What should I do if an unvented cylinder discharge pipe is dripping?

Do not block, cap or adjust a dripping unvented-cylinder discharge. If it is safe to use the normal user control, stop the heat source and contact a competent engineer qualified for unvented hot-water systems. Keep people away from the outlet because discharged water may be hot.

Approved Document G requires an unvented storage system to have independent safety devices and pipework that makes a safety discharge visible and carries it to a place where it will not endanger people. A visible drip can therefore be evidence that a control or safety device needs professional attention, not a nuisance outlet to silence.

Do not turn safety valves, recharge expansion components or dismantle the tundish or discharge pipe. Record when the discharge occurs and whether it is continuous, then pass that information to the engineer.

If the cylinder is heated by a gas boiler, any work on the boiler, its gas pipework or gas controls is also restricted to a Gas Safe registered engineer. A suitably qualified engineer may hold both competencies; check the scope of their qualifications.

Tell the engineer whether the water appears at the tundish, at the outside termination or both; whether it is hot; and whether it happens only during heat-up. Do not touch the discharge to test its temperature. Photograph the visible flow from a safe position and note the cylinder make and model. A normal service history does not make a current discharge safe to ignore. If the flow increases, threatens people or enters the building, stop using the heat source and seek urgent professional help.

Sources

• Approved Document G — GOV.UK, accessed 14 November 2026.
• Approved Document G PDF — GOV.UK, accessed 14 November 2026.
• Gas safety: who can do the work? — HSE, accessed 14 November 2026.

On Leatmere

Read unvented cylinder basics and the unvented cylinder definition.