Signs Your Hot Water Cylinder Needs Replacing

Is a leaking hot water cylinder worth repairing?

If water is coming from the body of the cylinder rather than from something screwed or soldered into it, the cylinder is finished. A copper vented cylinder pinholes, usually low on the shell where sediment has sat, and the first sign is a damp patch that comes back within a day of being wiped dry. A stainless unvented cylinder tends to go at a weld seam or around the immersion heater boss, and because the vessel is wrapped in sprayed foam the water tracks down inside the casing and appears at the base as if it came from a pipe. Copper that has gone at one point has thinned across a wide area, so patching buys weeks at best, and on an unvented cylinder there is no legitimate repair to the vessel at all because it is a sealed pressure vessel.

It is worth working out which it is, because several leaks that look terminal are not, and tracing where a leaking hot water cylinder is actually losing water is a job you can do yourself in an hour. Water at the immersion heater is usually the fibre gasket under the element. Water at a compression fitting on the cold feed or hot draw off is a fitting. Water running from the tundish and out through the discharge pipe is a valve, either doing its job or failed. A wet floor tells you nothing on its own. Dry everything, put kitchen roll at each candidate point, and come back in an hour. Whichever paper is wet is where you are actually leaking from. A tundish dripping intermittently belongs in the second group rather than the first, because it is a valve either doing its job or failing at it, and the vessel behind it may be perfectly sound.

If an unvented cylinder is leaking from the vessel, turn off the cold supply to it and shut down the immersion and the boiler feed before you do anything else. A leaking pressure vessel with a heat source still firing into it is the version of this that becomes dangerous rather than just wet.

How old is too old for a hot water cylinder?

There is no honest single number for when a cylinder should be replaced, which is why the question of how long a hot water cylinder lasts only ever gets answered in ranges. A well made copper cylinder in a soft water area, lightly used, can sit in a cupboard for decades and still be sound. A stainless unvented cylinder in hard water, feeding a busy household and heating twice a day, can be worn out in a fraction of that. Age is a probability, not a diagnosis.

What age does is stack the odds. The sacrificial anode fitted to some unvented cylinders is consumed over time, and once it is gone the vessel corrodes instead of the anode. Older foam insulation slumps. Immersion bosses corrode where dissimilar metals meet. Coils and elements accumulate scale that never comes off on its own. Find the data plate anyway, usually on the front of the casing with the model, capacity and year, because it tells you whether spare parts are still made for that unit. A cylinder nobody stocks a thermostat or valve for has effectively made the decision for you.

Corrosion and the marks that give it away

On a copper cylinder, look for green or blue staining on the shell and around fittings. That is verdigris, and it means water has been present long enough to react with the copper. White crusty deposit at a joint is the same story with the mineral left behind after the water evaporated. Neither proves the vessel is failing, but both mean something has been weeping and should be traced rather than wiped.

On a foam lagged unvented cylinder you cannot see the vessel, so you read the casing instead. Rust streaks below the immersion boss, a rippled or discoloured patch of casing, or foam that feels damp when pressed all point at water inside the jacket. Brown hot water with clear cold water is the other quiet sign, because the discolouration is coming from inside the hot side rather than from the main.

• Green or blue staining on copper, especially low on the shell.
• White mineral crust at joints and around the immersion boss.
• Rust streaks down the casing of a foam lagged cylinder.
• Foam that feels damp, or a cupboard that smells of damp.
• A damp patch that returns within a day of being dried.

Scale, slow reheats and why London water shortens a cylinder

Most of London is supplied with hard water, and what that does to a hot water cylinder over its life has a direct effect on how long it stays useful. Every time water is heated, calcium carbonate drops out of solution and sticks to the hottest surface it can find. In an indirect cylinder that surface is the coil the boiler heats through. In a direct cylinder it is the immersion element.

Scale on a coil is an insulator. The boiler still sends hot water through it, but less heat gets into the tank, so the cylinder takes longer and longer to recover. Before blaming the tank, it is worth knowing how long hot water should take to heat up on a healthy system, because the gap between that and what yours does now is the measurement that matters. If a full reheat used to take about an hour and now takes most of an afternoon, and the boiler itself has been checked and is fine, a furred coil is the usual explanation. There is no practical way to clean the inside face of a coil in a fitted cylinder, and one that has lost most of its heat transfer is not coming back.

Scale on an immersion element is more obvious. The element runs hotter because it cannot shed heat into the water, the scale layer cracks off in flakes, and eventually the element fails, often taking out the breaker with it. Replacing an element in a heavily scaled cylinder is reasonable once. Doing it every year or two is the cylinder telling you what the inside of it looks like.

