Immersion Heaters and Hot Water Cylinders — Sizing, Installation, and Controls

Immersion heaters remain one of the most common hot water heat sources in UK housing — as primary heating in all-electric properties, backup in solar thermal and heat pump systems, and emergency supply in properties where the boiler has failed. This guide covers element selection and sizing, thermostat types and settings, wiring requirements to BS 7671, common faults and diagnosis, and the cylinder types a plumber or electrician will encounter on site.

What Is an Immersion Heater?

An immersion heater is an electric resistance element fitted directly into a hot water cylinder. It heats domestic hot water in open-vented or unvented cylinders without relying on a boiler — making it a primary heat source in all-electric homes and a backup in systems with solar thermal or heat pump primary heating.

Immersion heaters are covered by BS EN 60335-2-73 and must be installed by a competent person. In England and Wales, unvented cylinder work requires a Part G notification; wiring is Part P.

Immersion Heater Types

Single Immersion (Top Entry)

A single top-entry element heats the full cylinder volume. This is less efficient for everyday use — the entire cylinder must reach temperature — but is simple and cheap to install. Common in direct electric cylinders and as emergency backup in combination solar cylinders.

Dual Immersion (Top and Bottom Entry)

A bottom element heats only the lower portion (typically 30–60 litres) for everyday use; the top element heats the full cylinder when high demand is expected. This is the most energy-efficient arrangement for domestic properties. The bottom thermostat is set lower (typically 55°C) and the top higher (typically 60–65°C).

Economy 7 Immersion

Economy 7 tariffs provide cheaper overnight electricity. Cylinders fed by Economy 7 use the bottom element to charge overnight and the top element as a top-up during the day. Both elements must be separately fused and switched. Controls should include a boost timer so the top element only runs for limited periods.

Megaflow and Unvented Cylinder Immersions

Unvented cylinders (Megaflow, Gledhill, Telford, etc.) use flanged immersion heaters to maintain pressure integrity. These are typically 2¼ inch BSP and must be de-energised before draining. Always use the manufacturer's specified heater — a wrong-spec element can damage the flange boss or void warranty. See our guide to Unvented Hot Water Cylinders: G3, Safety Devices, and Installation for full compliance requirements.

Sizing Immersion Heaters

Power Rating

Domestic immersion heaters are rated 1kW, 2kW, or 3kW. The standard is 3kW (13A fused spur). Higher-capacity cylinders benefit from 3kW to minimise recovery time. 1kW elements are used for overnight Economy 7 charging where recovery time is less critical.

Length and Entry Configuration

Element length must suit the cylinder height. Common lengths: 11 inch (280mm), 14 inch (355mm), 18 inch (457mm), 27 inch (686mm). Measure from flange boss to cylinder base, then select the next shorter element — the element must not touch the base.

Thread Standard

UK cylinders use 2¼ inch BSP (British Standard Pipe) as the standard thread. Always confirm thread type before ordering — BSPT (tapered) threads are occasionally found on older cylinders and require a matching element boss.

Thermostat Types and Settings

Rod Thermostats

The most common type. A bimetal rod inside the element stem senses water temperature. Fixed or adjustable; standard range 40–75°C. Replaceable without draining on most cylinders — the thermostat fits in a pocket within the element boss.

Capillary Thermostats

A remote-sensing capillary bulb clips to the cylinder wall. Used where the element boss is inaccessible or on solar pre-heat cylinders. More accurate than rod types but requires correct positioning of the sensing bulb.

Temperature Settings

• Minimum 60°C — required by HSE L8 to control Legionella risk. Water stored below 60°C for prolonged periods can support Legionella pneumophila growth.
• Maximum 65°C — above this, limescale deposits accelerate significantly in hard water areas, reducing element life and efficiency.
• Overheat cutout — all immersion heaters include a manual-reset thermal cutout (typically 82°C). A tripped cutout indicates a thermostat fault or blocked circulation — do not simply reset without investigation.

Wiring an Immersion Heater

Immersion heaters must be wired on a dedicated 20A radial circuit from the consumer unit, protected by a 20A RCBO (preferred) or RCD. The immersion heater connection unit (switched, double-pole, 20A rated, with neon indicator) must be within 2 metres of the cylinder.

Cable: 2.5mm² twin-and-earth is minimum; 4mm² is preferred for runs over 20 metres. For dual-immersion installations, each element requires its own connection unit and FCU.

Dual Immersion Wiring Arrangement

• Top element: permanent live, thermostat-controlled, 20A radial
• Bottom element: Economy 7 supply (off-peak), or separate radial for boost control
• Both must be double-pole switched — do not share a switch

Common Faults and Diagnosis

No Hot Water — Element or Thermostat?

Use a clamp meter at the connection unit to confirm current draw. If current flows but water does not heat, suspect a sheathing failure allowing water ingress to the element. If no current flows with thermostat calling for heat, suspect the thermostat, overheat cutout, or wiring fault.

Tripped Thermal Cutout

Manual-reset via a small reset button on the element. If it trips repeatedly, the thermostat is failing to cut out at temperature — replace the thermostat, not just the cutout. On unvented cylinders, also check the expansion relief valve is operating correctly. Refer to our guide on Sealed Heating System Pressure: Gauges, Filling Loops, and How to Repressurise a Boiler for pressure-related checks.

Brown/Discoloured Hot Water

Corrosion inside the cylinder or element failure. On copper cylinders this is often early-stage pitting. Check sacrificial anode condition if the cylinder is fitted with one (stainless steel unvented cylinders do not have anodes, but some manufacturers recommend an in-line sacrificial anode on the cold feed).

