Unvented cylinder — pressure 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 →⚠️ Do not attempt to cap or block the discharge pipe. It is a critical safety device. Call a qualified unvented cylinder engineer.
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Most likely cause & what to check
Unvented cylinders have several safety valves that release water if the system over-pressurises. The discharge pipe (usually 15mm copper exiting through an outside wall) will drip when any of these operate.
A small amount of dripping during and immediately after heat-up can be normal — this is the expansion vessel absorbing the pressure increase as water is heated. If the discharge pipe flows continuously or heavily, there is a fault.
The most common cause is a failed expansion vessel: the bladder inside perishes, the vessel can no longer absorb pressure changes, and the PRV lifts instead. An engineer can recharge or replace the vessel (costs £80–£200).
The second common cause is a faulty PRV: the valve develops a small weep even at normal pressure. An engineer will replace the PRV — never try to tighten or adjust it yourself.
If the discharge is extremely hot and flowing heavily, turn off the cylinder (isolate the heating circuit and switch off the immersion heater) and call an engineer as an emergency.
Unvented cylinders must only be installed and repaired by G3-qualified engineers (Unvented Hot Water Storage Certificate). This is a legal requirement — standard plumbers cannot work on pressurised cylinders.
Annual servicing by a G3 engineer is essential and required by most manufacturers to maintain warranty. The service checks expansion vessel pressure, valve operation, and thermostat settings.
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