Hot Water Cylinder: Repair or Replace?

Can you tell repair from replace by the symptom?

Not reliably. The same symptom, such as no hot water or water on the floor, can come from a small, cheap part or from a cylinder that has reached the end of its life. That is why the order matters: find the fault first, then decide. A quote for a new cylinder given before anyone has looked at the old one is a guess.

What the symptom can do is narrow the search. Lukewarm water from an electric cylinder points at the immersion element or its thermostat. Water dripping from the discharge pipe of an unvented cylinder points at the valve group or the expansion vessel. A damp patch on the cylinder body or its base could be a connection, or it could be the shell, and only a methodical check separates the two. The hot water cylinder diagnosis page sets out how that check runs.

If you can see steam or very hot water coming out of the discharge pipe of an unvented cylinder, do not wait to work out which category you are in. Turn off the heat source at the isolator, keep clear and call an engineer.

Which cylinder faults are normally a repair?

A fault in a replaceable part leaves the cylinder itself sound. If the shell is dry and the rest of the cylinder is in good order, replacing the part is almost always the better route. These are the usual cases, with the evidence an engineer looks for in each.

• Immersion element: no heat from one or both elements. The engineer checks supply at the element terminals, then continuity and insulation resistance on the element. A tripped over-temperature cut-out is reset only after the cause is found. See immersion heater replacement.
• Thermostat: water too cool, or too hot, with a healthy element. The thermostat is tested on its own, separate from the element and its wiring.
• Motorised valve or cylinder stat on an indirect cylinder: the boiler heats but the cylinder does not. The valve actuator, its microswitch and the wiring are checked before anything on the cylinder is touched.
• Valve group on an unvented cylinder: pressure reducing valve, expansion relief valve or combination valve passing water, or low flow at the taps. The valve is tested and, if worn, replaced with its compression nuts and olives.
• Expansion vessel: its air charge has fallen, so the relief valve lifts and the tundish drips each time the water heats. The vessel is re-pressurised or, if the diaphragm has failed, replaced. See TPRV and expansion vessel repair.
• Tundish or discharge pipework: scale, a partial blockage or a failed joint. The pipe run is traced and cleared or renewed.

Which faults usually mean replacing the whole cylinder?

Some faults sit in the vessel, and a vessel cannot be patched from outside. A shell leak, a failed heating coil and heavy internal scale are the three that most often end a cylinder’s life.

A leak through the cylinder wall, as opposed to a leak at a connection, is the clearest case. On a copper cylinder it is usually pitting or a seam failing from inside. On a stainless cylinder it can be a weld. Neither is a repair a householder or a plumber should rely on. Tape, resin and sealant hold for a short while at best, and a vessel that weeps once is likely to weep again somewhere nearby.

A failed indirect coil shows itself in two ways. The cylinder stops heating from the boiler, or water from the heating circuit mixes with the stored water, which can show as boiler pressure that keeps changing with no visible leak. The coil is welded or brazed into the vessel, so it is not a part you swap on its own.

Heavy scale is the borderline case. Thick scale on the element and the cylinder floor means the water has been heating the limescale, not the water, and the element fails repeatedly. If the vessel is otherwise sound, a new element and a clean may be reasonable. If the scale has already started a shell leak, or the cylinder has lost much of its capacity, it is not. The cylinder water hardness and scale guide covers where scale builds first.

What does the engineer check to decide?

The decision follows from a short sequence of checks, taken in an order that avoids replacing a cylinder that was never at fault.

• Find where the water or the failure actually is. Dry the cylinder, the pipework and the floor, then watch which joint or surface wets first. Water that runs down a pipe can appear a long way from its source.
• Test the controls before the vessel. A supply fault, a blown fuse, a stuck motorised valve or a programmer timing error looks exactly like a dead cylinder. A blown fuse at the immersion spur is a good example of a dead-looking cylinder that was not the faulty part.
• Test the heat source: element resistance, thermostat operation, coil flow and return temperatures.
• Inspect the vessel: the data plate for age and type, seams, the base, the area beneath the cylinder for staining or corrosion, and any scale on parts brought out for inspection.
• Write down what was found, part by part, and which parts are repairable.

Is it worth repairing an old cylinder?

The question people search is whether a tank that is 15 or 20 years old is worth another repair. Age is a prompt to look closer, not a verdict. Manufacturers publish warranty terms, not service lives. A warranty says how long the maker stands behind the vessel under its conditions, and the parts around it are usually covered for much less. They do not promise that your particular cylinder, in your water and your cupboard, will last that long. The lifespan and signs of failure guide goes through what to expect.

A workable decision rule is to repair when three things are true together: the vessel is dry and sound, the failed part is a standard replacement, and the repair does not need work that you would be doing again within the next year or two if the cylinder were replaced. Replace when the vessel is leaking or corroded, when the part is no longer available, or when the same cylinder has needed two unrelated repairs close together and the rest of it is showing the same wear.

There is one more factor. A cylinder swap carries a regulation burden that a part replacement does not. Fitting a new unvented cylinder falls under Part G of the Building Regulations, is notifiable work and calls for an installer with the skills for it, so it is a bigger job than a repair. That is a reason to be sure of the diagnosis. It is not a reason to keep repairing a vessel that has failed.

What to ask before agreeing to either

Whichever way the finding goes, a fair diagnosis can be written down. Ask which part is at fault, how that was established and what evidence there is, such as a photograph of the leak source or a meter reading from the element. Ask whether the cylinder itself is sound and what that is based on.

If replacement is recommended, ask why repair is not enough, which parts would be replaced with the cylinder (the valve group, tundish and expansion vessel are usually renewed together on an unvented swap) and what the old cylinder showed when it came out. The cylinder replacement quote checklist lists the lines a quote should carry, and what happens on replacement day explains the sequence once the decision is made. For a like-for-like swap, the like-for-like cylinder replacement page sets out the job.