Unvented cylinder expansion relief valve dripping
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →⚠️ 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.
An unvented hot water cylinder is a clever system that delivers mains-pressure hot water throughout your home — but it's also pressurised, which means it needs to manage expansion safely. When water heats up, it expands, and a small rubber diaphragm vessel (the expansion vessel) normally absorbs this extra volume. However, when that vessel fails — usually after 8–12 years — the excess pressure has nowhere to go, so it's released through a relief valve, which drips water as a warning sign. This is most common in older systems or homes where the expansion vessel has been neglected during servicing. If you're seeing water pooling beneath a small valve on your cylinder, or noticing regular drips, it's a safety issue that needs professional attention. This guide explains what's happening, what you can do temporarily, and when to call a G3-qualified engineer.
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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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Frequently asked questions
Why is my expansion vessel failing? Is it my fault?
No, it's not your fault — diaphragm vessels naturally degrade over time as the rubber weakens, typically after 8–12 years. They're a consumable part, like a boiler heat exchanger. Regular servicing can spot early signs, but eventual failure is normal wear and tear.
How much will it cost to fix?
Replacing an expansion vessel typically costs £150–£350 total (parts and labour), depending on your location and how accessible the vessel is. If your cylinder is over 10 years old, an engineer might recommend a full service or cylinder replacement during the visit, which would cost more. Always get a quote before agreeing to work.
Can a plumber fix this, or does it have to be a Gas Safe engineer?
Only a Gas Safe registered engineer with G3 qualification can legally work on the expansion vessel, relief valves, or internal safety components of an unvented cylinder. These are high-pressure safety devices, and the rules are strict for good reason. Your household insurance may also require this certification.
Will replacing the expansion vessel fix the dripping permanently?
Yes, in most cases. Once a new diaphragm vessel is fitted and pressurised correctly, the relief valve should stop dripping immediately. However, if the cylinder itself is very old or faulty, the engineer might discover other issues during the job and recommend further work.