Signs Your Hot Water Cylinder Is Failing

A hot water cylinder almost never fails without warning. It gives you months, often years, of small signs first, and the useful part is that each sign points somewhere different. Rusty hot water means one thing, running out sooner than it used to means another, and damp at the base means the clock has genuinely started. Before any of that though, it is worth establishing whether the cylinder is even involved.

That check takes a minute. Run the cold tap and compare it against the hot. If both are discoloured, the problem is in your supply rather than the cylinder. If only the hot side is affected, the cylinder or the immersion heater is where to look. We are a Gas Safe registered and OFTEC certified business based in Bearsted, and what follows is what each sign actually means, in roughly the order they turn up.

The four places a cylinder fails

A cylinder is a simple object with a small number of things that go wrong, and they tend to fail in a fairly predictable order. Knowing which part a symptom belongs to is most of the diagnosis, and whether yours is vented or unvented changes which of them are even possible.

Discoloured or rusty hot water

Brown or orange water on the hot side only is the classic early sign. On a copper cylinder it is usually not the vessel itself, because copper does not rust, so it is more often the immersion heater corroding or debris arriving from steel pipework elsewhere in the system. On a steel or enamel lined cylinder it can genuinely be the tank, and that is the more serious version.

The isolating check earns its keep here. If the cold tap runs discoloured as well, nothing is wrong with your cylinder at all and the cause is upstream, sometimes just mains work happening nearby. We have found a good number of people replace an immersion heater on the strength of brown water without ever running the cold tap alongside it first.

It runs out sooner than it used to

A cylinder that used to manage two showers and now manages one has lost usable capacity, and that is almost always scale or a control fault rather than the tank somehow shrinking. Limescale on the coil insulates it, so the boiler heats the water more slowly, and a cylinder that reheats slowly feels smaller even though it holds exactly the same amount as it always did.

The other candidate is the thermostat. If it has drifted low the cylinder is simply storing cooler water, and cooler water needs less cold blending in at the shower, so it runs out faster. Both of those are cheap to put right. Neither gets put right if nobody looks, and both get blamed on the boiler.

Noises from the airing cupboard

A cylinder that rumbles or bangs as it heats is telling you about scale. The noise is water boiling in pockets against a scaled surface, the same mechanism as kettling in a boiler, and it means the coil or the immersion element is coated. It is not dangerous in itself, but it is a fair marker of how far the scaling has already gone.

Ticking as it heats and cools is a different thing entirely and usually nothing at all. That is expansion, most often pipework clipped tightly against a joist with nowhere to move. Annoying at four in the morning, but not a fault.

Water where it should not be

Damp, staining or a slow drip at the base is the sign that changes the conversation, because a cylinder does not recover. Corrosion works from the inside out, and a pinhole is simply the first place it got all the way through. It is not the only place the wall is thin.

In our experience, replacing a cylinder that has started weeping at the base works better than patching it, because the corrosion that produced one pinhole has been working on the whole vessel just as long, and a repair buys months while a failure empties the lot through a ceiling.

Worth separating from that is water at the tundish, or dripping from a discharge pipe outside on an unvented system. That is a safety valve doing precisely what it is there for, usually because the expansion vessel has lost its charge, and it is a repair rather than a replacement. People panic about that one and shrug at damp under the tank, which is exactly the wrong way round.

The cupboard test nobody does

Put your hand on the cylinder. A modern foam lagged cylinder should be barely warm on the outside even when it is completely full of hot water. If the airing cupboard is noticeably hot, that is not a bonus airing cupboard, it is failed insulation, and you are paying to heat a cupboard every hour of every day.

On an older cylinder with a separate quilted jacket the fix can be as simple as a new jacket. On a foam lagged one the insulation is bonded to the tank, so a hot cylinder means a replacement. From working with customers across Kent, this is the sign people are most surprised by, because nothing is broken, nothing leaks and nothing has stopped working. It is just quietly expensive.

Repair or replace

There is no fixed age at which a cylinder has to go, and anybody quoting you one is guessing. What decides it is which part has failed and what the vessel is made of. A thermostat, an immersion heater, an expansion vessel or a failed jacket are all repairs, and none of them are expensive ones. The vessel itself is not a repair.

The awkward middle is a cylinder that is structurally sound but scaled and slow. It still works, it simply works a little worse every year, and whether it is worth descaling or replacing depends on what else is happening. If the boiler is coming up for replacement anyway, doing both together usually costs less than doing them separately, because a good deal of the labour overlaps.

The one case where we will say replace without much hesitation is a cylinder that is weeping. The one where we will usually say do not bother is a cylinder somebody has been told to replace purely because it is old. Age on its own is not a fault, and a copper cylinder in soft conditions can outlast several of the boilers that fed it.

If it does come to a swap, it is worth knowing that the cylinder is rarely the awkward part. Getting the old one out of an airing cupboard that was built around it is a real consideration in older houses, and so is where the new pipework runs. The system has to be drained, so there is a stretch with no hot water, and that is far better known the week before than at nine on the morning itself.

What to check before you call

Four things, and none of them need tools or take more than a couple of minutes.

• Run the cold tap alongside the hot. If both are discoloured, it is not the cylinder.
• Put your hand on the cylinder itself and on the cupboard wall beside it.
• Look at the floor around the base, and at the pipework where it enters and leaves the tank.
• If you have an immersion heater, try heating on that alone. Hot water on the immersion but not from the boiler points at the coil or the controls rather than the tank.

Cylinders give plenty of notice. Discoloured water and slow reheating are early and cheap to deal with. Noise tells you how far the scale has got. Damp at the base is the one that means the end, and it is a great deal better found in August than discovered through a ceiling in January. We cover Maidstone and the rest of Kent from Bearsted, with free home visits and free quotes across Kent. If yours is showing any of these, get in touch and we will tell you honestly whether it needs replacing or just needs attention.