Hot Water Cylinder Leak Repair: Locate the Source First

Where is my hot water cylinder leaking from?

Water finds the lowest point, so a puddle at the base tells you very little about where the leak starts. The source can be several centimetres above and run down the insulation or the pipework before it drips.

Cylinder leaks fall into a short list, and each has a different outcome. The list below runs from the most common source to the one that cannot be repaired.

• Valve group and compression joints: a weeping nut or olive, a failed seal on the combination valve or a passing strainer cap.
• Immersion boss: a perished fibre washer or a scaled element thread letting water seep round the element.
• Coil connections: unions or compression joints on the flow and return of an indirect cylinder.
• Discharge route: the tundish and its pipe, where a relief valve is passing water that then appears to come from the cylinder.
• Shell seam or base: a seam weeping at a weld or corrosion through the base. This is the one source that cannot be repaired in place.

What should I do while water is leaking?

Isolate the electrical supply to any immersion heater at the isolator or fused spur before touching anything near a wet cylinder, because mains voltage and water together are a shock risk. If the isolator is wet or sits in the path of the water, switch the circuit off at the consumer unit instead, and do not touch it with wet hands. Then shut the cold supply to the cylinder at its isolating valve if you can reach it safely, or the main stopcock if you cannot.

Catch what you can in a container and move anything stored beneath the cylinder. Do not remove covers, tighten relief valve connections or put a cap on a discharge pipe. A blocked discharge on an unvented cylinder stops it releasing pressure.

Which leak sources can be repaired?

Almost everything outside the shell can be fixed in place. A compression joint is remade with a new olive and nut, and a combination valve or temperature and pressure relief valve is replaced with its fittings as a set. A scaled immersion boss usually means a new element and washer, as covered in immersion heater replacement. Part-by-part repair is set out in cylinder fault repair.

A leak that appears only when a valve discharges is often not a leak at all. The tundish is doing its job, and the fault is the valve or the expansion vessel behind it. That branch is covered in TPRV and expansion vessel repair.

Prolonged water at the cylinder base can rot the timber base and stain walls, so the cause is fixed before parts are replaced and any damp is dried out. That matters because a repair on top of a wet base only hides the problem.

When does a leaking cylinder need replacing?

If water is seeping from a weld seam, from the side of the shell beneath the insulation or from the base, the vessel itself has failed. Patching a pressure vessel is not a repair a householder or a competent installer should attempt, and it does not return the cylinder to a safe, watertight state.

Before that call is made, the other sources are ruled out, because a weeping valve joint can look exactly like a failed shell. The way the cylinder is tested in order is set out on hot water cylinder diagnosis.

Where replacement is the answer, the same-for-same swap and the choices around it are in cylinder replacement.