Noisy Operation — Kettling

Limescale on the element causes kettling noise, as trapped water superheats in scale pockets. Common in hard water areas (TDS above 250 ppm). Descale with an appropriate citric acid solution, or fit a scale inhibitor on the cold feed. Polyphosphate dosing is effective for mains-pressure unvented cylinders.

High Electricity Bills

Extended recovery time due to scaled element increases run hours. A 3kW element coated in 6mm of scale can lose up to 40% efficiency. Replacing a scaled element typically pays back within 6–12 months on electricity savings alone.

Cylinder Types and Selection

Open-Vented Cylinders (Direct and Indirect)

Traditional vented copper cylinders (BS 1566) are fed by a cold-water storage cistern in the loft, operating at low pressure (typically 0.5–1.0 bar at the tap). Direct cylinders are heated only by the immersion heater. Indirect cylinders include a heating coil for primary (boiler) circulation, with the immersion heater as backup.

Suitability: vented cylinders are tolerant of poor water quality, easy to maintain, and lower cost. Disadvantage: dependent on header tank for pressure — often inadequate for power showers without a pump. For details on cylinder selection, see our guide to Unvented Hot Water Cylinders: G3, Safety Devices, and Installation.

Unvented Cylinders (High-Pressure)

Unvented cylinders (Megaflow, Gledhill, Telford Cylinders, OSO, Tribune, etc.) connect directly to mains cold water supply and deliver hot water at mains pressure. This eliminates the need for a header tank and provides balanced hot and cold pressure — essential for thermostatic shower valves and combi-equivalent performance.

Mandatory fittings: expansion vessel (or internal air gap), pressure reducing valve, expansion relief valve, temperature and pressure relief valve (Tundish to safe drainage). Installation requires G3 Building Regulations notification in England and Wales and must be carried out by a competent person registered with a relevant body.

Solar Thermal Pre-Heat Cylinders

Twin-coil cylinders allow a solar thermal collector to heat the lower zone (via the lower coil) with the boiler or immersion heater topping up the upper zone. The immersion heater in a solar cylinder should be set to run only as a backup — using a proportional thermostat or temperature-differential controller rather than a simple rod thermostat — to avoid overriding solar gains.

Heat Pump Cylinders

Air source and ground source heat pumps require large-volume, low-flow-temperature cylinders. Typical volumes: 200–300 litres for a 3–4 bedroom home. The heat pump coil is sized for low-temperature operation (45–55°C primary, 50°C stored). The immersion heater provides emergency backup and periodic Legionella pasteurisation cycles (run to 60°C for 1 hour at least once per week if stored temperature is normally below 60°C). For a full overview of heat pump system design, see our guide to Air Source Heat Pumps: Design, Installation, and MCS Certification.

Replacing an Immersion Heater — Step-by-Step

1. Isolate the circuit at the consumer unit (lock off where possible)
2. Prove dead with a voltage tester at the connection unit
3. Turn off the cold feed to the cylinder (or drain the cylinder if the boss is mid-cylinder)
4. For top-entry elements, partial drain is usually not needed — use an immersion heater spanner with a rubber insert to prevent slipping. Apply steady torque — 100–150 Nm typical
5. Withdraw the element carefully — residual water will drain from the boss
6. Inspect the boss threads and seating face for corrosion or damage
7. Fit a new fibre washer (never reuse the old washer)
8. Insert the new element and torque to manufacturer's specification
9. Restore cold feed, check for leaks before energising
10. Reinstate the circuit, set the thermostat, and verify operation

Legionella Risk Management in Cylinder Systems

HSE ACOP L8 and HSG274 Part 2 cover domestic hot water systems. Key requirements for cylinder systems:

• Store hot water at 60°C or above
• Distribute at 50°C or above (measured at the outlet within 1 minute of running)
• Cold water to cylinders should be below 20°C at the inlet
• Infrequently used outlets (holiday lets, second homes) should be flushed weekly and pasteurised on return from periods of low use

Key Standards and Regulations

• BS EN 60335-2-73 — immersion heaters for household use
• BS 1566 — open-vented copper cylinders
• BS EN 12897 — unvented hot water storage
• Building Regulations Part G — sanitation, hot water supply, water efficiency (England and Wales)
• Building Regulations Part P — electrical safety in dwellings
• BS 7671:2018+A2:2022 — 18th Edition IET Wiring Regulations
• HSE ACOP L8 — Legionella risk management

Products for Immersion Heater Installation

Tesla Immersion Heater c/w Thermostat – Incoloy/Copper 11" UK

MK 13A 1-Gang Switched Fused Connection Unit with Neon Indicator Gloss White

Related Guides

• Article #129 — Electric Water Heaters: Point-of-Use, Under-Sink, and Instant Hot Water
• Article #29 — Expansion Vessels Explained: Heating System and Unvented Cylinder Vessels
• Article #87 — Unvented Hot Water Cylinders: G3, Safety Devices, and Installation
• Article #45 — Sealed Heating System Pressure: Gauges, Filling Loops, and How to Repressurise
• Article #15 — Zone Valves and Motorised Valves: The Heating Engineer's Complete Guide
• Article #102 — Combi vs System vs Regular Boilers: A Trade Guide to Boiler Selection
• Article #148 — Pressurisation Units and Automatic System Filling for Sealed Heating Systems
• Article #146 — Air Source Heat Pumps: Design, Installation, and MCS Certification

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