Expansion Vessels Explained: A Practical Guide for Heating Systems
Expansion vessels are a critical component in both domestic and commercial heating and hot water systems. Designed to regulate system pressure and protect key components, they play an essential role in maintaining safe, efficient, and reliable operation.
Flowstar supplies high-quality heating expansion vessels to OEMs and distributors worldwide. From upgrading expansion vessels in large commercial buildings to supplying new and replacement vessels for residential properties, our products are trusted by plumbing and heating professionals across the industry.
This guide explores how expansion vessels work, their role in different heating systems, and how to select the right vessel for your application. If you need further advice, our specialist team is on hand to help with all expansion vessel and component enquiries. For further help or information, please contact us on +44 (0)1482 601030 or email sales@flowstar.co.uk.
Expansion vessels available at Flowstar
Contents
What Is an Expansion Vessel and Why Is It Important?
An expansion vessel, sometimes referred to as an expansion tank, is designed to manage and regulate water pressure within closed heating and hot water systems.
As water heats up, it expands. Without a means of absorbing this increase in volume, system pressure can rise dramatically, leading to safety valve discharge, pipework damage, component failure, or even system shutdown. Expansion vessels prevent these issues by safely accommodating changes in water volume and pressure.
They are particularly important in closed-loop heating systems, where fluctuations in temperature and variations in mains water supply can otherwise lead to costly repairs and reduced system lifespan.
How Expansion Vessels Work in Heating Systems
An expansion vessel consists of a sealed pressure tank divided into two chambers by a flexible synthetic rubber diaphragm.
One side of the vessel connects to the heating system and contains system water. The other side is pre-charged with air or nitrogen gas. As system water heats and expands, it pushes against the diaphragm, compressing the air or nitrogen within the vessel. This absorption of excess pressure keeps the system operating within safe limits.
When the water cools and contracts, the compressed gas pushes the water back into the system, maintaining consistent pressure levels.
Key Components of an Expansion Vessel
Vessel Body
The outer steel container that houses all internal components.
Diaphragm
A flexible rubber membrane that separates system water from the gas chamber.
Air or Nitrogen Chamber
Pre-charged to absorb pressure fluctuations. Expansion vessels can be nitrogen-charged to extend vessel life and reduce corrosion risk.
Air Valve
Allows pressure checks and adjustments during routine maintenance.
Water Inlet/Outlet
The system connection point that allows water to enter and leave the vessel as required.
Expansion Vessels in Domestic & Combi Boiler Systems
Combination (combi) boilers rely on expansion vessels to manage pressure changes caused by heating and cooling water. Because combi boilers provide both central heating and domestic hot water, they are particularly susceptible to pressure fluctuations.
As temperatures change across the system, expansion vessels stabilise pressure, protecting boilers, radiators, pipework, and valves from excessive stress. This reduces the risk of breakdowns, costly repairs, and system downtime, helping to maintain comfort for occupants.
Expansion vessels are therefore widely used in homes and small commercial properties such as shops, cafés, offices, and restaurants.
Flowstar offers a comprehensive range of expansion vessels specifically designed for domestic and small-scale plumbing and heating systems. Contact our team for expert guidance and product support.
Expansion Vessels in Commercial & System Boiler Applications
Larger commercial, industrial, and public buildings typically use system boilers to supply central heating and high-volume hot water storage. These systems experience significant thermal expansion due to higher operating temperatures and larger water volumes.
Commercial expansion vessels are designed to accommodate these increased demands, regulating system pressure during both peak usage and low-demand periods. Without correctly sized expansion vessels, pressure spikes can damage equipment, compromise safety, and reduce system efficiency.
Expansion vessels are widely used in environments such as hotels, hospitals, shopping centres, manufacturing plants, apartment complexes, universities, and government buildings.
For tailored advice on commercial expansion vessel selection, speak to our specialist team today.
How to Choose the Correct Expansion Vessel
Choosing the correct expansion vessel is essential for safe, efficient, and long-term system performance. Key considerations include:
System Volume and Vessel Size
The vessel must have sufficient capacity to absorb the expected expansion of water within the system.
Maximum Operating Temperature
Ensure the vessel is rated for the system’s temperature range.
Static Head
The vertical distance between the expansion vessel and the highest point in the system affects required pre-charge pressure.
Pressure Rating
The vessel must be rated equal to or higher than the maximum operating pressure of the system.
At Flowstar, we supply expansion vessels directly to plumbing companies and independent contractors, alongside a wide range of heating and plumbing accessories for maintenance and installation projects.
Expansion Vessel Installation and Best Practices
A failed expansion vessel can cause several system issues, including:
- Excessive system pressure
- Inefficient or inconsistent heating
- Unusual noises such as banging or kettling
Common Expansion Vessel Failures and Their Effects
- Diaphragm Rupture or Fatigue
- Loss of Pre-Charge Pressure
- Internal or External Corrosion
- Physical Damage to the Vessel
Regular inspection and correct sizing are key to preventing premature failure and maintaining system reliability.
For further help or information about these products, please contact us on +44 (0)1482 601030 or email sales@flowstar.co.uk.