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✅ HelpfulBoiler & Heating System
The Expansion Vessel
The expansion vessel absorbs the extra volume of water as it heats and expands. When it fails, boiler pressure fluctuates dramatically — high when hot, low when cold.
⚙️ What it does
A pressurised metal tank (usually red, inside or near the boiler) with a rubber diaphragm and nitrogen gas charge. As water expands when heated, the vessel absorbs the extra volume and prevents pressure from rising to dangerous levels.
🔍 Classic signs of failure
Pressure reads correctly when cold but rises to 3+ bar when the heating has been on for 30–60 minutes. The PRV repeatedly discharges. You need to repressurise frequently because the PRV keeps releasing water.
This pattern — high when hot, low when cold — is the classic sign of expansion vessel failure.
🛠 What to tell an engineer
"My boiler pressure is fine when cold but gets very high when the heating's been on — I think the expansion vessel might have failed." Expansion vessel replacement typically costs £150–£300 including labour.
Good to know: Expansion vessels have a valve similar to a bicycle tyre valve. The gas pressure can sometimes be recharged — a Gas Safe engineer can check this before deciding whether replacement is needed.
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General guidance only — not a substitute for professional advice. Always consult a qualified engineer if you are unsure. Gas work must only be carried out by a Gas Safe registered engineer. Unvented (pressurised) hot water systems must be worked on by a G3-qualified engineer. We accept no liability for any loss, damage, or injury arising from the use of this information.
Up next in Your Boiler & Heating System
The Circulation Pump