Question 1

What is an immersion heater?

Accepted Answer

An immersion heater is an electric resistance heating element - essentially a kettle element of much larger size - screwed into the wall of a hot water storage cylinder. When energised, current flows through the nichrome element, which heats up by resistance and warms the surrounding water. A thermostat on the immersion heater head controls the element, typically set to 60-65 degrees C. Standard immersion heaters are 3kW and require a dedicated 20-amp radial circuit. Most cylinders have two immersion heater bosses - a long element from the top (heats the full cylinder) and a short element near the bottom (heats only the lower section for economy use).

Question 2

How expensive is it to run an immersion heater?

Accepted Answer

A 3kW immersion heater uses 3 units (kWh) of electricity per hour of operation. At typical UK electricity prices (approximately 24-28p per kWh as of 2024-2025), this costs around 72-84p per hour. Heating a typical 150-litre cylinder from cold takes approximately 2-3 hours, costing around 1.50-2.50 pounds per full heat-up. Running an immersion heater as the sole source of hot water for a household costs significantly more than heating water with a gas boiler (gas is typically 4-6p per kWh versus 24-28p for electricity). This is why immersion heaters are best used as backup or for the legionella pasteurisation cycle, not as the primary heat source, unless on an Economy 7 or smart tariff that provides cheaper overnight electricity.

Question 3

Does an immersion heater require a dedicated circuit?

Accepted Answer

Yes. An immersion heater must be connected to a dedicated 20-amp radial circuit (not shared with any other load) from the consumer unit, protected by a 20-amp MCB (or RCD+MCB combination). The circuit should use 2.5mm2 twin and earth cable and must be installed by a qualified electrician and notified to Building Control (as notifiable electrical work under Part P of the Building Regulations). The immersion heater must be controlled by a double-pole isolating switch with a red neon indicator (so it can be seen when the heater is on) mounted within reach of the cylinder - typically a 20-amp DP switch with neon indicator mounted on the wall adjacent to the cylinder.

Question 4

Why do heat pump systems need an immersion heater?

Accepted Answer

Heat pumps typically heat water to 45-55 degrees C for space heating and domestic hot water. However, Legionella bacteria can survive and multiply in warm water below 60 degrees C. Building Regulations and Health and Safety guidance (HSG274) require that stored domestic hot water be raised to at least 60 degrees C regularly (at least weekly) to pasteurise the water and prevent Legionella growth. Most heat pumps cannot efficiently heat water to 60 degrees C in a single cycle. The immersion heater in the hot water cylinder is therefore used for the weekly legionella pasteurisation cycle - typically an automated function in the heat pump controller that fires the immersion heater to reach 60 degrees C and hold for a period, then returns to normal heat pump operation.

Immersion Heater

Just so you know...