How Long Do Immersion Heaters Last? Repair or Replace

What decides how long an immersion heater lasts

Four things account for nearly every early failure, and only one of them is the quality of the part.

Water hardness comes first. Hard water drops limescale on any surface that heats it, and the hottest surface in the cylinder is the element. A crust of scale acts as insulation, the metal underneath runs hotter than it was designed to, and eventually the sheath cracks or the heating wire inside fails. London is a hard water city across most of its area, which is why elements here have a reputation for short lives.

The sheath metal is second. Plain copper elements are the cheapest and the least tolerant of scale. Incoloy and titanium sheathed elements handle it much better and generally last longer in the same water. If yours has been replaced before, it is worth knowing what went in. Our guide to which immersion heater do I need explains the differences.

Running the element dry, even briefly, is third. An element that is not fully covered by water can burn out within minutes. That happens when an immersion is switched on during a drain down, after a cylinder has been partly emptied, or in a vented system where the loft tank has stopped filling.

How hard it works is the last. An immersion that is the only heat source, running every day on a flat with no gas, simply does more hours than one kept as a backup for a gas boiler. More heating hours in hard water means more scale and a shorter life.

The three parts that fail, and which one has gone

An immersion heater is really three parts working together, and they wear out independently. Treating any failure as a dead element is how people end up paying for parts they did not need.

The element is the heating rod itself. When it fails it usually goes open circuit, which means no heat at all, or it breaks down to earth, which trips the circuit breaker or the RCD when it switches on. A leak around the head is a different fault again, covered in immersion heater leaking.

The thermostat is the rod that sits in the centre of the element and switches it off at the set temperature. When it fails open, the element never gets power and the water stays cold. When it fails closed, the element keeps heating past the set point until the cut-out stops it.

The cut-out is the safety device that shuts the element off if the water gets far too hot. Once it trips it usually has to be reset by hand. A cut-out that trips once may be a one off; one that keeps tripping means the thermostat is not doing its job.

• Water stays cold, switch light on, breaker not tripping: element or thermostat open circuit, or cut-out tripped
• Breaker or RCD trips when the immersion switches on: element breaking down to earth
• Water scalding hot, then cold after a while: thermostat stuck, cut-out tripping
• Heats, but takes far longer than it used to: heavy scale on the element or in the tank

A tripped cut-out can be reset, but it tripped for a reason. Resetting it again and again without finding out why is how a cylinder gets boiled. Our guide on how to reset an immersion heater covers what is safe to check yourself.

Signs an immersion heater is near the end

Most elements give some warning. The one people notice first is time: a cylinder that used to be hot in an hour or so now needs the immersion on for much longer to reach the same temperature, because scale is standing between the element and the water.

Noise is the second. A crust of scale traps small pockets of water against the hot element, and they flash into steam and collapse, which you hear as crackling, fizzing or knocking from the cylinder while the immersion is on. The same scale is often what you see as grit or flakes in the hot water.

The third is the electrics. An element that has started to break down will sometimes trip the RCD only on a damp day or only after it has been running a while, long before it fails completely. That is worth having tested rather than living with, because the breaker is telling you something real.

Repair or replace: element, or the whole cylinder?

For a single failure in a sound cylinder, replacing the failed part is almost always the right answer. An element and thermostat change is routine work, and on a vented cylinder it is usually done in one visit once the water level has been lowered below the boss.

The picture changes when failures repeat. If the cylinder is on its second or third element in a short time, the scale and sediment that killed the last one are still in the tank and will do the same to the next. A better sheath metal buys time, but it does not remove what is already inside.

It also changes when the problem is the cylinder rather than the immersion. Green or white crusting around the boss, damp that returns after a new washer, or a pinhole in the copper near the element are faults in the cylinder wall, and no new element will cure them. That is when the useful question becomes: is it worth repairing an old cylinder at all? Our guide on signs your cylinder needs replacing sets out what to look for.

On an unvented cylinder the immersion is part of the safety design, and the replacement has to be the element the maker specifies, fitted by someone qualified to work on unvented systems. That is also a good moment to have the other safety controls checked, since the cylinder is already drained.

• One failure, cylinder sound: replace the failed part
• Repeat element failures in a few years: look at scale and the tank, not just the element
• Leaking or corroded boss, pinholes in the copper: the cylinder is the problem
• Unvented cylinder: maker's part only, fitted by a qualified engineer

How to make the next one last longer

Choose the sheath metal for the water. In most of London that means Incoloy or titanium rather than copper when the element is replaced. The extra cost is small next to a repeat visit.

Keep the stored water at a sensible temperature. Around 60 degrees is hot enough to control Legionella without driving scale formation as hard as a thermostat turned right up. Our guide on what temperature should a hot water cylinder be explains why that figure is used.

Never switch the immersion on when the cylinder might not be full. If the water has been off, the system drained, or a vented system has stopped filling from the loft, run a hot tap until the flow is steady first.

Deal with the scale, not only the element. A scale reducer on the incoming cold supply, or a softener where it makes sense, slows the build up for every hot surface in the house. Our guide to hard water and hot water cylinders in London covers what actually helps.

Get a failed immersion tested

If the water is cold right now, the step by step checks in immersion heater not working are the place to start. When you call, tell us what happens: whether the switch light comes on, whether the breaker trips, and whether the cylinder is vented or unvented. An immersion heater repair visit tests the element, the thermostat and the cut-out on a meter before anything is replaced, so you only pay for the part that has actually gone. If the cylinder itself is the problem, we will say so plainly and explain the options.

Call 020 4652 5111. Our rates are on the pricing page and are agreed before we attend.