Expansion Vessels Explained: Types, Sizing, and Troubleshooting
Expansion Vessels Explained: Types, Sizing, and Troubleshooting
What Is an Expansion Vessel?
An expansion vessel is a crucial safety and performance component in heating and potable water systems. Its main purpose is to absorb increases in water volume caused by temperature changes, preventing over-pressurisation that could damage components or cause leaks.
How Do Expansion Vessels Work?
Water reaches its lowest density at 4°C (not 0°C as commonly thought). Above or below this temperature, its volume expands, whether turning to ice or heating up.
When system water expands, it needs a safe space to go. An expansion vessel provides this space via a sealed air chamber with a pre-charge of air. As water volume increases, it flows into the vessel. As the system cools, the pre-charge pushes the water back into the system.
Key factors in vessel sizing:
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System temperature
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System volume
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Safety relief valve (SRV) setting
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Pre-charge setting
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Application type (heating or potable)
Types of Expansion Vessel
1. Heating Vessels
Water sits directly inside the metallic body, separated from the air charge by a diaphragm.
2. Potable Vessels
Designed for wholesome (drinking) water, these vessels store water in a rubber bladder to prevent contact with the metal, protecting against oxygenation damage.
3. Flow through vessels
These operate like potable vessels but include a mushroom-shaped insert that encourages constant water movement, preventing stagnation. Some models feature top and bottom connections for direct flow-through.
Sizing Your Expansion Vessel
Correct sizing ensures your system can safely accommodate water volume changes when heated from 10°C to its maximum operational temperature (or higher under fault conditions).
Top Tip: Use Altecnic’s vessel sizing calculator for accurate results.
Best Practice for Installation
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Position close to the pump inlet to maintain positive pressure throughout the system.
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Install in a dry, accessible location for easy inspection, maintenance, and pre-charge testing.
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Ensure correct orientation — most vessels are designed to be installed vertically, but always follow manufacturer guidance.
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Fit isolation valves to allow servicing or replacement without draining the entire system.
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Minimise pipe runs between the vessel and the system connection to reduce pressure loss.
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Check pre-charge before commissioning to ensure optimal performance from day one.
Correct installation reduces wear, ensures accurate pressure management, and prolongs vessel lifespan.
Troubleshooting Common Issues
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Loss of Air Charge: Check and recharge if needed.
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Damaged/Worn Diaphragm: May require vessel replacement.
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Incorrect Sizing: Install the correct vessel for your system.
Quick Answers for Common Expansion Vessel Questions
Q1: How do I know if my expansion vessel needs replacing?
If the system pressure rises rapidly on heating, the vessel may have lost its air charge or the diaphragm may have failed. Test the pre-charge and check for water on the air side.
Q2: Can I use a heating vessel for potable water?
No. Potable water vessels have a rubber bladder to prevent direct contact between oxygenated water and metal surfaces, avoiding corrosion and contamination.
Q3: How often should I check my vessel’s pre-charge?
At least annually during routine system servicing. More frequent checks are recommended in high-use or critical systems.
Q4: Where is the best place to install an expansion vessel?
As close as possible to the pump inlet on the return pipework, in a dry, accessible, and well-ventilated location.
Q5: What happens if my expansion vessel is too small?
An undersized vessel will cause frequent pressure relief valve discharge, increasing water loss, system stress, and energy waste.
Why Choose Altecnic?
With decades of engineering expertise and a full range of heating, potable, and flow-through expansion vessels, Altecnic provides reliable solutions for domestic, commercial, and industrial applications.