What is an expansion vessel?
An expansion vessel is a sealed tank fitted to closed hot water systems that absorbs pressure increases as water expands when heated, preventing dangerous pressure buildup.
In a closed hot water system (one where the water cannot escape to the atmosphere), heated water expands as its temperature rises. Without somewhere for that extra volume to go, pressure builds rapidly inside the pipe network and tank. An expansion vessel is a small sealed chamber, usually positioned near the hot water system's pressure relief valve, that accepts this expanded water and prevents pressure from climbing beyond safe limits.
The vessel contains a flexible internal diaphragm or bladder that separates the water side from a pre-charged air chamber. As the water warms and expands, it pushes the diaphragm, compressing the air. When the system cools, the air pressure pushes the water back. This cycle repeats daily without loss of water or pressure spikes.
An expansion vessel is a mandatory requirement under the National Construction Code and Australian Standards AS/NZS 3498 for any closed hot water system fed from a mains cold water supply. Without one, excessive pressure can damage pipes, fittings, and the storage tank itself, and may trigger the pressure relief valve to discharge water repeatedly. Most Perth plumbers fitting closed systems will specify and install an expansion vessel sized to match the system's capacity. The device itself requires minimal maintenance but should be checked periodically to ensure the diaphragm remains intact and the air charge is correct.
For advice on expansion vessel installation or testing on your hot water system, consult a licensed plumber.