Electrical

Immersion Heater Installation and Wiring

Name: TradePlanr
Rating: 5 (3 reviews)
Author: TradePlanr

An immersion heater is a fixed, high-load appliance that needs its own dedicated circuit, heat-resisting flex, and local double-pole switching — wired and tested to BS 7671.

Updated 2 Sep 20267 min read

An electric immersion heater in a hot water cylinder is a straightforward but heavy load — a 3 kW element drawing around 13 A continuously when it's heating. It needs its own dedicated circuit, the correct heat-resisting final connection, and proper double-pole switching, all installed and tested to BS 7671.

This guide covers the dedicated circuit and cable sizing, the heat-resisting flex to the element, switching and isolation, and the thermostat and safety cut-out that keep it safe. It assumes a new or replacement circuit installed with safe isolation before work begins.

Key takeaways

An immersion heater needs its own dedicated circuit — not a spur off a socket ring.
Size the circuit for a continuous load (typically 3 kW / ~13 A) plus its cable and device.
Use heat-resisting flex for the final connection to the element.
Switch and isolate with a double-pole switch — commonly a 20 A DP flex-outlet switch.
The element relies on a thermostat plus a thermal cut-out for safety — check both.

The dedicated circuit

An immersion heater is a fixed appliance drawing a sustained heavy current, so it takes its own circuit from the consumer unit — typically a 16 A or 20 A radial in 2.5 mm² cable, sized for the specific load, run and installation method. It should not be run as a spur off a socket circuit; a continuous 13 A load has no business sharing a ring.

Because the load is continuous when heating, size conservatively and check the cable's rating against the installation conditions rather than assuming the minimum.

Size it for continuous load

Unlike a socket that sees intermittent use, an immersion runs at full current for long periods. Size the cable and device for that sustained load, allowing for grouping and insulation on the run.

Switching and isolation

The immersion is controlled and isolated by a double-pole switch — commonly a 20 A double-pole switch with a flex outlet, mounted within reach of the cylinder but clear of it. Double-pole switching breaks both line and neutral, which matters on a fixed heating appliance for safe isolation during maintenance.

Where an immersion has both a top and a bottom element (for economy heating), each is switched and often controlled separately. Label the switching clearly so the user — and the next electrician — knows what does what.

Doing this on a job? Minor Works Certificate — part of TradePlanr. Free to start, no card needed.

Heat-resisting flex to the element

The final connection from the switch to the immersion element must be heat-resisting flex, because the top of the cylinder gets hot and ordinary PVC flex would degrade over time. Use the correct high-temperature flex rated for the location and connect it properly at both ends, with the CPC connected to earth the element.

Keep the flex clear of the hottest parts where you can, and don't substitute ordinary flex to save a few pence — it's a known cause of failures at the element.

Certify the circuit

Issue the appropriate certificate from TradePlanr — a Minor Works Certificate for a like-for-like replacement, or an EIC for a new circuit — with the test results, so the immersion install is documented.

Thermostat, safety cut-out and testing

The immersion element has a thermostat to control the water temperature and, critically, a thermal cut-out (a non-self-resetting safety device) to prevent the element continuing to heat if the thermostat fails. On a replacement or fault, check the cut-out is present and functional — a common cause of "no hot water" is a tripped thermal cut-out, and its tripping means the thermostat should be investigated too.

Test the circuit fully — continuity, insulation resistance, polarity, Zs and RCD operation where fitted — and certify it. A new dedicated circuit is notifiable; a like-for-like replacement is recorded on a Minor Works Certificate.

From guidance to action

Cable sizing calculatorSize the immersion circuit for its continuous load and run.Minor Works CertificateCertify a like-for-like immersion replacement on site.

Frequently asked questions

Does an immersion heater need its own circuit?

Yes. It's a fixed appliance drawing a continuous heavy current, so it needs a dedicated circuit from the consumer unit — typically a 16 A or 20 A radial — not a spur off a socket ring.

Why does an immersion need heat-resisting flex?

The top of the hot water cylinder gets hot, and ordinary PVC flex would degrade and eventually fail there. Heat-resisting flex rated for the temperature is used for the final connection to the element.

My immersion has no hot water but the switch is on — why?

A common cause is a tripped thermal cut-out on the element, which trips when the thermostat fails to switch off. The cut-out is a safety device — if it has operated, the thermostat should be investigated rather than simply resetting it.