Do unvented cylinders leak?
Do unvented cylinders leak? The short answer is yes, unvented cylinders can leak just like vented cylinders can leak, but under normal conditions, they absolutely should not. Because unvented cylinders operate at full mains water pressure, any leak can quickly escalate from a minor drip into a significant flood.
When diagnosing a “leak”, we look at three distinct areas:
1. Safety Valve Discharges (The “Fake” Leak) Most of the time, what a homeowner thinks is a leak is actually a safety valve doing its job. If you see water dripping from the tundish (the visible air-break funnel) or the external discharge pipe, the Temperature and Pressure (T&P) relief valve has likely lifted. This isn’t a broken cylinder; it’s a symptom of an underlying issue, most commonly a failed expansion vessel or a faulty Pressure Reducing Valve (PRV).
2. Pipework and Connection Leaks Because the system is under constant high pressure, a poorly made joint, a loose valve, or a degraded O-ring can spray water forcefully. Leaks frequently occur at the mains inlet connections, the expansion vessel valves, or the coil connections.
3. Vessel Failure Actual leaks from the cylinder vessel itself are rare, as modern units are typically made from high-grade stainless steel. However, it can happen due to a manufacturing defect, severe external corrosion, or if the system water quality violated UK Water Regulations (e.g., incorrect inhibitor use on an indirect unvented cylinder). If the vessel itself is leaking, it cannot be repaired and must be replaced.
Please refer to the following webpages that detail our range of stainless steel unvented cylinders.
our indirect pressurised hot water cylinders are detailed here –
or our direct electric pressurised hot water cylinders are detailed here –
UK Regulatory Context
Under Part G of the Building Regulations, any discharge from safety valves must be safely routed outside via a correctly sized, dedicated discharge pipe. This legal requirement prevents scalding and property damage. Furthermore, all components must be WRAS-approved to comply with UK water fittings regulations.
What to do if you see a leak
If you suspect a leak, please locate the cylinder’s isolation valve and shut off the mains water supply immediately. Due to the extreme pressure and temperature involved, please do not attempt to tighten joints or repair the vessel yourself. Always call a G3-qualified engineer to safely diagnose and rectify the issue.
