 Bar) is designed for sealed heating systems and chilled water applications, providing reliable accommodation of thermal expansion and contraction within closed-loop systems. By maintaining consistent system pressure, it helps protect boilers, pumps, valves, and other essential…

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  • Sale! BOSS 18L Expansion Vessel 3 Bar for Sealed Heating & Chilled Water Systems

    BOSS 18L Expansion Vessel 3 Bar for Sealed Heating & Chilled Water Systems

    Original price was: £114.73£95.61.Current price is: £52.93£44.11. £52.93£44.11 inc VAT Incl. VATExcl. VAT

    BOSS Expansion Vessel 18L (3 Bar) – Sealed Heating & Chilled Water Systems The BOSS Expansion Vessel 18L (3 Bar) is designed for sealed heating systems and chilled water applications, providing reliable accommodation of thermal expansion and contraction caused by temperature changes. By maintaining stable system pressure, it helps protect boilers, pumps, valves, and other…

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  • Sale! BOSS 25L Expansion Vessel 3 Bar for Sealed Heating & Chilled Water Systems

    BOSS 25L Expansion Vessel 3 Bar for Sealed Heating & Chilled Water Systems

    Original price was: £125.98£104.98.Current price is: £58.13£48.44. £58.13£48.44 inc VAT Incl. VATExcl. VAT

    BOSS Expansion Vessel 25L (3 Bar) – Sealed Heating & Chilled Water Systems The BOSS Expansion Vessel 25L (3 Bar) is a high-performance sealed expansion vessel designed for closed-loop heating systems and chilled water applications. Engineered to accommodate thermal expansion and contraction caused by temperature fluctuations, it helps maintain stable system pressure, protect heating and…

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  • Sale! BOSS 35L Expansion Vessel 3 Bar for Sealed Heating & Chilled Water Systems

    BOSS 35L Expansion Vessel 3 Bar for Sealed Heating & Chilled Water Systems

    Original price was: £255.17£212.64.Current price is: £117.73£98.11. £117.73£98.11 inc VAT Incl. VATExcl. VAT

    BOSS Expansion Vessel 35L (3 Bar) – Sealed Heating & Chilled Water Systems The BOSS Expansion Vessel 35L (3 Bar) is a high-performance sealed expansion vessel designed for closed-loop heating systems and chilled water applications. Built to accommodate thermal expansion and contraction caused by changing water temperatures, it helps maintain stable system pressure, protect key…

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  • Sale! BOSS 50L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    BOSS 50L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    Original price was: £306.70£255.58.Current price is: £141.50£117.92. £141.50£117.92 inc VAT Incl. VATExcl. VAT

    BOSS Expansion Vessel 50L (6 Bar) – Sealed Heating & Chilled Water Systems The BOSS Expansion Vessel 50L (6 Bar) is a high-performance sealed expansion vessel designed for closed-loop heating systems and chilled water applications. Engineered to accommodate thermal expansion and contraction caused by temperature changes, it helps maintain consistent system pressure, protects equipment from…

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  • Sale! BOSS 80L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    BOSS 80L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    Original price was: £443.99£369.99.Current price is: £204.85£170.71. £204.85£170.71 inc VAT Incl. VATExcl. VAT

    BOSS Expansion Vessel 80L (6 Bar) – Sealed Heating & Chilled Water Systems The BOSS Expansion Vessel 80L (6 Bar) is a high-performance sealed expansion vessel designed for closed-loop heating systems and chilled water applications. Engineered to accommodate thermal expansion and contraction caused by temperature fluctuations, it helps maintain stable system pressure, protect system components,…

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  • Sale! BOSS 100L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    BOSS 100L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    Original price was: £599.17£499.31.Current price is: £276.46£230.38. £276.46£230.38 inc VAT Incl. VATExcl. VAT

    BOSS Expansion Vessel 100L (6 Bar) – Sealed Heating & Chilled Water Systems The BOSS Expansion Vessel 100L (6 Bar) is a high-performance sealed expansion vessel designed for use in closed-loop heating systems and chilled water applications. Engineered to accommodate thermal expansion and contraction caused by temperature fluctuations, it helps maintain stable system pressure, protects…

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  • Sale! BOSS 150L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    BOSS 150L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    Original price was: £674.90£562.42.Current price is: £311.40£259.50. £311.40£259.50 inc VAT Incl. VATExcl. VAT

    BOSS Expansion Vessel 150L (6 Bar) – Sealed Heating & Chilled Water Systems The BOSS Expansion Vessel 150L (6 Bar) is a high-performance sealed expansion vessel designed for closed-loop heating systems and chilled water applications. Built to accommodate thermal expansion and contraction caused by temperature fluctuations, it helps maintain stable system pressure, protect system components,…

