What are common unvented cylinder problems?
What are common unvented cylinder problems? While unvented cylinders are excellent at delivering high-pressure hot water, their complex, pressurised nature means they can develop specific issues. Here are the most common problems we encounter:
1. Dripping from the Tundish This type of issue is the most frequent call-out. The tundish is a visual safety indicator. While a few drops during the heating cycle are normal as water expands, continuous dripping means a safety valve has lifted. This condition is usually caused by:
- Loss of expansion vessel charge: The internal or external expansion vessel has lost its air cushion, leaving nowhere for the expanding hot water to go.
- Faulty Pressure Reducing Valve (PRV): Allowing mains pressure into the cylinder to rise too high.
- Faulty Temperature Control Thermostat: Allowing the domestic hot water to overheat.
2. Low Hot Water Pressure or Flow Rate Since these cylinders rely entirely on your mains supply, a drop in pressure usually points to:
- A blocked inlet strainer/filter (often caused by debris or grit from the mains).
- A failing PRV that isn’t opening fully.
- A general drop in the local mains water pressure.
3. No or Insufficient Hot Water If the water isn’t getting hot, the cylinder vessel itself is rarely the culprit. The issue usually lies with the heat source.
- Indirect unvented cylinders, e.g., boiler-fed: a faulty motorised zone valve, pump issue, or boiler breakdown.
- Direct unvented cylinders, i.e., immersion heaters (electric): A failed thermostat, a tripped high-limit reset switch, or a burnt-out heating element.
4. Noisy Operation Banging, ticking, or “kettling” noises are usually due to thermal expansion (pipes expanding and rubbing against joists), water hammer from quick-closing mixer taps, or heavy limescale buildup on the electric elements.
Crucial Safety Warning
Because unvented cylinders operate at high pressures and temperatures, homeowners must never attempt to repair, adjust, or replace the internal safety components (such as the T&P relief valves, PRVs, or expansion vessels). Under Part G of the Building Regulations, a G3-qualified engineer must carry out any work on these pressure components. Attempting DIY repairs on an unvented system is incredibly dangerous and illegal if not properly certified. If you suspect a fault, isolate the mains water supply to the cylinder and call a professional.
