Immersion Heater Replacement CR4

What is an immersion heater

An immersion heater is an electric heating element fitted directly into a hot water cylinder — either a vented copper cylinder in an airing cupboard or an unvented cylinder (Megaflo or similar). It works on the same principle as a kettle element: electrical resistance heats the element body, which transfers heat directly to the surrounding water. A dedicated 16A or 20A circuit powers the element, which is typically rated at 3kW.

The thermostat controls the element, cutting power when the water reaches the set temperature. The minimum recommended thermostat setting is 60 degrees Celsius to prevent Legionella bacteria growth in the cylinder. Stored water at temperatures between 20 and 45 degrees Celsius is the primary risk range for Legionella proliferation.

Types of immersion heater element

Top-entry elements are the most common type in UK cylinders. The element enters the cylinder through a 2.25-inch BSP boss at the top of the cylinder and hangs vertically into the water. Side-entry elements enter through a boss on the side of the cylinder and are more common in older vented copper cylinders. Both types use the same basic element construction but differ in the thread and fitting arrangement.

Dual immersion cylinders have two element positions. The bottom element heats the full cylinder volume — typically 120 to 180 litres — overnight on Economy 7 off-peak electricity. The top element heats only the upper portion of the cylinder (approximately 50 litres) for a faster daytime boost when small volumes of hot water are needed without heating the full cylinder.

When to replace the immersion element

The element requires replacement when it fails to heat water despite the thermostat being correctly set and the circuit breaker being in the on position. A failed element is confirmed by checking continuity with a multimeter across the element terminals. Zero continuity (open circuit) confirms element failure. Elements also require replacement when the flange shows visible corrosion or when scale deposits have built up to the point that heating efficiency is severely impaired.

Immersion element replacement process

Replacing an immersion element requires isolating the electrical supply at the consumer unit and confirming with a voltage tester that the circuit is dead before touching any wiring. The cold feed to the cylinder is shut off and the cylinder is drained to below the element position — either fully for a top-entry element or partially for a side-entry element positioned above the cold feed inlet. The element wiring connections are photographed and disconnected. An immersion heater spanner — typically an 11-inch socket type — is used to unscrew the element body from the BSP boss.

The new element is fitted with a new fibre washer under the flange and PTFE thread sealant on the male thread. The element is torqued finger-tight plus a quarter turn. Wiring is reconnected to the original polarity. The cylinder is refilled and checked for leaks at the element flange. The circuit breaker is restored and the thermostat operation is confirmed.

Unvented cylinders — additional requirements

Unvented hot water cylinders (Megaflo, OSO, Santon, Heatrae Sadia) operate under mains water pressure. Any work on the cylinder, including immersion heater replacement, requires the engineer to check the expansion vessel pre-charge pressure and inspect the temperature and pressure relief valve (T and P valve). Building Regulations Part G requires this work to be carried out by a person holding the G3 unvented systems qualification. Prestige Engineers engineers hold this qualification.

The expansion vessel is a small pressurised vessel connected to the cylinder that accommodates the expansion of water as it heats. If the vessel loses its nitrogen pre-charge (typically 3 bar), the expansion is not accommodated and the T and P valve opens repeatedly, discharging water through the tundish. We check and recharge the vessel at every unvented cylinder service visit.

Economy 7 and off-peak immersion heating

Many London flats and houses use Economy 7 electricity tariffs, which offer a lower per-unit rate for electricity used during a 7-hour overnight window. A dual immersion cylinder with a timer set to the overnight Economy 7 window heats a full cylinder of water at the cheaper night rate. A boost switch activates the top element during the day for additional hot water when required. We set up Economy 7 immersion timers, check the wiring, and advise on optimal timer settings for the cylinder size and household hot water demand.