INSTALLATION GUIDE

Immersion Heater Installation UK: Electrical Requirements & Costs

Everything electricians need to know about immersion heater installation — dedicated 20A circuit requirements, thermostat settings for Legionella prevention, element types, timer controls, solar diverter compatibility, and 2026 costs.

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11 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

  • 1An immersion heater must be supplied from a dedicated 20A radial circuit protected by a 20A MCB or fuse. It must not be shared with any other circuit or appliance. A double-pole isolating switch rated at 20A must be located within reach of the cylinder.
  • 2Thermostats must be set to at least 60°C (and ideally 60–65°C) to prevent the growth of Legionella bacteria in the stored water. Regular heat cycling — heating to 60°C at least once per week — is recommended even when economy tariffs are used.
  • 3Immersion heaters are available with top-entry elements (heat the whole tank), bottom-entry elements (heat a smaller volume for economy use), and dual-element configurations (one element at each position for flexible control).
  • 4A 3kW immersion element at 60°C takes approximately 2–3 hours to heat a standard 120-litre cylinder. Running costs depend on the electricity tariff: at 25p/kWh, a full heat cycle costs approximately 22–27p per cycle on a 3kW element.
  • 5Solar PV diverters (such as myenergi Eddi or Immersun) can route surplus solar generation to the immersion heater, significantly reducing water heating costs for properties with solar PV systems.
01 · Installation Guide

Immersion Heater Installation: Overview

An immersion heater is an electric resistance heating element installed directly into a hot water cylinder. It heats stored water in the same way as a kettle element, providing hot water independently of (or as backup to) a central heating boiler. Immersion heaters are found in properties without gas boilers, as backup systems in gas-heated properties, and increasingly alongside solar PV diverter systems.

From an electrical perspective, immersion heater installation involves a dedicated 20A radial circuit, correct thermostat setting (critical for Legionella prevention), and appropriate controls. All installation work must comply with BS 7671:2018+A4:2026 and is notifiable under Building Regulations Part P for new circuits.

  • Part P notification — new immersion heater circuits are notifiable under Building Regulations Part P. Registered competent persons self-certify automatically. Non-registered persons must submit a building notice and have the work inspected.
  • BS 7671 compliance — the circuit must comply with all relevant sections of the IET Wiring Regulations, including circuit protection, cable sizing, earthing and bonding, and RCD protection requirements.
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02 · Installation Guide

Dedicated 20A Radial Circuit Requirement

A standard domestic immersion heater draws 3kW (13A at 240V). BS 7671 requires the circuit to be sized for the load and the installation method. A dedicated 20A radial circuit is the standard specification for immersion heater supply in UK domestic premises.

  • MCB rating — a 20A Type B or Type C MCB at the consumer unit. A 16A MCB is acceptable for a 3kW element (13A continuous load) but 20A provides more headroom for inrush current and is standard practice. Do not use a 13A fused spur — a dedicated circuit to the consumer unit is required.
  • Cable sizing — 2.5mm² twin and earth for most domestic runs where the cable is clipped to a surface or in conduit (current capacity 23A in free air, 18.5A clipped to surface, 18A in conduit). For runs where cable passes through insulation, 4mm² twin and earth is required (thermal insulation significantly dereates cable capacity).
  • Isolating switch — a 20A double-pole switch with a pilot light (to indicate when the element is energised) must be installed adjacent to the cylinder and accessible without moving the cylinder. The switch must be within reach of the element for safe isolation. A flex outlet plate connecting the switch to the element is the standard arrangement.
  • RCD protection — Regulation 411.3.3 requires 30mA RCD protection for socket-outlet circuits. For fixed appliance circuits such as immersion heaters, an RCBO providing both overcurrent and RCD protection is the preferred solution and provides the highest level of shock protection.
03 · Installation Guide

Element Types & Selection

Immersion heater elements are available in a range of lengths, wattages, and connection configurations. Selecting the correct element for the cylinder is essential — the wrong element length will not heat the cylinder correctly and may damage the element.

