Immersion Heaters: Copper vs Incoloy, Sizing, and Wiring Explained

Immersion Heaters: Copper vs Incoloy, Sizing, and Wiring Explained

The immersion heater is one of the most straightforward pieces of kit in a domestic or light commercial hot water system — a resistive heating element screwed into a 2¼" BSP boss on the cylinder, controlled by a thermostat. But specifying the wrong element material, wrong length, or wiring it incorrectly leads to callbacks, failed cylinders, and unhappy customers. This guide covers everything you need to get the selection and installation right first time.

How an Immersion Heater Works

An immersion heater is simply an electric resistance element submerged in the water. When current flows through the element, it generates heat which transfers directly to the water by conduction and convection. A thermostat clipped to or built into the element monitors water temperature and cuts power when the set point is reached.

Most domestic immersion heaters are rated at 3kW. Some larger units run at 1kW (economy/anti-legionella mode) or 6kW (fast recovery commercial), but 3kW is the standard for UK residential hot water cylinders.

Copper vs Incoloy Elements

The element sheath material is the most important selection decision, and it depends on water hardness:

• Copper elements — correct for soft water areas. Copper transfers heat efficiently and is cheaper to produce. In hard water areas, limescale builds up rapidly on copper, insulating the element and causing overheating, element failure, and reduced efficiency.
• Incoloy elements — a nickel-iron-chromium alloy with a smooth, non-porous surface that resists limescale adhesion. Correct for hard water areas (most of England east of a line from the Wash to the Solent). Incoloy elements last significantly longer in hard water and are now the recommended choice for most UK installations.
• Titanium elements — the premium option, used in aggressive water conditions and unvented cylinders where longevity is critical.

London is a hard water area. At APM we recommend Incoloy as the default for London and the South East. If you are working in a soft water area (parts of Wales, Scotland, the Lake District), copper is fine.

Element Lengths and Where They Are Used

Immersion heaters are available in several lengths, each suited to different cylinder positions and applications:

• 11" (280mm) — top entry, short element. Used as a "boost" heater at the top of an indirect cylinder where the primary heat source (boiler coil) heats the bulk of the cylinder. Heats only the top third of the water for a quick boost shower when the boiler is off. Also used as the primary heater in direct cylinders where the cylinder itself is not too deep.
• 14" (355mm) — general purpose. Fits most standard cylinders as a top-entry primary or backup element.
• 18" (455mm) — longer top-entry element, heats a greater volume of the cylinder.
• 27" (685mm) — bottom entry, long element. Used as the primary heater in direct cylinders, heating the full volume of water from the bottom up. More efficient for full-cylinder heating than a top-entry element.

On a standard indirect copper cylinder (the most common type in UK homes), the usual configuration is a short 11" element in the top boss as a boost heater, with the coil at the bottom of the cylinder providing primary heat from the boiler. The immersion heater provides hot water during summer when the boiler is off, or as a backup when the boiler fails.

Direct vs Indirect Cylinders

It is important to understand which type of cylinder you are working on:

• Direct cylinder — heated solely by immersion heater(s). No boiler coil. Often used in properties without a gas supply or with off-peak electricity tariffs. Usually fitted with a bottom-entry long element as primary and a top-entry short element as boost.
• Indirect cylinder — has a boiler coil as primary heat source. Immersion heater is secondary/backup. The element sits in the top boss and heats only the upper portion for quick top-up.

Thermostat Settings and Legionella

The thermostat on an immersion heater should be set to 60–65°C for domestic use. At 60°C, Legionella bacteria (which cause Legionnaires disease) are killed within minutes. Below 45°C, Legionella can multiply rapidly. The NHS and HSE recommend domestic hot water stored at 60°C and delivered at no less than 50°C at outlets.

Do not set thermostats to low temperatures "to save energy" on stored hot water systems — this creates a Legionella risk. If energy saving is the goal, use a timer to heat water only when needed, but always heat to 60°C when energised.

Overheat thermostats (also called safety thermostats or ECO switches) are a separate, non-resetting thermal cutout that trips if the main thermostat fails and temperature exceeds around 80–85°C. These require manual reset via a screwdriver through the access hole in the thermostat cap. If the overheat trips repeatedly, the main thermostat needs replacement.

Wiring Requirements

A 3kW immersion heater draws 13A at 230V. The wiring requirements under BS 7671 (18th Edition) are:

• Dedicated radial circuit from the consumer unit — do not share with other loads.
• 20A MCB or fuse protecting 2.5mm² cable (though 3kW = 13A, derating factors and the dedicated circuit requirement push this to 20A protection as standard practice).
• Double pole switch at the cylinder — either a 20A DP switch with neon indicator, or a timer with DP switching action.
• Flex connection from the switch to the immersion heater element: heat-resistant flex rated for the temperature environment inside an airing cupboard (minimum 85°C rated flex, typically 3-core 1.5mm² or 2.5mm²).

On Economy 7 or time-of-use tariff installations, the immersion is typically wired to a dedicated off-peak supply circuit that energises overnight. This is a separate circuit from the main consumer unit, controlled by a separate meter tails arrangement — an electrician must confirm the supply arrangement with the DNO before installing.

Fitting an Immersion Heater

The immersion heater screws into a 2¼" BSP female boss on the cylinder. A dedicated immersion heater spanner (typically C-spanner or box spanner style for 2¼" BSP) is required — a standard wrench will damage the element head. Always use a new fibre washer on the thread to ensure a watertight seal. Do not over-tighten — the boss is often copper and can crack.

Drain the cylinder before removing the old element. On a vented system, isolate the cold feed and open a hot tap to break the siphon. On an unvented cylinder, isolate the supply and use the pressure relief valve to depressurise before opening any connections.

Products We Stock

We hold Tesla and Barco immersion heaters and replacement thermostats at our Acton branch:

• Tesla 11" Incoloy/Copper Immersion Heater with Thermostat
• Tesla 14" All Incoloy Immersion Heater
• Tesla 11" Replacement Thermostat (50–80°C)
• Heatrae Sadia Megaflo Replacement Thermostat 11"
• Barco 11" Immersion Heater Thermostat

Same-day collection: 24 Western Avenue, Acton W3 7TZ. Call 020 8702 8080.

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