Immersion Heater Not Working: Fault Finding Guide

An immersion heater is a self-contained electric heating element fitted directly into a hot water storage cylinder. It operates at 230V and typically draws 3kW, heating the water around it through direct conduction. Most cylinders have one or two heater bosses — the upper heater heats a small volume quickly for washing up, whilst the lower heater heats the full cylinder capacity.

The immersion heater circuit is a dedicated radial circuit from the consumer unit, wired in 2.5mm² twin and earth cable, protected by a 16A MCB and typically an RCD. The heater is switched via a 20A double-pole switch with a neon indicator, mounted on the airing cupboard wall.

The safe isolation procedure for an immersion heater circuit follows the same steps as any electrical isolation:

These steps apply whether you are a qualified electrician carrying out a repair or a competent person inspecting the heater head. Members of the public should not remove the heater head cover without completing safe isolation first.

Thermostat failure is the most frequently encountered immersion heater fault. The symptoms depend on the mode of failure:

Thermostat replacement does not require draining the cylinder. After safe isolation, the heater head cover is removed, the thermostat is unclipped from the element sheath, and the replacement is clipped in. The job typically takes 30 to 60 minutes for an electrician. Thermostat parts cost £10 to £25; labour is £40 to £80, making total repair cost approximately £50 to £105.

A burned-out heating element is the second most common immersion heater fault. Elements fail due to age, limescale build-up in hard water areas, or electrical breakdown of the insulation. The element typically fails by developing a fault to earth, which trips the RCD.

In hard water areas (most of England south and east of a line from Exeter to the Humber), consider fitting a titanium or incoloy element rather than copper. Titanium elements resist limescale significantly better and typically last twice as long as copper elements in hard water.

Many unvented hot water cylinders and some vented cylinders include a sacrificial anode — a rod of magnesium or aluminium that corrodes preferentially to protect the cylinder walls and the heating element. Anodes are particularly important in hard water areas where electrochemical corrosion is accelerated.

Not all cylinders have an accessible sacrificial anode. If yours does not, consider a powered anode system (impressed current cathodic protection) if you are replacing the cylinder, particularly in coastal or industrial areas with aggressive water chemistry.

Wiring faults on immersion heater circuits are less common than element or thermostat failures but should be investigated if element and thermostat tests are satisfactory.

Deciding whether to repair or replace an immersion heater depends on the age of the cylinder, the nature of the fault, and the local labour rates. Here are typical UK costs for 2026:

As a general rule: if the cylinder is under 10 years old, repair is usually the right choice. If the cylinder is over 15 years old or has had multiple repairs, full replacement offers better value and significantly reduces the risk of a catastrophic cylinder failure.

Immersion heater repair and replacement is routine domestic work for electricians. It falls under Part P of the Building Regulations in England and Wales as notifiable electrical work in a bathroom or kitchen (the airing cupboard is treated as a kitchen for Part P purposes if adjacent to a kitchen). Registration with a competent person scheme such as NICEIC or NAPIT means you can self-certify the work without local authority notification.

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