By Patrick Gordon, consultant at Splashworks
In the world of plumbing and heating systems, pressure control is critical – not just for performance, but for safety and longevity.
One of the key tools in achieving this balance is the expansion vessel. But when exactly should you use one? And how do they work? Let’s break it down.
What is an expansion vessel?
An expansion vessel is a sealed container divided by a flexible membrane (or bladder). One side contains water, the other a compressed gas – typically nitrogen or air. As water heats and expands, it exerts pressure on the membrane, compressing the gas and absorbing the pressure increase. This simple concept helps protect systems from dangerous pressure spikes.
Where are expansion vessels used?
Expansion vessels serve a range of functions depending on the type of system:
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In closed heating systems
In closed-loop heating systems, water is circulated continuously.
As it heats up, it expands – but since water is incompressible, the pressure rises. Without an expansion vessel to absorb this pressure, relief valves would constantly discharge and system components could be damaged.
Key points:
- Installed on the return line (where water is coldest)
- Helps maintain consistent system pressure
- Extends the life of components like pumps and valves
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In pumped domestic water systems (hydro-pneumatic wells) In pressurised domestic water systems, an expansion vessel is often used as a hydro-pneumatic tank to reduce the frequency of pump cycling.
How it works:
- As the pump fills the system, water compresses the air inside the vessel
- When a tap is opened, the compressed air pushes water out, maintaining pressure without activating the pump
- This significantly extends the pump’s lifespan and provides more stable water pressure
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To prevent water hammer
Water hammer occurs when fast-closing valves abruptly stop water flow, creating a sudden pressure surge (and sometimes a loud bang). Expansion vessels act as shock absorbers, compressing momentarily to dampen these surges and protect pipes and fittings from damage.
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Not for use in hot water cylinders (according to SANS 10254) In domestic hot water cylinder installations, the use of expansion vessels is generally prohibited under SANS 10254 clause 1.3(c). Instead, the following components are required:
- Pressure control valve
- Expansion control valve
- Safety valve (temperature and pressure control)
- Vacuum control valve
All of these must comply with SANS 198 standards. This regulation ensures safety and consistency in hot water systems and overrides the need for expansion vessels in these specific setups.
Installation guidelines
To ensure effective operation:
- Always install the expansion vessel on the coolest part of the system (typically the return line in heating systems)
- The pre-charge pressure of the vessel must match the system pressure when cold
- The vessel must be correctly sized to the system’s volume and pressure characteristics
Incorrect sizing or installation can render the vessel ineffective – or worse, dangerous.
Final thoughts
Expansion vessels are a vital component in many plumbing and heating applications. They help regulate pressure, prevent system failures and ensure efficient pump operation. However, it’s crucial to follow local standards and best practices – especially in domestic hot water installations where expansion vessels are restricted.
