Whether you're replacing a failed element in an existing hot water cylinder or specifying immersion heating for a new installation, getting the right combination of element material, length, boss thread and thermostat type is what determines whether the job lasts five years or twenty. Our range covers copper, Incoloy and titanium immersion heaters from Tesla UK and OSO Hotwater, alongside compatible thermostats, thermostat pockets and box spanner keys.
Choosing the right element material
Element material is matched to water hardness, not personal preference:
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Copper — the standard choice for soft to moderately hard water areas and general domestic use. Cost-effective and widely compatible, but prone to accelerated scale build-up and reduced service life in hard water.
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Incoloy — a nickel-chromium alloy sheath offering improved resistance to limescale and pitting corrosion versus copper. The recommended minimum specification for moderate to hard water areas.
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Titanium — the most corrosion- and scale-resistant option, advised for hard to very hard water areas, for cylinders that rely on the immersion as the sole or primary heat source (rather than boiler/heat pump backup), and for high-use settings such as HMOs and student accommodation where element life under continuous cycling matters most.
A critical compatibility point often missed at spec stage: thermostat pocket material must match the cylinder material. Fitting a copper thermostat pocket into a stainless steel cylinder creates a galvanic couple between dissimilar metals, driving electrolytic corrosion at the pocket. Stainless steel cylinders require an Incoloy or stainless steel thermostat pocket — never copper. Always check the cylinder manufacturer's spec before ordering.
Getting the length and thread right
Immersion heaters are sized by element length in inches — common lengths in our range include 11", 14", 18" and 27" — and fitted via a threaded brass boss, typically 2¼" as standard, with 1¼" and 1¾" variants used on some OSO cylinders. Length and thread must match the original unit exactly; a mismatched length won't seat correctly in the tank, and a mismatched thread won't engage the boss. If you're unsure of your existing spec, the length and thread size are usually stamped on the immersion boss itself or listed in the cylinder manufacturer's spares documentation.
One length worth noting: shorter elements built with a denser coil (rather than a single straight run) can offer the effective heating surface area of a much longer standard element while fitting a shallower cylinder — useful where cylinder height is constrained.
Rating and installation basics
- Standard domestic immersion heaters are rated 3kW at 240V, drawing roughly 13A — install on a dedicated 16A circuit as standard practice.
- Higher-output single and twin-element configurations (commonly up to 4.5kW+ per element, or dual 3kW+3kW arrangements) are used where the immersion serves as primary heating rather than backup, and may need a dedicated 20A+ circuit — confirm against the manufacturer's electrical spec.
- All units in this range are manufactured to EN 60335-2-73 and are BEAB approved, with an integral dual safety thermostat (operating + high-limit cut-out) as standard.
- Unvented cylinder installations fall under Part G of the Building Regulations and must be carried out or signed off by a G3-qualified engineer — this applies to the whole cylinder system, not just the immersion element.
Extending element life
Regardless of material, a few practices reduce scale-related failure: fit the shortest element length that suits the cylinder rather than defaulting to the longest available, set the cylinder thermostat no higher than necessary, and avoid running the immersion continuously where a boiler or heat pump can share the load. Running an element without full water coverage — for example during draindown or a low-water fault — can anneal and split the sheath, which is a common cause of premature failure unrelated to water hardness.
Accessories in this range
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Replacement thermostats — rod and standard pattern, matched to element type and cylinder material.
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Thermostat pockets — copper, Incoloy and stainless steel, selected to match cylinder material and avoid galvanic corrosion.
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Box spanner keys — sized to the immersion boss for safe removal and installation without damaging the boss or element sheath.
Related collections: Direct Unvented Cylinders · Solar Immersion Controllers · Electric Water Heaters
