Hot Water Cylinder Leaking: Find the Source First

What should I do first if my cylinder is leaking?

Stop the water and the heat before you look for the cause. Close the stopcock that feeds the cylinder, or the isolation valve on its cold supply. On a vented cylinder that is the gate valve on the feed from the loft tank. Switch the immersion heater off at its isolator and set the boiler hot water to off. Heat on a cylinder that is losing water can overheat an element that is no longer fully covered.

Then deal with the water on the floor. Water near a cylinder is often near electrics, so keep clear of the immersion wiring, and do not stand in water to reach the isolator. If the water is spreading and you cannot safely reach the isolator, switch off at the consumer unit. Move stored items, soak up what you can, and put a bowl or tray under the suspected point.

You can still use cold water in the house once the cylinder supply is closed, but the hot taps will run dry once the stored water is gone. Do not leave the cylinder drained and heated.

Where is the water actually coming from?

Water runs downhill and along pipes, so the wet patch is rarely directly under the leak. Dry the cylinder and its pipework with a towel, wrap a piece of dry toilet paper around each suspect joint, and watch for a few hours. The first spot to go dark is the source. Check from the top down.

• Valve group and pipework: the compression joints on the combination valve, pressure reducing valve and the isolating valves on an unvented cylinder. A weeping nut shows a bead of water and a green or white crust.
• Temperature and pressure relief valve: if water runs from the discharge pipe into the tundish, the valve is discharging. That is a different problem from a leak and is covered below.
• Immersion boss: water seeping around the immersion heater flange or collar, often with a limescale ring. A failed seal or a corroded boss lets water track down the cylinder side.
• Coil connections: on an indirect cylinder, the two boiler flow and return unions on the side. On a sealed heating system a weep here may show as falling boiler pressure.
• Discharge pipe and tundish: a drip into the visible tundish, running down the pipe to a drain.
• Shell and seams: water on the lower third of the cylinder, behind the insulation or running from the base, with no leak at any fitting above. This is the one that matters most.

Why is my unvented cylinder discharging through the tundish?

A steady flow into the tundish is not a pipe leak. It means a safety valve is opening. Two things make that happen: water pressure above the valve rating because the expansion vessel has lost its air charge, or water temperature above the valve limit because a thermostat or cut-out has failed. Both are engineer faults and neither is a reason to block the pipe. Our guide to a dripping temperature and pressure relief valve covers the difference between a one-off drip on heat-up and a constant discharge.

An occasional drip while the cylinder heats, which stops as it cools, usually points to an expansion vessel that has lost its charge. A constant flow, especially while the cylinder is cold, can point to a valve seat that no longer seals, often because of debris or scale, or to incoming pressure that is higher than it should be. Only an engineer can tell which.

Can I still use the water, and can the leak be repaired?

You can use cold water elsewhere in the house. Leave the hot taps unused and the heat source off until the source is known, because a cylinder that is losing water should not be heated.

Repairability depends on the source. A weeping compression joint on the valve group can be remade with new olives and nuts. A discharging valve can be replaced. An immersion boss seep can often be cured with a new element and seal. A coil union can be reseated or replaced. These are repairs, and the cylinder stays.

A shell leak is different. When water comes from the cylinder body, the copper or stainless steel has failed from corrosion, and no fitting can fix it. A weeping seam on a copper cylinder means the metal is thinning. Patching it is not a lasting repair, and a cylinder that has leaked from the shell will usually need replacing. Our cylinder leak repair page sets out how the source is confirmed and handled.

Repair or replace: what decides it?

The rule is simple. If the water comes from a fitting, a valve, a seal or a union, the cylinder is usually fine and the leak is repaired. If it comes from the shell, the cylinder is finished. Age is the tiebreaker where it is unclear: a young cylinder with a leaking boss is repaired, while an old one with a corroded boss and a weeping base is more likely at the end of its life.

There is a trap in this. Long-term discharge from a valve can soak the base of the cylinder and the timber under it, which makes the cylinder look like it is leaking from the bottom when the valve above is the real fault. The engineer traces the water back to its origin before making any call, and checks the base and the timber beneath it for damp.

If you cannot tell the source, or the leak returns after a repair, the next step is a hot water cylinder diagnosis, where each joint and valve is checked in turn, rather than a guess at a part.