How to Wire an Immersion Heater and Cylinder: Single, Dual and Economy 7 Setup

An immersion heater is an electric heating element inside a hot water cylinder. It can serve as the primary hot water source in flats without gas, or as backup to a boiler. Wiring an immersion correctly requires understanding element ratings, control methods, and dual-element cylinder rules.

This guide covers single and dual immersion wiring, timer connections, Economy 7 setups, and cable requirements.

Immersion Heater Types

Single element

One heating element at the cylinder top. Heats the top 40–50 litres only.

• Rating: 3 kW (13 A)
• Suitable for small households, backup heating

Dual element

Two heating elements:

• Top element (1 kW or 3 kW) — quick heat for small volume
• Bottom element (3 kW) — heats full cylinder overnight

Dual cylinders are standard for Economy 7 installations.

Power Calculations

I = P / V = 3,000 / 230 = 13 A

A 3 kW element draws 13 A continuously. Use fused connection unit or dedicated circuit, not a plug.

Wiring a Single Immersion

FCU with boost switch

Supply (via FCU from ring or dedicated)
        |
        v
[Fused Connection Unit 13A, switched]
        |
        +--- 2.5 mm² cable ---+
        |                      |
        |                      v
        +------------- [Immersion 3kW]
        |                      |
        +--- Return ------------+

Components:

• 13 A FCU — fuse protects cable
• 2.5 mm² twin and earth — rated for 20 A
• Thermostat — built-in, typically 60–70°C

Dedicated circuit

For primary heating:

• 16 A RCBO at consumer unit
• 2.5 mm² cable
• 20 A double-pole switch with neon near cylinder

Wiring a Dual Immersion

Economy 7 setup

Off-peak supply (Economy 7)
        |
        v
[Timer or ripple receiver]
        |
        +--- 2.5 mm² --- [Bottom Element 3kW]

Normal supply
        |
        v
[FCU or switch]
        |
        +--- 1.5 mm² --- [Top Element 1-3kW]

How it works:

• Bottom element runs overnight on cheap Economy 7
• Top element runs on peak rate for quick top-up

Related: How a Timer Switch Works and How to Wire One

Control Methods

Built-in thermostat

• Rod thermostat clips to element
• Temperature: 50–80°C
• Safety cut-out: manual reset if primary fails

Timer control

• Mechanical — 24-hour or 7-day
• Digital — programmable schedules
• Ripple control — utility-controlled for off-peak

Boost switch

• Manual override forces immersion on
• Often near cylinder or in kitchen
• Label clearly to avoid unintended peak-rate use

Cable Sizing

Most dual cylinders only run one element at a time via selector switch.

Part P Notification

Installing or replacing an immersion is notifiable if:

• New circuit or significant alteration
• Work in a bathroom/affected location
• New FCU installed

Use a Part P-registered electrician or notify Building Control.

Key Points

• 3 kW = 13 A — wire via FCU or dedicated circuit, not a plug
• Dual immersion typically has separate off-peak and peak supplies
• 10 mm² cable for 7 kW+, 6 mm² for 3 kW
• Economy 7 uses bottom element overnight, top element for boost
• Built-in thermostat controls temperature with safety cut-out
• Part P notifiable for new installations