Hard Water and Hot Water Cylinders in London: What Scale Actually Does

Why London water does this and softer water does not

The hardness in London water is mostly temporary hardness, which is calcium bicarbonate. It is stable while the water is cold, which is why a cold pipe run is still clean after thirty years. Heat is what breaks it. Warm the water and the bicarbonate decomposes, carbon dioxide is driven off, and what is left is calcium carbonate, which will not stay dissolved. It precipitates onto whatever surface is doing the heating.

The reaction is slow at low temperatures and gets noticeably faster past about sixty degrees, which is why the temperature section further down matters more than anything you can buy. A house on soft water genuinely does not have this problem: the same cylinder, element and boiler simply last longer there. That is geology rather than a maintenance failing, and worth knowing so that an engineer telling you an eight year old element has scaled up is not telling you a story.

What limescale does to an immersion heater

An immersion element is a metal sheath with a resistance wire inside, and it works by getting hot and passing that heat outward into the water. Scale forms on the sheath because the sheath is the hottest thing in the cylinder, and once there it acts as insulation. The heat produced inside the element is unchanged, but it can no longer escape at the same rate, so the element runs hotter and hotter behind its own deposit. That is what kills it. The wire is not designed to sit at that temperature indefinitely and eventually fails, usually as an open circuit, sometimes to earth, which trips the breaker. Whether that ends as an immersion heater repair or as a straight element replacement is decided by what the test at the terminals shows rather than by the age of the cylinder.

Scale that cracks off does not vanish. It drops to the bottom of the cylinder as grit and slabs, and in an older cylinder the element ends up half buried in it. That layer is why a cylinder can still take too long to heat after a new element is fitted, and it is the first thing to check when a replacement does not restore the performance the customer remembers. Where the tank never gets hot at all rather than slowly, that is a different diagnosis, and our page on a hot water tank not heating works through the electrical and control causes in order.

• Longer reheat times year on year with no other change to the system.
• Ticking or popping from the cylinder while the immersion is running.
• An element that trips the breaker the moment it is switched on, which usually means it has failed to earth.
• White or grey flakes in the bath after the cylinder has been drained and refilled.

What it does to the boiler coil, and why reheat times creep up

In a cylinder heated by a boiler, primary water runs through a coil inside the cylinder and never mixes with the water you wash in. From the stored water side, the outside of that coil is the hot surface, so carbonate settles on it exactly as it settles on an element. A coil buried in scale transfers less heat for the same flow of primary water. The boiler runs longer for the same tank, the return temperature climbs, and on a condensing boiler that alone costs efficiency because it stops condensing properly. Customers notice it as a cylinder that used to reheat in a comfortable half hour and now wants an hour. If you are not sure whether yours has drifted, knowing how long hot water should take to heat up for your size of cylinder is the quickest way to judge it, because the comparison is against a normal figure rather than against memory.

There is a second mechanism worth separating out so nobody spends money in the wrong place. Black iron sludge from the heating side fouls the inside of the same coil and produces the same slow reheat. Scale is white or buff and sits on the outside of the coil, sludge is black and sits inside it, and the fix for one does nothing for the other. An engineer should tell you which they think it is, and why, before anything is proposed.

Why scale makes relief valves drip into the tundish

On an unvented cylinder, several safety components seal by pressing a soft seat against a machined face, and both surfaces spend their lives in hot hard water. Once a thin crust of carbonate builds on the seat, the valve can still open but can no longer close cleanly, so you get a trickle rather than a dramatic failure. It appears in the tundish, the small open funnel in the pipework near the cylinder, and is usually mistaken for a fault that has just arrived when it is really years of water quality showing itself. A dripping tundish has its own diagnosis and scale on a valve seat is only one of the possible causes, so the cause should be proved rather than assumed.

The other components hard water reaches are the ones you touch daily. Tap aerators and shower heads block from the inside, thermostatic shower cartridges seize on their sliding parts, and flexible hoses fur until flow through them drops noticeably. None of those are cylinder faults, and they are worth ruling out before anyone opens a cupboard, because a blocked shower head is a five minute job you can do yourself.

Water running through the tundish is a safety valve telling you it has operated. Scale is a common cause but not the only one, and the pipe must never be capped or plumbed into a drain to hide it. Get the cause found.

What temperature to set a cylinder in a hard water area

Stored water temperature is the lever. Carbonate comes out of solution faster the hotter the surface is, so a cylinder run at the top of the thermostat range scales dramatically faster than one run sensibly, for no benefit you can feel at the tap.

The right setting for a stored cylinder is sixty degrees. That is not arbitrary and it is not a target to beat. Store below it and you move into the range where legionella bacteria can multiply in stored water, which is why the figure exists. Store well above it and you are paying to make limescale faster while creating a real scalding risk at the taps. So the advice is narrow: set it to sixty, leave it there, and if a previous occupant has wound the stat to maximum chasing a hotter bath, wind it back. If the water then feels too hot at the basin, the answer is a thermostatic blending valve on the outlets, not a colder cylinder.

