Immersion Heaters Explained: Types, Wiring, and How to Pick the Right Element
What an immersion heater is, the difference between copper and Incoloy elements, why the thermostat stays at 60C, and how it should be wired on a 20A radial circuit.
Replacing a failed immersion heater sounds like a ten-minute job until you discover the plumber fitted a copper element in a hard-water area. Three years of limescale later, the rod is caked solid, the thermostat trips constantly, and you are draining a 150-litre cylinder again to swap in the element that should have gone in the first time. The part costs less than a takeaway. The wrong part costs you a day, a soaked airing cupboard, and a second call-out fee.
An immersion heater is one of the cheapest components in your hot water system and one of the easiest to get wrong. This page explains what it does, why element material and thermostat temperature matter more than brand, and how it needs to be wired so it does not cook your fuse spur.
What it is and what it's for
An immersion heater is an electric resistance heating element, much like the element in a kettle, that screws into a threaded port on a hot water storage cylinder and heats the stored water directly. "Resistance heating" simply means electricity passes through a coiled metal rod, the rod heats up, and that heat transfers straight into the surrounding water.
The element threads into a fitting called a boss, which is a reinforced socket welded or riveted into the cylinder wall. UK cylinders use a standard 2.25-inch (57mm) boss, so almost any mainstream immersion heater will physically fit. A built-in thermostat senses the water temperature and cuts the power once the water reaches its set point, then switches back on as the water cools.
Most homes have an immersion heater as a backup to the boiler. If your gas boiler fails in January, the immersion will still give you a tank of hot water from the electricity supply. In homes with no gas, an immersion may be the primary water heater, often paired with an Economy 7 tariff that charges cheaper electricity overnight. Either way it is governed by the wiring rules in BS 7671, the UK wiring regulations, and the appliance itself is built to the BS EN 60335 safety standard for household electrical appliances.
A standard element is rated 3kW at 240V. Lower ratings of 1.8kW or 2kW exist for slow, economy-focused installations, but 3kW is the default because it gives a sensible recovery time: roughly 45 to 60 minutes to heat a 120 to 150-litre cylinder from cold.
Types, sizes, and specifications
Three things distinguish one immersion heater from another: where it enters the cylinder, how long the element is, and what metal the element is made from. The last one is the decision that wrecks installations.
| Type | Where it fits | What it heats | Best for |
|---|---|---|---|
| Top-entry (long element) | Through the top boss, hanging down | The full cylinder, top to bottom | Vented cylinders, primary or backup heating |
| Side-entry (mid/bottom) | Through a boss in the side, lower down | Only the water above the element | Economy 7 lower element, faster small draw-offs |
| Incoloy element | Either entry point | Same as its entry type | Hard-water areas (scale resistant) |
| Copper element | Either entry point | Same as its entry type | Soft-water areas only |
Top-entry is the most common fit, especially in older vented cylinders, where the element hangs down through the top and heats the whole tank. Side-entry (sometimes called bottom-entry) heats from a lower point, so it warms only the water sitting above it. That sounds like a downside, but it is exactly what you want for an Economy 7 lower element: you can cheaply heat the bottom half of the tank overnight and leave a separate top element for a quick daytime boost.
Element length matters for top-entry units. A 27-inch element suits a tall cylinder; an 11 or 14-inch element suits a short, squat one. Buy too long for the cylinder and it will not seat; too short and the bottom third of your water never gets properly heated.
The element material is where money gets wasted. Copper elements conduct heat well and are cheaper, but in hard water they scale up fast. Limescale acts as an insulating jacket around the rod, the element runs hotter to push heat through it, and it burns out, often within a couple of years. Incoloy is a nickel-iron-chromium alloy that resists scale far better and tolerates running hot. In any hard-water area (most of the South East, East Anglia, and the Midlands), fit an Incoloy element and do not think twice. Copper is only the right choice in genuinely soft-water areas such as parts of Scotland, Wales, and the North West.
Warning
Fitting a copper element in a hard-water area is the single most common immersion mistake. The element will scale up, overheat, and fail prematurely, and you will pay to drain the cylinder and swap it again. If you are unsure of your water hardness, check your water company's website, then default to Incoloy.
The thermostat is built into the element head and is normally factory-set to around 60C. That figure is not arbitrary. Stored water below 60C can let Legionella bacteria multiply, and L8 / BS 8558 guidance (the UK rules on managing Legionella risk in water systems) calls for stored hot water to reach 60C to kill it. Some heads also include a timer or are paired with an external Economy 7 timer so the element only runs during cheap overnight hours.
How to work with it
An immersion heater is light, the element rod weighs almost nothing, but the job around it is wet and electrical, which is why a competent installation matters more than the part itself.
Replacing or fitting one means draining the cylinder, or at least dropping the water level below the boss for a side-entry unit. For a top-entry element on a vented cylinder you usually need to drain only down to the element's seal. The old element is unscrewed with an immersion heater spanner (a wide box spanner sized for the hex fitting, around 38 to 86mm), then the new one is fitted with a fresh fibre washer and screwed home firmly but not over-tightened, since the boss can distort.
