Immersion Heater Repair and Replacement

What an immersion heater actually is, and what fails inside it

An immersion heater is an electric heating element screwed into a threaded boss on your hot water cylinder, sitting directly in the stored water. Most domestic elements are 3kW and are either 11 inches long for a top boss or around 27 inches for a bottom boss that heats the whole tank. Under the round plastic cap on the head you will find a terminal block where the flex lands, a rod thermostat pushed down a dry pocket in the centre of the element, and a thermal cut-out, which is a safety device that kills the supply if the water gets past roughly 85 degrees.

The element is a resistance wire packed in magnesium oxide inside a metal sheath. When it fails it either goes open circuit, meaning current stops flowing and nothing heats, or the insulation inside the sheath breaks down and it starts leaking current to earth, which trips your RCD. Neither is fixable. The sheath is crimped and sealed at the factory and no part of it can be opened up.

The thermostat is a rod that expands with temperature and opens a set of contacts when the water reaches the dial setting. It fails either by sticking open, so the element never gets switched on and you get no heat at all, or by sticking closed, so the element never switches off and the water carries on climbing until the cut-out has to intervene. The cut-out itself is a one-shot device with a small reset button, and it is meant to trip, so a tripped cut-out is a symptom rather than the fault.

• Element open circuit: no heat, no noise, no trip, switch light may still glow
• Element leaking to earth: the RCD or RCBO trips the moment you switch the immersion on
• Thermostat stuck open: no heat, but the element itself tests fine
• Thermostat stuck closed: water far too hot, cut-out trips, tundish may discharge on an unvented cylinder
• Burnt terminals at the head: intermittent heat, smell of hot plastic, discoloured flex
• Perished head gasket: water weeping from the boss down the side of the cylinder

An element must never be switched on when the cylinder is empty or only part full. Running one dry destroys it in seconds and can distort the boss. If the cylinder has been drained for any reason, make sure the immersion switch is off and stays off until the tank is refilled and vented.

Immersion heater not working: no hot water at all

If the immersion switch is on and the water is stone cold hours later, work backwards along the supply before assuming the element. An immersion is normally fed from its own circuit at the consumer unit through a double pole switch, often with a heat resisting flex from the switch down to the element head. A tripped breaker, a blown fuse in a spur, or a switch whose contacts have burnt out will all give you exactly the same cold tank that a dead element does.

Once power at the head is confirmed, the element gets tested two ways. Continuity across the two element terminals should read somewhere close to 18 ohms for a 3kW element on a 230 volt supply, because resistance is voltage squared divided by wattage. A reading in the high teens means the element is intact. An open circuit, where the meter reads nothing at all, means the element has gone and needs replacing.

The second test is insulation resistance between either terminal and the element body, done with an insulation tester rather than a multimeter. A healthy element reads effectively infinite. A failing one reads in the low megohms or worse, and that is the element that has been tripping your RCD every time you flick the switch on. An element can pass continuity and still fail insulation, which is why both tests get done rather than one.

If the element passes both tests, the fault is upstream of it: thermostat, cut-out or wiring. That is the good outcome, because those parts come out of the head without touching the water in the tank.

If the same cylinder is also heated by a boiler coil and neither source is getting it hot, the element is unlikely to be the whole story. A tank that stays cold on both sources points at the controls rather than the immersion, and the walk through on a hot water tank not heating covers the motorised valve, cylinder thermostat and wiring centre side of that.

The immersion trips the electrics as soon as you switch it on

A breaker that trips instantly on switch-on is the classic signature of an element that has failed to earth. Water has found its way into the magnesium oxide packing inside the sheath, usually through a pinhole caused by corrosion or by a hot spot under heavy scale, and the earth leakage is enough for the RCD to disconnect. Once that has happened the element is finished, and no amount of drying it out brings it back.

Do not keep resetting the breaker. Each reset sends fault current through the earth path, which on older installations with borderline earthing is exactly the condition that puts a voltage somewhere it should not be. Switch the immersion off at its own switch and leave the breaker off.