The second free change is to stop the cylinder reheating when nobody needs it. A cylinder held at temperature around the clock is scaling around the clock.

• Set the cylinder thermostat to sixty degrees and leave it alone.
• Never set stored water below sixty to slow scale. That trade is not one to make.
• Use the timer rather than leaving the cylinder on permanently.
• On a secondary return in a flat, run the pump on a time clock rather than continuously.

Does a water softener protect a hot water cylinder

Yes, and it is the only thing on the market that stops new scale forming rather than slowing it slightly. An ion exchange softener passes incoming water through a resin bed that swaps calcium and magnesium for sodium, and sodium salts stay in solution when heated. Nothing precipitates, so nothing furs up downstream.

It is a real installation rather than a gadget. The unit sits on the rising main after the kitchen cold tap has been teed off, because at least one hard drinking water tap should be kept unsoftened. It needs a drain for regeneration waste, space, and salt topping up. Softened water also works gently on existing deposit, so a system furred for fifteen years cleans up slowly rather than instantly. Worth saying plainly: softeners treat hardness only, they do not filter anything.

Do magnetic scale reducers and inline cartridges work

Inline phosphate dosing units, the small cartridges fitted to the cold feed, do have a real effect. They do not remove hardness. They interfere with how the carbonate crystals form so they are less inclined to bond hard to hot surfaces, which is why they are widely fitted to protect boilers and heat exchangers. The cartridge is a consumable and does nothing once it is spent.

Electromagnetic and magnetic conditioners are the ones to be careful with. The claims are large, the independent evidence is thin and inconsistent, and engineers regularly find heavily scaled elements on systems that have had one fitted for years. They are not a substitute for a softener and should not be sold as one.

Two things that definitely do not work: pouring descaler down a tap hoping it reaches the cylinder, and turning the thermostat up because the water feels cooler. The first never arrives in any useful concentration, and the second accelerates the exact process causing the problem.

How to tell how scaled your own system is

Some of this you can check yourself in ten minutes, and it tells you whether you are dealing with scale or something else entirely. Look inside the kettle: whatever is caked in there is a fair sample of what is happening in the cylinder, because it is the same water and the same reaction. Unscrew the shower head and look through it against the light. Then compare how long the cylinder now takes to reach temperature with what you remember, because the change over the years is the most reliable indicator there is. One symptom that scale rarely explains on its own is no hot water but the heating works, which usually points at a control or a motorised valve rather than at the deposit inside the vessel.

• Kettle furred solid: the cylinder is in the same water and doing the same thing.
• Reheat noticeably longer than a few years ago: scale on the coil or the element.
• Hot side flow poor while the cold side is fine: blocked outlets and hoses rather than the cylinder.
• Dripping from the tundish that began with no other change: a scaled relief valve seat is a likely cause.

Descale, replace the element or replace the cylinder

Where the problem is a scaled or failed immersion element, replacing it is the straightforward repair, and it is the moment the loose deposit in the base can be drawn out as well, because the immersion boss is the only sensible access. Doing both together is the difference between restoring performance and putting a clean element into a bucket of grit.

Where the coil is the problem the calculation changes. A heavily scaled coil can sometimes be chemically cleaned, but access is limited and the result is not always worth the disruption. Against a cylinder already well into its second decade, replacement is often the more honest answer, and an engineer should say so rather than sell a clean that buys eighteen months. Where the cylinder body itself has started to weep, repair is not on the table and any sealant sold as a fix for a hot water vessel should be declined. The other signs a cylinder needs replacing are worth checking against yours before money is spent on the scale alone, because scale is rarely the only thing that has aged inside an older unit.

One point specific to unvented cylinders: they are sealed and pressurised, so the safety components are not optional and the annual check exists to prove they still work. Hard water is precisely what stops those components working, which is why skipping that service in London is a worse idea than skipping it elsewhere. Work on the sealed side is notifiable and must be done by an engineer qualified for it. An unvented cylinder service is that annual check, and on hard water it is the appointment that decides whether the safety components are still capable of doing their job.

The rate is fixed before we attend and the total depends on how long the job takes. What decides the cost of a scale job is access and how much comes out of the base, not the brand on the jacket.

Why scale hits harder in London flats and conversions

The building stock plays a part too. Victorian conversions tend to have long distribution runs and old pipework, so there is more surface for deposit and more chance of a partially blocked run masquerading as a cylinder fault. Flats above shops and purpose built blocks often run secondary circulation, which keeps hot water moving continuously and gives the reaction far more opportunity than a house where the cylinder sits still overnight. Cylinders in these properties also live in cramped cupboards, which is why base deposit is so often left in place during an element swap. It is still worth insisting on, because it decides whether the new element lasts.

The brand on the jacket makes less difference than people expect. A well specified unvented cylinder from any established maker, a Megaflo for instance, still has an element and still has valve seats, and those are the parts London water attacks. What changes the outcome is storage temperature, whether the water is treated, and whether the annual check actually happens. Those three do more to set how long a hot water cylinder lasts in London than anything printed on the jacket.