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  • Sale! BOSS 200L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    BOSS 200L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    Original price was: £886.73£738.94.Current price is: £409.14£340.95. £409.14£340.95 inc VAT Incl. VATExcl. VAT

    BOSS Expansion Vessel 200L (6 Bar) – Sealed Heating & Chilled Water Systems The BOSS Expansion Vessel 200L (6 Bar) is a high-performance sealed expansion vessel designed for closed-loop heating systems and chilled water applications. Engineered to accommodate thermal expansion and contraction caused by temperature fluctuations, it helps maintain stable system pressure, protect critical components,…

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  • Sale! BOSS 300L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    BOSS 300L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    Original price was: £1,273.20£1,061.00.Current price is: £587.46£489.55. £587.46£489.55 inc VAT Incl. VATExcl. VAT

    BOSS Expansion Vessel 300L (6 Bar) – Sealed Heating & Chilled Water Systems The BOSS Expansion Vessel 300L (6 Bar) is a high-performance sealed expansion vessel designed for closed-loop heating systems and chilled water applications. Engineered to accommodate thermal expansion and contraction caused by temperature fluctuations, it helps maintain stable system pressure, protect critical components,…

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  • Sale! BOSS 400L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    BOSS 400L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    Original price was: £1,680.05£1,400.04.Current price is: £775.18£645.98. £775.18£645.98 inc VAT Incl. VATExcl. VAT

    BOSS Expansion Vessel 400L (6 Bar) – Sealed Heating & Chilled Water Systems The BOSS Expansion Vessel 400L (6 Bar) is a high-performance sealed expansion vessel designed for closed-loop heating systems and chilled water applications. Engineered to accommodate thermal expansion and contraction caused by temperature fluctuations, it helps maintain stable system pressure, protect critical system…

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  • Sale! BOSS 500L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    BOSS 500L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    Original price was: £1,621.93£1,351.61.Current price is: £748.36£623.63. £748.36£623.63 inc VAT Incl. VATExcl. VAT

    BOSS Expansion Vessel 500L (6 Bar) – Sealed Heating & Chilled Water Systems The BOSS Expansion Vessel 500L (6 Bar) is a high-performance sealed expansion vessel designed for closed-loop heating systems and chilled water applications. Engineered to accommodate thermal expansion and contraction caused by temperature fluctuations, it helps maintain stable system pressure, protect critical system…

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  • Sale! BOSS 600L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    BOSS 600L Expansion Vessel 6 Bar for Sealed Heating & Chilled Water Systems

    Original price was: £1,844.81£1,537.34.Current price is: £851.20£709.33. £851.20£709.33 inc VAT Incl. VATExcl. VAT

    BOSS Expansion Vessel 600L (6 Bar) – Sealed Heating & Chilled Water Systems The BOSS Expansion Vessel 600L (6 Bar) is a high-capacity sealed expansion vessel designed for commercial heating systems, HVAC installations, and chilled water applications. Engineered to accommodate thermal expansion and contraction caused by temperature fluctuations, it helps maintain stable system pressure, protect…

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Key Features of BOSS Expansion Vessels

  • Suitable for heating (HTG) and chilled water (CHW) systems
  • Factory pre-charged for quick installation
  • Long-life diaphragm membrane design
  • Compatible with glycol solutions (up to 50%)
  • Available in multiple capacities (2L to 1000L+)
  • Designed for sealed closed-loop systems
  • Complies with PED pressure equipment safety standards

Applications

BOSS expansion vessels are widely used in:

  • Domestic central heating systems
  • Commercial boiler plant rooms
  • Chilled water HVAC systems
  • Underfloor heating systems
  • District heating networks
  • Industrial process systems

Heating vs Chilled Water Expansion Vessels

Heating Systems (HTG)

Used in hot water heating systems where water expands as it is heated by a boiler or heat source.

Chilled Water Systems (CHW)

Used in cooling systems where temperature fluctuations still cause volume and pressure changes within closed loops.

BOSS expansion vessels are designed to perform in both environments.

3 Bar vs 6 Bar Expansion Vessels

Choosing the correct pressure rating is essential:

3 Bar Expansion Vessels

  • Standard domestic heating systems
  • Low-pressure sealed systems
  • Residential boilers

6 Bar Expansion Vessels

  • Commercial heating systems
  • Chilled water applications
  • Higher pressure HVAC systems
  • Industrial installations

How to Choose the Right Expansion Vessel Size

Correct sizing depends on several system factors:

  • Total water volume in the system
  • Maximum operating temperature
  • System pressure rating
  • Type of application (heating or chilled water)
  • Glycol concentration (if used)

Incorrect sizing can lead to pressure instability, system faults, or premature component failure.

For critical applications, vessel sizing should be confirmed by a qualified mechanical engineer or system designer.