  • Top-entry (long) element — enters the cylinder from the top and extends most of the way down the tank. Heats the entire cylinder contents. Available in various lengths (typically 11", 14", 18", 27") to suit different cylinder depths. This is the standard element for single-element cylinders. Supply and fit cost: £50–£120.
  • Bottom-entry (short) element — enters the cylinder from the side near the bottom. Available in short lengths (typically 11"). Used as the primary element in dual-element cylinders to provide economy heating on overnight tariffs.
  • Dual-element cylinders — fitted with both a top (short, typically 1kW or 1.5kW) and a bottom (long, typically 3kW) element. The bottom element is used for full overnight heating on an Economy 7 tariff; the top element provides a daytime boost to the upper portion of the tank only (more economical than heating the entire cylinder for a small hot water demand).
  • Titanium elements — standard copper-sheathed elements in hard water areas accumulate limescale rapidly. Titanium or Incoloy elements resist limescale significantly better and are recommended for hard water areas. They cost more (£30–£80 vs £15–£40 for copper) but last considerably longer.
04 · Installation Guide

Thermostat Settings & Legionella Prevention

The immersion heater thermostat setting is not merely a comfort or economy choice — it is a health and safety requirement. Incorrect thermostat settings create conditions that allow Legionella bacteria to proliferate in stored hot water.

  • Legionella risk — minimum 60°C — Legionella bacteria proliferate in water stored between 20°C and 45°C and are killed at temperatures above 60°C. The HSE recommends stored hot water be maintained at 60°C or above. Setting the thermostat to 50°C or lower creates a genuine risk of Legionella growth, particularly in cylinders that are not frequently used or have stagnant sections.
  • Recommended setting: 60–65°C — this temperature kills Legionella and all other waterborne pathogens while not being so high as to cause excessive limescale deposition or create a scalding risk at unblended outlets.
  • Thermostatic mixing valves (TMVs) — where the hot water is stored at 60°C+ but outlets (particularly baths and showers accessible to children or elderly people) must deliver water at a safe temperature, a TMV rated to TMV2 or TMV3 should be fitted at the point of use to limit delivered hot water to 43°C (bath/shower) or 38°C (bidet/wash basin where required).
  • Weekly heat cycling — for properties using economy tariffs where the cylinder may not reach 60°C every day, the HSE recommends heating to 60°C at least once per week. A timer boost circuit set for weekly pasteurisation is the cleanest solution.
05 · Installation Guide

Timer, Boost & Smart Controls

Appropriate controls significantly improve the efficiency and running cost of an immersion heater installation. At minimum, a timer should be fitted to prevent the heater running continuously.

  • 24-hour mechanical or digital timer — programmes the immersion heater to operate during off-peak tariff periods (e.g., Economy 7 overnight periods). Typically fitted at the isolating switch position using a combined timer/switch plate (e.g., Timeguard TRT035 or equivalent).
  • Boost facility — a timer with a manual boost override allows the immersion to be turned on for a set period (typically 30 or 60 minutes) for on-demand hot water heating between scheduled periods. Essential for any installation that relies on off-peak heating.
  • Smart controls — Wi-Fi-connected immersion heater controllers (such as the Sunamp or myenergi Eddi app-controlled units) allow remote monitoring and control, automatic response to dynamic tariff pricing (Octopus Agile, etc.), and integration with solar diverter systems. The wiring is identical to a standard isolating switch; the intelligence is in the controller.

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06 · Installation Guide

Solar Diverter Compatibility

Solar PV diverter devices offer an elegant way to reduce water heating costs for properties with solar PV systems. Instead of exporting surplus generation to the grid at the Smart Export Guarantee rate (typically 3–15p/kWh), the diverter routes this surplus to the immersion heater, effectively converting solar electricity to hot water at no cost.

