Unvented cylinder expansion relief valve dripping
Check the steps below first — if you\'re not confident, get it fixed safely today.
Find a Gas Safe engineer near you →⚠️ Unvented cylinders operate at mains pressure and are a safety-critical system. Only a G3-qualified engineer should carry out any work on the safety devices, expansion vessel, or internal components of an unvented cylinder.
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Most likely cause & what to check
The expansion vessel on an unvented cylinder absorbs the increase in water volume as water heats. When it fails, excess pressure is discharged through the temperature/pressure relief valve (TPRV) or expansion relief valve — causing it to drip.
A dripping TPRV is a warning sign that should not be ignored. It is the cylinder's last line of defence before a potential explosion. Call a G3-qualified plumber immediately.
As a temporary measure, reduce the cylinder temperature setting — lower temperature means less expansion and may reduce the frequency of discharges while you wait for an engineer.
The most common fix is replacing the expansion vessel (typically a 2–8 litre diaphragm vessel on the cold feed pipe) — cost £50–£150 for parts, total £150–£350 with labour.
If the vessel has failed and the cylinder is over 10 years old, it may be worth discussing a full cylinder service or replacement with the engineer.
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