Repair or replace: when more parts stop being worth it

Cylinders rarely die in one go. They wear out through a run of small faults, and what decides repair against replacement is not how many parts have failed but why. A thermostat that failed because thermostats eventually fail is a part. A thermostat that failed because its pocket is packed with scale will fail again, and so will the next one.

The same logic runs across the unit. A pressure relief valve that passes because grit from the tank lodged on the seat will pass again once the new one meets the same grit. An element that burns out in a scaled tank burns out again. A motorised valve is a heating control rather than part of the cylinder and should not be counted against it at all.

The question to put to whoever is looking at it is simple: is this a sound vessel with a worn component on it, or a worn vessel making its components fail? An engineer who has drained it, seen what came out and looked at the immersion boss can answer that, and on an unvented cylinder the annual service is where most of that evidence surfaces anyway. If it is the second, more parts is money going in the wrong direction. That is the judgement an unvented cylinder repair turns on as well: a valve or a thermostat renewed on a sound vessel is money well spent, and the same part renewed on a vessel that keeps destroying it is not.

We do not quote from a description over the phone. What a repair or a replacement comes to depends on what is found, how long it takes and what has to be altered around it. The rate is fixed before we attend, so nothing moves once the job has started.

Insulation and what a cylinder costs you standing still

Every cylinder loses heat while it sits there doing nothing, and how much depends largely on when it was made. An older copper cylinder with a separate quilted jacket strapped round it was insulated as an afterthought, and where that jacket has slipped or been cut around a pipe the loss from the bare area is real. A modern cylinder is foamed at the factory, and the difference in standing loss between the two is not marginal.

The test costs nothing. Put a hand flat on the cylinder an hour or two after it has finished heating. It should feel barely warm through the insulation. If it is properly warm, or the cupboard is noticeably hot, you are paying to heat that cupboard every day of the year. Poor insulation alone is not a reason to scrap a sound cylinder, and an extra jacket over an old one is a cheap and sensible thing to do. It does tip a decision that is already close.

The wrong size or the wrong type for the house now

A cylinder can be in perfect condition and still be the wrong answer. Households change and cylinders do not. If a second bathroom went in, or the shower was upgraded to something with real flow, or there are more people in the house than there were, a cylinder correctly sized ten years ago now runs cold halfway through the second shower. Nothing is broken. It is too small, and working out what size the household actually needs now is a separate exercise worth doing before anyone quotes for a replacement. The opposite costs money too: a large cylinder in a flat with one occupant is reheated to maintain a volume nobody draws.

Type is the other half of it. A vented cylinder fed from a loft tank delivers hot water at whatever pressure the height of that tank gives you, which is why the hot side of a mixer can feel weak against the cold. An unvented cylinder, of the kind made by Megaflo, Gledhill, Telford and others, works at mains pressure and gives balanced hot and cold at the outlets, at the cost of needing the mains flow to support it and an annual service to stay right. If the complaint that started all this was pressure rather than quantity, replacing like for like will not fix it. Unvented work is notifiable and has to be done by an engineer holding the unvented qualification, so confirm that before anybody starts. In practice that means a G3 qualified plumber attending, and the ticket is worth seeing rather than taking on trust.

• Hot water runs out partway through normal use: too small for the household.
• Reheats several times a day for very little draw off: larger than it needs to be.
• Weak hot pressure on a mixer with strong cold: a vented system doing what vented systems do.
• A new bathroom or a high flow shower added since the cylinder went in.

Repair or replace: the short version

Put crudely, leaks from the vessel mean replace, and everything else is a judgement about how much life is left. That judgement gets easier once you separate the vessel from the things bolted to it. Elements, thermostats, valves and gaskets are consumables. The tank is the asset. Money spent on consumables is fine. Money spent on consumables that keep failing because of the state of the tank is not. Put as a question rather than a rule: is it worth repairing an old cylinder that has already eaten two elements, or is that money better put towards the vessel itself?

Before you call anyone, spend ten minutes gathering what an engineer will ask for anyway. Find the data plate and note the make, model, capacity and year. Work out where the water is actually coming from with the kitchen roll test. Note how long a full reheat takes now against how long it used to. Check whether the cylinder is warm to the touch when it should be cool. List what has already been replaced and when. That is most of a diagnosis handed over in one go, and plenty of cylinders booked in as replacements turn out to need an element and a gasket. A hot water cylinder replacement is the right answer often enough, but it should be the conclusion that list arrives at rather than the offer that opens the conversation.

• Water from the vessel body: replace.
• Water from a fitting, element or valve: repair.
• Reheat time stretched badly with a healthy boiler: scale, and usually replace.
• The same component failing repeatedly: the vessel is making it fail.
• Warm to the touch when idle: standing loss, a factor rather than a verdict.
• Runs out or lacks pressure for the household: right condition, wrong specification.