Run your immersion on an off-peak tariff.

Why the immersion is worth timing

A typical immersion heater is rated at 3kW, the same as a kettle, but it can run for hours to heat a full cylinder. Left on a basic switch or an old mechanical timer, it often heats water at peak rates in the middle of the day.

Most suppliers now offer tariffs with a cheaper overnight window, from traditional Economy 7 to smart and EV tariffs. If your immersion heats the tank inside that window, you pay the lower rate for the same hot water. The saving depends on your tariff and how much hot water you use, but the immersion is usually one of the easiest loads in the house to shift.

Economy 7, smart tariffs and your meter

How you time the immersion depends on the tariff and meter you have.

• Traditional Economy 7: the cheap hours are set by the meter, often a seven hour window overnight. Some older homes have a separate off-peak circuit or a dual immersion with a top element and a bottom element.
• Smart time-of-use and EV tariffs: the cheap window is usually shorter and set by your supplier, for example late evening to early morning. There is no separate switched supply, so a timer does the work.
• Times can change: suppliers do adjust windows. A Wi-Fi timer can be updated from your phone in seconds, while a mechanical one needs someone in the airing cupboard.

Check the exact cheap hours on your bill or supplier app before setting any schedule.

Wi-Fi timers versus standard immersion timers

An immersion timer replaces the switch that controls the heater, usually a 20 amp double-pole switch or a fused spur near the cylinder. There are three common types.

• Mechanical dial timers: low cost and simple, but they drift after power cuts and are awkward to change.
• Digital timers: more precise, with several on and off times a day and a boost button.
• Wi-Fi timers: set and change schedules from an app, boost from anywhere, and some keep running their schedule if the Wi-Fi drops. Several well-known UK brands make them as a direct replacement for an immersion switch.

Whichever you choose, it must be rated for a continuous 3kW resistive load. A cheap plug-in timer or smart plug is not suitable, as it is not designed to carry that current for hours at a time.

What an electrician checks before fitting one

Fitting a timer is usually a short job, but the checks matter because an immersion runs at full load for long periods.

• The circuit is dedicated to the immersion, with the right cable size and protective device
• The existing switch, connections and back box show no sign of overheating or scorching
• The flex from the switch to the heater is heat-resistant, as standard flex can degrade in a hot airing cupboard
• The timer's rating suits the heater, and there is a sensible place for it to be installed
• The Wi-Fi signal reaches the airing cupboard, or a different position is agreed
• The cylinder thermostat works, so the water does not overheat

Does it need Part P or a certificate?

Replacing an existing control switch with a timer on the same circuit is generally not notifiable under Part P of the Building Regulations in England. The work still has to meet BS 7671, and a good electrician will test the circuit and give you a Minor Electrical Installation Works Certificate.

If the immersion does not yet have its own circuit, or the existing one needs replacing, that becomes new circuit work. New circuits are notifiable, and as an NICEIC Approved Contractor we self-certify them and register the work for you.

Keeping the water hot and safe

Heating only overnight works best with a well-insulated cylinder. If you run out by the evening, a short afternoon boost, or a second cheap window if your tariff has one, usually solves it.

Keep the cylinder thermostat at around 60°C so stored water gets hot enough to control bacteria such as legionella. If you have young children or elderly people at home, a thermostatic mixing valve on the hot water outlet can lower the temperature at the taps without lowering it in the tank.

Solar diverters and other options

If you have solar panels, a solar diverter can send spare daytime generation into the immersion instead of exporting it, with the timer as an overnight top-up. A heat pump or a modern cylinder with smart controls are bigger projects worth considering if you are replacing the whole system. See solar and battery storage.

Common questions

This guide is general information for properties in England and is not a substitute for an inspection by a qualified electrician.