Tip
Buy the immersion spanner before you start, not halfway through. The hex on the element is far wider than any normal spanner, and trying to grip it with adjustable pliers usually rounds the fitting and turns a simple swap into a cylinder replacement.
The electrical side is where homeowners must stop and bring in an electrician. An immersion heater is a fixed appliance on a dedicated radial circuit (a circuit that runs to one point and stops, not a ring), wired in 2.5mm twin-and-earth cable. It must be isolated by a double-pole switch, meaning a switch that breaks both the live and the neutral together. A standard single-pole light-type switch only breaks the live and is not acceptable for an immersion.
That isolating switch is a 20A double-pole switch fuse mounted within reach of the cylinder, so the supply can be safely killed before anyone touches the element. The graph entity for that device is the switch fuse, which also covers the smaller 13A fused connection units used for towel rails and extractor fans. Do not confuse the two: a 13A fused connection unit is the wrong device for a 3kW immersion.
This is electrical work in a wet environment and falls under Part P of the Building Regulations. A new circuit must be installed and certified by a registered electrician, and the work notified to building control or self-certified through a competent person scheme. Swapping a like-for-like element on an existing, sound circuit is a smaller job, but the moment a new cable, switch, or circuit is involved, it is electrician territory.
How much do you need
For almost every domestic installation, the answer is one element. A single 3kW top-entry element heats a typical 120 to 210-litre cylinder. You only need two when running an Economy 7 setup, where a lower side-entry element heats the bulk of the tank overnight on cheap electricity and a separate upper element gives a daytime top-up.
To size the element, measure the cylinder's internal height and pick an element length that reaches into the lower third without touching the base. As a rough guide, a 900mm-tall cylinder takes a 27-inch element; a 600mm cylinder takes around 14 inches. When in doubt, measure the old element you are removing and match it.
There is no meaningful wastage allowance here. Buy one correctly sized element of the right material, plus a spare fibre washer, and you are done.
Cost and where to buy
The element itself is cheap. A standard 3kW immersion heater costs £20 – £60 to supply, with Incoloy versions sitting at the upper end of that range and copper at the lower. Brands worth searching for include Heatrae Sadia, Backer, Sunhouse, Intatec, and MTS. They are all sold by Screwfix, Toolstation, and plumbing merchants, and the differences between mainstream brands are minor next to getting the length and material right.
Installation is where the real cost sits. Retrofitting an element to an existing cylinder, which means draining the tank, swapping the element, refilling, and re-energising, typically runs £80 – £150 in labour. Fitting a brand-new dedicated circuit with its own switch fuse costs more, because that is notifiable electrical work that must be tested and certified.
If you are replacing the whole cylinder rather than just the element, that is a separate and much larger job. A new unvented cylinder supplied on its own runs £350 – £700, and supplied and fitted by a G3-registered installer it comes to £800 – £1,800.
External resource
DWI: Water hardness by area
The Drinking Water Inspectorate and your local water company both publish hardness data for your postcode, which tells you whether to fit a copper or Incoloy element.
dwi.gov.uk
Alternatives
The immersion heater's job, heating stored water, can be done other ways, and which one suits you depends on what energy you have.
A gas or oil boiler heating the cylinder through an internal coil (an indirect cylinder) is cheaper to run per unit of heat than an immersion in most of the UK, which is why the immersion is usually a backup rather than the main heater in gas homes. If you have no gas, a heat pump feeding a cylinder is far more efficient than a direct immersion, though it costs much more to install. An instantaneous electric water heater or an electric combi avoids stored water altogether, but cannot deliver the same flow to multiple outlets at once.
For most homeowners the immersion is not really an alternative to anything; it is the resilient backstop that gives you hot water when the primary heat source is down. Keep it, keep the element material right, and keep the thermostat at 60C.
Where you'll need this
- First-fix plumbing - positioning the hot water cylinder and its supply before walls close up
- First-fix electrics - running the dedicated 2.5mm radial and double-pole switch fuse to the cylinder
- Kitchen plumbing provisions - planning hot water capacity when a utility room or new cylinder is added
An immersion heater turns up on any project that touches the hot water system, whether you are adding a utility room, replacing an old vented cylinder with an unvented one, or simply fitting a backup element. The decisions stay the same across every extension or renovation: right element material for your water, right length for your cylinder, and a properly wired and isolated supply.
Common mistakes
The errors that cost real money are predictable and avoidable.
Warning
Never reduce the thermostat below 60C to save energy. Below 60C, stored water can let Legionella bacteria multiply, and the few pence saved is not worth the health risk. If your hot water is scalding at the tap, fit a thermostatic blending valve at the outlet rather than turning the cylinder thermostat down.
Beyond the thermostat, the recurring mistakes are using a copper element in hard water (covered above, and the most expensive of the lot), and using a 13A fused connection unit instead of a 20A double-pole switch fuse. A 13A fuse passes at most 3120W, and a 3kW element at 240V draws about 12.5A. That is right on the limit for a load that runs for an hour at a time, which scorches the faceplate and causes nuisance faults. The graph guidance is unambiguous on protection: use a 20A protective device on the dedicated radial, not a 13A spur. The last common error is buying the wrong element length, leaving the lower third of the cylinder full of cold water that never gets heated.