Occasionally a trip is not the element at all. Water tracking down the inside of the terminal cap, usually from a weeping gasket or from a leak higher up the cylinder dripping onto the head, will trip an RCD just as convincingly. So will a flex that has been pinched against the boss and worn through. Those are repairable without a new element, which is another reason for the insulation test before anything is condemned.

If the immersion trips the whole consumer unit rather than just its own breaker, stop using it and leave it isolated. That points to an earthing or RCD arrangement that needs attention in its own right, not just a new element.

Water heats but never gets properly hot, or runs out quickly

Lukewarm water that never reaches a proper temperature is usually a thermostat that has drifted low rather than a failed element. Immersion thermostats are adjusted with a small screw or dial under the cap, and the sensible setting for a domestic cylinder is around 60 degrees. That is hot enough to keep the stored water safe and cool enough not to scald or to scale the element unnecessarily. Thermostats that have been in service for years commonly drift well below their marked setting, so the water tops out at 45 degrees and every bath feels short.

Before assuming a fault at all, it is worth establishing what normal looks like on your own cylinder. How long does hot water take to heat up is one of the questions we are asked most often, and the answer is set by the rating of the element, the volume of the store and how cold the water was when the cycle started, not by how long it used to take when the system was new. A cylinder that has slowed down gradually over several winters is telling you about scale on the element rather than about the element having failed, and that is a different repair with a different urgency.

Running out of hot water quickly with a top-mounted element is different and is often not a fault at all. A short element sitting in the top boss only heats the water above it, which on a typical cylinder is a third of the contents. That is deliberate: it is the boost element, designed to give you enough for a wash without paying to heat the whole tank. If your cylinder has two elements, the lower one is the one that heats the full volume, and on an Economy 7 setup it is wired to the night rate. Households often discover that the lower element failed months ago and they have been living off the boost without realising.

• Lukewarm only: thermostat drifted low or stuck, check the setting before changing parts
• Hot but very little of it: top boost element working, lower element likely failed
• Hot then cold within minutes: cylinder volume reduced by scale, or a hot water draw-off fault
• Scalding hot and no control: thermostat welded closed, isolate and get it changed

Why immersion elements fail faster in London

Most of London sits on a hard water supply drawn from chalk aquifers, and hard water is the single biggest reason immersion elements here do not last as long as they do in the north and west of the country. The calcium in the water stays dissolved while the water is cold, but as soon as it touches a surface above roughly 60 degrees it comes out of solution as limescale. The hottest surface in your cylinder by a wide margin is the element sheath, so that is where the scale forms first and thickest.

Scale on an element is not just untidy, it is an insulating blanket. The element is designed to shed its heat straight into the water, and a scale layer stops it doing that. The resistance wire inside carries on producing the same 3kW, the heat cannot escape at the same rate, and the core temperature climbs. Eventually the sheath develops a hot spot, the packing degrades, and the element either goes open circuit or starts leaking to earth. That is the whole life story of a scaled element, and it is why a London immersion often fails years earlier than the same part in a soft water area.

You can usually hear it coming. A scaling element makes a distinctive kettling or crackling noise as it heats, caused by water flashing to steam in the gaps under the scale and collapsing again. A cylinder that has started to sound like a kettle in the airing cupboard is telling you the element is furred. Scale also breaks off in flakes and settles in the base of the cylinder, which is why draining an old London tank often produces a startling amount of grit.

The element is only the first casualty. Hard water leaves the same deposit on the boiler coil and in the base of the tank, which is why our longer read on hard water and hot water cylinders in London is worth ten minutes if the element you are replacing is not the first one.

Immersion heater repair or immersion heater replacement

Repair and replacement mean different things depending on which part has failed, and it is worth being precise about it because the two jobs are not comparable in effort.

