Six faults account for almost every unvented cylinder call-out: a lost expansion vessel charge, a failed pressure-reducing valve, a blocked inlet strainer, a scaled immersion element, a stuck cylinder thermostat and a spent anode. Five of the six are service items rather than tank failures, and an annual service — required to keep the warranty valid — prevents most of them.
What goes wrong with unvented cylinders most often?
Not the tank. In a properly installed system the vessel itself outlasts everything bolted to it; the faults live in the safety group, the controls and the heating element. This table covers the great majority of service calls.
| Symptom | Most likely cause | Fix |
|---|---|---|
| Water dripping from the tundish | Expansion vessel has lost its air charge | Repressurise to match the PRV setting, or replace the vessel |
| Sudden drop in hot water pressure | Pressure-reducing valve cartridge failed | Replace the cartridge or the valve |
| Poor flow, hot and cold | Blocked inlet strainer | Isolate, remove and clean the mesh |
| Hot water lukewarm, boiler runs | Coil airlock or failed diverter valve | Vent the primary, check the mid-position valve |
| No hot water on electric only | Immersion element scaled or tripped | Reset the thermal cut-out, descale or replace |
| Water too hot, thermostat overrun | Cylinder thermostat not seated in its pocket | Reseat the sensor, verify against a thermometer |
| Rumbling or kettling noise | Scale on the element or coil | Descale; consider softening or dosing |
| Rusty water, tank weeping | Magnesium anode consumed | Replace the anode; inspect the vessel |
Why does water come out of the tundish?
Because the expansion vessel can no longer absorb the volume increase of the heated water. Water expands about 1.7% between 10 °C and 60 °C, so a 200 litre cylinder generates roughly 3.5 litres of expansion on every full reheat. If the vessel's air charge has leaked away, pressure climbs until the expansion relief valve lifts at around 6 bar and the surplus runs to the tundish.
A drip that appears only during reheat is expansion; a continuous flow with a hot discharge pipe is the temperature-and-pressure relief valve and is a genuine safety event. The full diagnostic sequence is set out in why an unvented cylinder discharges through the tundish.
Why is my hot water pressure or flow suddenly poor?
Three components stand between the mains and the tap, and each fails in a recognisable way. The inlet strainer clogs with debris after mains work and reduces both hot and cold flow at every outlet fed through it. The pressure-reducing valve fails gradually, so pressure falls over weeks rather than overnight. Scale narrows the outlet fittings and the immersion boss, and shows up first at the shower.
Test in that order: isolate and inspect the strainer, then gauge the pressure downstream of the PRV against the incoming static pressure. If the cold mains at the kitchen tap is strong but the unvented outlets are weak, the fault is in the safety group, not in the supply.
Why is there no hot water even though the boiler runs?
The heat is not reaching the water. On an indirect system the usual culprits are an airlock in the primary circuit, a mid-position valve stuck on heating, a failed cylinder thermostat, or a pump running but not circulating. Feel the primary flow and return pipes at the cylinder: if the flow is hot and the return cold, the coil is transferring nothing and the problem is inside the tank; if both are cold, the fault is upstream.
On a heat pump system add one more possibility — an undersized coil. A cylinder built for a boiler cannot absorb heat pump output at 50 °C, and the symptom is exactly this: the heat source runs, the tank warms slowly, and the household runs short. That is the case explained in can I reuse my existing hot water cylinder with a heat pump.
How do I stop these problems happening?
Five habits cover nearly all of it:
- Service annually. Check the expansion vessel charge, operate both relief valves, clean the strainer. This is a condition of most warranties and recommended practice for G3 unvented systems in the UK.
- Inspect the anode every two years on an enamelled steel tank, and replace it once about 60% is consumed.
- Store at 60 °C, not 70 °C. Scale precipitation accelerates sharply with temperature, and a 1 mm scale layer adds around 7% to the energy needed for the same heat output.
- Fit a thermostatic mixing valve so a 60 °C store can be delivered safely at 43–48 °C.
- Keep the discharge visible. A tundish hidden in a cupboard turns an early warning into a flooded floor.
Solimpeks builds its enamelled tanks with accessible anode and sensor ports precisely so that these checks stay a fifteen-minute job.
Frequently asked questions
How long should an unvented cylinder last?
Fifteen to twenty-five years for the vessel itself, provided the anode is maintained on an enamelled tank and the water is not aggressively soft or chloride-rich. The safety group is a shorter-lived assembly: expect to replace an expansion vessel or PRV cartridge once or twice over the tank's life.
Do unvented cylinders need to be serviced every year?
Yes. An annual service checks the expansion vessel pre-charge, exercises both relief valves and cleans the inlet strainer. Skipping it is the single biggest cause of tundish discharge, and most manufacturers make the warranty conditional on documented servicing.
Can I fix an unvented cylinder myself?
You can clean a strainer and repressurise an expansion vessel, but in the UK any work on the unvented safety group must be carried out by a person registered under the G3 competent-person scheme. Relief valves must never be adjusted, capped or removed.
Why is my unvented cylinder making a rumbling noise?
Almost always scale on the immersion element or the coil, which makes the water film next to the heating surface boil locally. Descale the element and consider softening or phosphate dosing if the supply is above about 200 mg/L of calcium carbonate.