Unvented hot water cylinders: common faults explained
The most common unvented cylinder faults are a discharging expansion or pressure relief valve, loss of air charge in the expansion vessel, a scaled or failed immersion element, and thermostat or motorised valve failure. A cylinder that discharges water from its outside pipe is telling you something is wrong and should be looked at rather than ignored.
Key facts
- Discharging outside pipe
- Expansion vessel or a failed pressure relief valve
- Legal position
- Unvented work requires a qualified engineer
- Annual service
- Recommended — vessel charge and valves are checked
- Urgent action
- Turn off the heat source and call if it is discharging continuously
Symptom to likely cause
| Symptom | Most likely cause |
|---|---|
| Water discharging from the outside tundish pipe | Lost expansion vessel charge, or an expansion or pressure relief valve passing. |
| Banging or knocking when taps close | Failed expansion vessel or an air issue in the system. |
| Hot water pressure falling over time | Scaled coil or a partially blocked balanced cold take-off. |
| Cylinder never reaches temperature | Failed thermostat, immersion element or a heating-side control fault. |
If it is discharging right now
- 1Turn off the heat sourceSwitch off the boiler's hot water schedule and any immersion heater.
- 2Do not cap the dischargeThe discharge pipe is a safety device. Never block, plug or reduce it.
- 3Check where the water comes fromNote whether it drips constantly or only while heating — it tells the engineer which valve is at fault.
- 4Call a qualified engineerUnvented cylinders must only be worked on by a competent qualified person.
What the symptoms usually mean
Unvented systems fail in fairly predictable ways.
- • Water dripping from the outside discharge pipe — usually a lost expansion vessel charge or a failing relief valve.
- • Banging or knocking when hot water is drawn — often expansion or a failing valve rather than the cylinder itself.
- • Hot water running out sooner than it used to — typically a scaled coil, a failed element or a thermostat problem.
- • No hot water at all with heating still working — look at motorised valves, the coil and controls.
- • Visible corrosion or weeping at fittings — needs attention promptly on a pressurised system.
Why annual servicing matters here
An unvented cylinder holds hot water under mains pressure, so its safety components — expansion vessel, pressure relief and temperature relief valves — are not optional extras. They are checked and recharged as part of a service, and in hard water areas scale makes that check more, not less, important.
What to do right now if it is discharging
If water is running from the discharge pipe continuously, turn off the immersion or hot water heating so the cylinder stops heating, leave the discharge pipe clear, and call. Do not cap or block the discharge pipe under any circumstances — it is a safety device.
Need this doing rather than reading?
We cover the GU, KT, SW, CR, RH and BN postcode areas, 24 hours a day, 7 days a week, including bank holidays.