If the thermostat or the cut-out has failed, the work is a repair in the ordinary sense. The cap comes off, the flex is disconnected, the old rod stat slides out of its pocket and a new one goes in. The cylinder stays full, the water stays where it is, and the whole thing is done in well under an hour. If the terminals have burnt or the flex has cooked, the same applies: new flex, new connections, everything else stays put.

If the element itself has failed, there is no repair. The element has to come out, which means isolating the cylinder, draining it down to below the level of the boss, unscrewing the old element with an immersion spanner and fitting a new one with a new gasket. That is a longer job and it carries the one real risk in immersion work, which is a seized element.

Elements that have been in a hard water cylinder for fifteen years do not always come out politely. Scale and corrosion effectively weld the element thread into the boss, and the force needed to break that seal can distort the boss or, on a thin walled cylinder, tear it. An experienced engineer can usually feel the difference between an element that is tight and one that is about to take the cylinder with it, and will stop rather than keep pulling. If a boss has gone beyond that point, the honest answer is a new cylinder rather than a new element, and it is better to hear that before the work starts than halfway through.

Anyone who quotes an immersion swap on an old cylinder without mentioning the possibility of a seized boss has not done many of them. It is a normal outcome on tanks of a certain age and the sensible thing is to plan for it rather than be surprised by it.

What an immersion heater replacement involves and how long it takes

A straightforward element swap on an accessible vented cylinder takes an engineer somewhere in the region of an hour to two hours, and most of that is water rather than spanner work. The cylinder is isolated electrically and proved dead, the cold feed is closed, and the tank is drained through the drain cock at the base until the level is below the boss. On a full cylinder that draining is the slow part, and on a cylinder with a corroded or missing drain cock it is slower still.

With the level down, the flex is disconnected and labelled, the old element is broken loose with a proper immersion spanner rather than anything improvised, and the boss face is cleaned back to bare metal. The new element goes in with a new fibre gasket, tightened to seal rather than to crush. The tank is refilled, vented, and checked for weeping at the boss before any power goes near it, because a wet element head is exactly what you do not want to energise.

Only then does the element get reconnected, the thermostat set, and the whole thing switched on and watched through a full heating cycle to confirm it heats, cuts out at temperature and does not trip anything. Skipping that last part is how an engineer leaves a job that fails the same evening.

On an unvented cylinder the sequence is similar but the isolation is more involved, because the tank is under mains pressure and has its own set of controls that have to be dealt with properly. On a sealed foam-lagged unvented cylinder the boss is also less forgiving, and the manufacturer will often specify a particular element length and material. That is not a job to improvise parts on. Where the element turns out to be one of several things wrong on a sealed tank, the unvented cylinder repair page sets out what else on that kind of cylinder is serviceable and what is not.

The rate is fixed before we attend and the total depends on how long the job takes, so a thermostat change and a full element swap on a stubborn old tank are not the same visit and should not be presented as if they were.

Choosing the right replacement element

Replacement elements are not all the same, and the two things that matter in a hard water area are the sheath material and the length.

Copper sheathed elements are the cheapest and are perfectly adequate in soft water. In London they scale quickly and they corrode, and a copper element in a hard water cylinder can have a disappointingly short life. Incoloy sheathed elements, sometimes described as titanium clad by merchants, use a nickel iron chromium alloy that tolerates the hot spots under scale far better and resists corrosion in aggressive water. For a London cylinder that is the sensible default, and the price difference between the two is trivial compared with the cost of doing the job twice.

Low watt density elements spread the same power over a longer heated surface so the sheath runs cooler for a given output. Cooler sheath means slower scale formation, which means longer life. On a cylinder that has already eaten one element early, that is the upgrade worth making.

Length matters because the element must sit in the water, not above it, and must not foul the coil, the baffle or the base of the tank. An element that is too long can touch the far wall and short against it. Too short and it heats a smaller volume than the cylinder was designed to deliver. The old element is the reference, and on a cylinder with two bosses both should be checked, because they are rarely the same length.

