Immersion Heater Replacement in London: A Complete Guide
If your hot water cylinder is no longer producing hot water and your heating is working fine, the immersion heater element or thermostat has most likely failed. This guide explains how immersion heaters work, what fails, and what replacement involves.
What Is an Immersion Heater
An immersion heater is an electric heating element installed directly inside a hot water storage cylinder. It heats the water in the cylinder in the same way a kettle element heats water — by electrical resistance. Unlike a combination boiler that heats water on demand, an immersion heater heats a stored volume of water (typically 100 to 200 litres) and keeps it hot until needed. It is controlled by a thermostat, which cuts power to the element when the water reaches the set temperature.
Immersion heaters are found in properties without a gas supply, in properties with a separate hot water cylinder (rather than a combi boiler), and as a backup heat source in properties where the primary heating is via a gas or oil boiler with a cylinder. In London flats and Victorian terraced houses, immersion heaters are common in properties that have not been converted to combi boiler systems.
Types of Immersion Element
Top-entry elements are the most common type in UK cylinders. They enter through a 2.25-inch BSP threaded boss at the top of the cylinder and hang vertically into the water volume. Side-entry elements enter through a boss on the side of the cylinder and are fitted horizontally or at an angle into the water body. Both types use the same basic construction — a stainless steel or copper tube containing a resistance wire embedded in magnesium oxide insulating material, sealed at the open end with a flange and thermostat pocket.
Dual immersion cylinders have two element positions — one near the bottom of the cylinder for heating the full tank volume, and one near the top for a faster boost of the upper portion only. The bottom element is typically used overnight on Economy 7 off-peak electricity to heat the full tank at reduced cost. The top element provides a quick daytime boost when additional hot water is needed without the time or cost of heating the full cylinder volume.
Diagnosing a Faulty Immersion Heater
The most common symptoms of immersion heater failure are: no hot water from taps despite the immersion being switched on, the circuit breaker for the immersion heater tripping repeatedly, water that is warm but not reaching the normal temperature, or the water becoming scalding hot (indicating a thermostat fault allowing the element to run continuously). To diagnose which component has failed, the engineer isolates the electrical supply, confirms the circuit is dead with a voltage tester, and then tests the element for continuity using a multimeter. Zero continuity across the element terminals confirms the element has failed. A thermostat that reads the correct set temperature but the element is not reaching it indicates a thermostat fault.
The Replacement Process
Replacing an immersion element requires isolating the electrical circuit at the consumer unit and confirming dead with a tester before opening any wiring. The cold water supply to the cylinder is closed and the cylinder is drained to below the element position. On a top-entry element in a direct cylinder, this means draining most of the cylinder. On a side-entry element, partial draining to below the element level is sufficient. An immersion heater spanner — a long socket type designed to engage the hexagonal flange — is used to unscrew the element. The new element is fitted with a new fibre or rubber sealing washer and PTFE thread sealant on the male thread. The cylinder is refilled, and the system is checked for leaks at the element flange before restoring the electrical supply.
Unvented Cylinders — G3 Qualification Required
Unvented hot water cylinders (manufactured by Megaflo, OSO, Santon, Heatrae Sadia, and others) operate under mains water pressure rather than gravity feed from a cold water storage tank. This means the cylinder contains water at typical London mains pressure of 2 to 4 bar. Any work on an unvented cylinder — including immersion heater replacement — requires the attending engineer to hold a G3 Building Regulations qualification. This qualification covers the safety checks specific to unvented systems: expansion vessel charge pressure, the temperature and pressure relief valve, the tundish and discharge pipe arrangement, and the cold water supply pressure and flow rate. Prestige Engineers engineers are G3 qualified for all immersion heater work across London.
Legionella and Thermostat Settings
The immersion heater thermostat must be set to a minimum of 60 degrees Celsius. Water stored at temperatures below 60 degrees Celsius — particularly in the range of 20 to 45 degrees — supports the growth of Legionella bacteria. A weekly thermal pasteurisation cycle at 65 degrees Celsius for one hour kills Legionella bacteria present in the cylinder and in the hot water distribution pipework. Smart timers and programmable thermostats make this routine automatic. We check and set the thermostat on every immersion heater visit and advise on pasteurisation timer setup. Contact Prestige Engineers for immersion heater replacement across all London boroughs, typically attended same day.