Immersion Heaters in London Homes: Wiring, Timers and Warning Signs

Many London homes, especially flats with electric heating and houses with a hot water cylinder, rely on an immersion heater for hot water. It is a simple device, but it draws a lot of power for long periods, so the wiring, switch and controls need to be in good condition. This guide covers the common problems and what an electrician checks.

How an immersion heater is wired

An immersion heater usually has its own circuit from the consumer unit, ending at a double-pole switch near the cylinder, often marked "water heater". From there a heat-resistant flex runs to the element in the cylinder. Some cylinders have two elements, one at the top for boosting and one at the bottom for cheaper off-peak heating.

Warning signs to look out for

• A switch or flex that feels hot, looks discoloured or smells of burning.
• The breaker or RCD for the water heater tripping.
• No hot water, or water that is only lukewarm.
• An old switch with a cracked front or a loose fit on the wall.
• A flex that is ordinary PVC cable instead of a heat-resistant type.

If you notice burning smells or scorch marks, switch the heater off at the consumer unit and call an electrician. See our guide on why an RCD keeps tripping.

Timers and controls

A timer or smart controller lets you heat water when electricity is cheaper, for example overnight on an off-peak or EV tariff, instead of leaving the immersion on all day. Some homes with solar panels use a diverter to send spare solar power to the immersion. These controls must be suitable for the load and fitted correctly. See our guide on electric storage heaters, which often share the same off-peak setup.

What an EICR checks

An EICR looks at the water heater circuit along with the rest of the installation: the protective device, the switch, the condition of the flex and connections, and earthing and bonding near the cylinder. Landlords need a satisfactory EICR for rented homes. See earth bonding explained.

Replacing an element or switch

Replacing an immersion element means draining and refilling the cylinder and is often done by a plumber. The switch, flex and circuit are electrical work and should be done by a qualified electrician, who will test the circuit afterwards.

Book a check

Energy North Ltd checks and repairs water heater circuits across North, East and West London with City & Guilds certified electricians. EICR from £100. See our EICR service.