• Incoloy sheath rather than copper for any London cylinder
• Low watt density where scale has already claimed one element
• Length matched to the boss and the tank, measured rather than guessed
• New fibre gasket every time, never the old one turned over
• Heat resisting flex if the existing one is hardened or discoloured

What to check yourself before calling an engineer

There are a few things worth checking first, and any engineer who tells you otherwise is not being straight with you. None of them involve taking a cover off anything live, and the symptom guide on an immersion heater not working runs through them in the order an engineer would.

Start with the switch. Immersion switches are usually a large double pole switch near the airing cupboard, often with a neon. If the neon is dark, either the switch is not getting power or the neon itself has failed, which happens. Then look at the consumer unit for a tripped breaker, at any fused spur feeding the circuit, and at the timer or programmer if the cylinder is also heated by the boiler, since a hot water channel switched off after a power cut costs nothing to put right.

If the property has an Economy 7 or off-peak arrangement, the lower element may only be live during the night rate window. No hot water at four in the afternoon on that setup is not necessarily a fault, and switching the boost on will tell you whether the top element is alive.

Do not remove the plastic cap on the element head to press the cut-out reset button unless the circuit is fully isolated and proved dead. The terminals inside are live at mains voltage and sit within easy reach of fingers. A cut-out that has tripped has tripped for a reason, and resetting it without finding out why simply sets the same fault up to happen again.

What happens on an immersion heater repair visit

The first job is establishing whether the fault is electrical supply, element, or control, because the answer determines everything after it. That means proving dead, opening the head, and testing rather than guessing. Continuity and insulation resistance on the element, continuity across the thermostat contacts, and a check of the cut-out and the terminal condition.

If the element is good and a control has failed, the part is changed and the cylinder is never drained. If the element has gone, the cylinder is drained and the element replaced, with an honest conversation first about the condition of the boss on an older tank.

While the head is open there are things worth checking anyway, because they are why many elements fail early: the thermostat setting, in case the tank has been run hotter than it needs; the condition of the flex and terminals, since burnt connections at the head are a fire risk rather than an inconvenience; and the state of the water that comes out during draining, because a bucket of scale flakes tells you what the next element is going to face.

The last step is always a full heating cycle with the engineer still there. Switch on, confirm current draw, confirm the tank heats, confirm the thermostat cuts out at temperature, confirm nothing trips and nothing weeps at the boss. Our engineers are qualified for the work described and will tell you plainly if what they find changes the picture.

When the cylinder is the real problem, not the element

Sometimes the honest answer is that the element is not worth replacing, and it is better to know that than to keep spending on a tank that is finished.

Repeated element failures in a short space of time usually mean the cylinder is heavily scaled and the new element is being killed by the same conditions that killed the last one. A tank in that state may be better descaled or replaced than fitted with a third element.

Corrosion around the boss, weeping that returns after a new gasket, or a boss that has already been strained by a previous removal all mean the joint is no longer reliable. A boss that leaks is not a small problem when it is at the bottom of a full cylinder in an airing cupboard above a bedroom.

When it is the vessel rather than the element that is finished, the conversation moves to hot water cylinder replacement, and the signs your cylinder needs replacing are usually visible for a year or more before anything comes through a ceiling. Knowing which of the two you are looking at is the point of testing before anyone starts quoting.

A smaller group of cupboards turn out not to contain a conventional immersion arrangement at all. Combined electric units of the Pulsacoil family store heating water and make domestic hot water on demand as it passes through, so the element in them is doing a different job and the fault finding follows a different route. Pulsacoil repair in London is its own piece of work with its own parts and its own quirks, so it is worth saying which unit you have when you book rather than having an engineer arrive equipped for a standard cylinder.

None of that means an element swap is wasted work in most cases. The majority of immersion faults are exactly what they look like: a part that has done its job for years and has come to the end of it, replaced in a morning, and the hot water back on the same day.