  • How diverters work — a current transformer (CT clamp) monitors current at the grid connection. When the PV system generates more than the household is consuming (surplus generation), the diverter begins routing current to the immersion heater in proportion to the surplus. The heater effectively absorbs the exact surplus — neither wasting it nor drawing from the grid.
  • Popular UK diverters — myenergi Eddi, Marlec Rutland Windcharger iBoost+, Immersun, and Power Diverter Pro are the most common UK solar diverter products. All work on the same principle but vary in features, app connectivity, and compatibility with home energy management systems.
  • Installation note — the diverter is wired between the consumer unit and the immersion heater isolating switch. The CT clamp is installed on the grid import/export cable at the consumer unit. The immersion thermostat must still be set to 60°C minimum, as the diverter controls the duration but not the temperature of heating.
07 · Installation Guide

Typical Immersion Heater Costs (2026)

Immersion heater installation and replacement costs depend on whether a new circuit is required and the accessibility of the cylinder. The following are typical costs for UK electrical contractors in 2026.

  • Immersion element only (supply) — copper element: £15–£40; Incoloy/titanium element: £30–£80. Premium dual immersion thermostats: £10–£25.
  • Like-for-like element replacement (labour) — £80–£150 depending on access. Includes draining sufficient water, removing old element, fitting new element with a new sealing washer, refilling, and testing. Minor Works Certificate recommended.
  • New immersion heater circuit (materials + labour) — £200–£400 for a straightforward domestic installation: consumer unit to cylinder, 20A RCBO, 20A double-pole switch with pilot light, flex outlet, element, and timer. Electrical Installation Certificate included. Longer cable runs or difficult access adds to cost.
  • Solar diverter installation — myenergi Eddi supply and installation: £400–£600 including CT clamp installation, wiring to immersion circuit, and commissioning. Payback typically 1–3 years depending on solar generation profile and hot water usage.
08 · Installation Guide

Installation Steps

The following steps cover a typical new immersion heater circuit installation in a domestic property. Always isolate and prove dead before working on any live circuit.

  • 1. Isolate and lock off — isolate the consumer unit main switch before any wiring work. Use a lock-off device and prove dead with an approved voltage indicator before commencing work.
  • 2. Install the circuit cable — run 2.5mm² (or 4mm² where passing through insulation) twin and earth from the consumer unit to the cylinder location. Route to avoid contact with hot water pipes. Clip or enclose in conduit throughout.
  • 3. Install the isolating switch and timer — mount the 20A double-pole switch (with timer if required) adjacent to the cylinder. Connect the circuit cable to the switch and run heat-resistant flex from the switch outlet to the immersion element boss.
  • 4. Install the element — ensure the cylinder is sufficiently drained or that the element boss is above the water line. Fit the element with a new fibre sealing washer, tighten to the manufacturer's torque specification, connect the flex, and set the thermostat to 60–65°C.
  • 5. Connect at consumer unit — install the 20A RCBO or MCB and connect the circuit cable. Restore power, test continuity, insulation resistance, polarity, and RCD operation. Record results and issue an Electrical Installation Certificate.
09 · Installation Guide

For Electricians: Immersion Heater Work

Immersion heater installation and element replacement is straightforward domestic work with a low parts cost and a short time on site. Solar diverter installations (particularly myenergi Eddi, which integrates with their EV charger ecosystem) are an increasingly common upsell for electricians already working on solar PV or EV charger installations.

Certificate on Site

Use the Elec-Mate certificate app to issue an Electrical Installation Certificate for new immersion heater circuits and a Minor Works Certificate for element replacements. PDF sent to the client before you leave — fully Part P compliant.

Upsell Solar Diverters

Any client with a solar PV system and a hot water cylinder is a candidate for a solar diverter installation. At £400–£600 fitted, with a typical payback of 1–3 years, these are easy to justify. A myenergi Eddi can also integrate with a myenergi Zappi EV charger if the client has or is considering an EV charger installation.

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