Immersion Heater Replacement Guide: Element Types, Wiring and Thermostat Settings

Immersion Heater Replacement Guide: Element Types, Wiring and Thermostat Settings

Immersion heater replacement is one of those jobs that crosses the trade boundary — plumbers deal with the cylinder and water side, electricians deal with the wiring and controls. On a straightforward replacement job, either trade will do it; on a new installation or full controls upgrade, you often need both. This guide covers element types, sizing, thermostat replacement, wiring requirements, and timer controls. All products are available same day from APM Electricals in Acton.

What an Immersion Heater Does

An immersion heater is an electric heating element fitted directly into a hot water storage cylinder. It heats water on demand or on a timed schedule, either as the primary heat source (in all-electric properties without a gas boiler) or as a backup to the main boiler system. Most UK domestic cylinders have a boss at the top and one at the side, allowing installation of a top element (for heating the full cylinder) or a side element (for heating the lower portion quickly for a smaller volume of hot water).

Immersion heaters are rated in kilowatts — the most common domestic rating is 3kW, though 2.25kW elements are used in properties with limited electrical supply or where the circuit is on Economy 7 overnight tariffs. A 3kW element in a well-insulated 120-litre cylinder will bring water from cold to 60°C in approximately two hours.

Incoloy vs Copper Elements: Which Do You Need?

The heating element inside an immersion heater is sheathed in either copper or Incoloy (a nickel-chromium-iron alloy). The choice depends on the water type in the local area:

Copper elements are suitable for soft water areas. They are less expensive and transfer heat efficiently, but in hard water areas the calcium carbonate scale that builds up on copper elements reduces efficiency rapidly and leads to premature failure. In London and much of the south-east, where water hardness is high, copper elements have a significantly shortened service life.

Incoloy elements are the correct specification for hard water areas and are the safer default choice if you are unsure of local water hardness. Incoloy resists scale build-up far better than copper and has a longer service life in the conditions found across most of England. For any installation in London or the Home Counties, specify Incoloy.

The Tesla Immersion Heater with Incoloy/Copper Element comes supplied with a thermostat and is suitable for both hard and soft water areas depending on the element selected. Tesla is a well-established UK brand in the immersion heater market with good availability of replacement parts.

Element Sizing: 11 Inch and 14 Inch

Standard immersion heater lengths in the UK are 11 inches and 14 inches. The correct length depends on the cylinder — specifically the depth of the boss (the threaded fitting in the cylinder wall). Measure the existing element before ordering: fitting a 14-inch element into a cylinder sized for 11 inches risks the element touching or sitting too close to the cylinder bottom, which affects efficiency and can cause damage.

The thread size is standard at 1 3/4" BSP for virtually all UK domestic cylinders manufactured since the 1980s. If you encounter an older cylinder with a non-standard boss, check before ordering.

Replacing the Thermostat

The thermostat clips or slots onto the element rod and is retained by the overheat cutout housing. It is a separate, replaceable component from the element itself. Before condemning the whole immersion heater, check whether just the thermostat has failed — a thermostat replacement is significantly cheaper and quicker than a full element change.

The Tesla 11" 50–80°C Immersion Thermostat is compatible with Tesla and most other standard UK immersion heaters. The 50–80°C adjustment range covers all domestic applications. The factory setting is typically 60–65°C — the recommended temperature for Legionella control. Do not set below 60°C in a cylinder that is used regularly; do not set above 65°C in a property with young children or vulnerable adults where scalding risk is a concern (or ensure a thermostatic blending valve is fitted at the point of use).

When replacing a thermostat, also check the overheat cutout (the ECO button). If it has tripped repeatedly, investigate why — common causes include a stuck or failed thermostat that allows the water to overheat, or a wiring fault.

Wiring Requirements

An immersion heater must be wired on a dedicated circuit from the consumer unit. The standard requirements for a 3kW immersion heater are:

• Circuit breaker: 16A MCB (Type B). Some older installations use a 20A fuse — this is acceptable for the load but check the cable sizing if upgrading.
• Cable: Minimum 2.5mm² twin and earth (T&E). If the cable run is long or the cable is buried in insulation, upsize to 4mm² to avoid voltage drop and heat build-up. Heat-resistant flex from the fused spur to the immersion heater head is required — standard PVC flex is not suitable near a hot cylinder.
• Control: A 20A double-pole fused spur or isolator switch adjacent to the cylinder. This must be double-pole (both live and neutral switched) and must be positioned so it can be safely reached without touching the cylinder or pipework.
• RCD protection: Required under BS 7671 (18th Edition). The circuit must be protected by an RCD or RCBO at the consumer unit.

Any new or replacement immersion heater circuit is notifiable work under Part P of the Building Regulations in England and Wales. If you are an electrician, this is self-certifiable under your competent person scheme. If you are a plumber replacing an element on an existing circuit, the electrical work must be notified via a building control application or carried out by a registered electrician.

Timers and Cylinder Stats: Reducing Running Costs

An uncontrolled immersion heater running continuously is expensive. Two controls make a significant difference to running costs:

Electronic 7-day timer: The Flush Immersion Heater Timer fits directly into the fused spur position and allows the immersion to run only during off-peak hours (overnight on Economy 7) or at set times ahead of when hot water is needed. A 7-day programme covers varying weekday and weekend demand patterns. Rated at 16A, suitable for standard 3kW immersion heaters.

Cylinder thermostat: The ESI Electronic Dual Cylinder Thermostat mounts on the outside of the cylinder and provides accurate temperature control for properties where the immersion is part of a more complex heating controls setup. The dual output model can control both heating and hot water circuits independently, making it suitable for use alongside a motorised valve setup.

The Right Tool for the Job

Removing a seized immersion heater from an older cylinder requires a proper immersion heater spanner — a standard wrench will not fit the large hexagonal head and applying force with the wrong tool risks damaging the cylinder boss. The Rothenberger Cranked Immersion Heater Spanner is the standard tool for this job. The cranked design provides clearance for the pipework and element connection housing when working in tight airing cupboards. Apply penetrating oil to a seized element the day before if possible.

Pick Up Today in Acton

We stock Tesla and Barco immersion heaters, replacement thermostats, timers, cylinder stats, and immersion spanners at our trade counter. Browse the full immersion heaters range and water heating products online, or collect same day from 24 Western Avenue, Acton, London W3 7TZ.

APM Electricals — 24 Western Avenue, Acton, London W3 7TZ. Tel: 020 8702 8080. Open to the trade.