Hot Water Cylinder Leaking? What It Means
Where the water appears tells you how serious it is, from a loose fitting to a cylinder at end of life.
From a fitting or valve
Drips at the immersion boss, a union or the temperature relief valve are usually seals and washers, quick repairs. A relief valve that keeps passing is a warning about pressure or a failed expansion vessel, so the cause matters more than the drip.
From the cylinder body
Weeping from the shell itself, or rust streaks down the insulation, means corrosion has made it through. There is no honest repair for a pinholed cylinder; plan the replacement before it chooses its own date.
The discharge pipe outside
On unvented systems, a dribbling external pipe is the safety system relieving pressure. It is doing its job, but doing it constantly means the expansion vessel or a valve needs attention at the next service.
Where it is leaking from matters enormously
From a fitting, an immersion boss or a valve: repairable, often straightforwardly. From the tundish or the discharge pipe on an unvented cylinder: that is a safety device doing its job, which means the expansion vessel or a valve has failed and needs urgent attention. From the body of the cylinder itself: not repairable, and replacement is the only honest answer.
Unvented cylinders discharge for a reason
Water at the tundish is never something to ignore or plumb away. It means the system could not absorb expansion, and the pressure had to go somewhere. The expansion vessel has usually lost its charge. On a mains-pressure cylinder these are the safety devices, and a seized temperature and pressure relief valve is genuinely dangerous.
What to do right now
Isolate the cold feed to the cylinder, switch off the immersion and the boiler, and run the hot taps to drop the level. On an unvented system do not attempt to cap or reroute a discharge pipe. Container underneath, and call, a cylinder holds well over a hundred litres and it is directly above your ceilings.
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