Immersion Heater Timer: Is It Worth Fitting One?

Do you need an immersion heater timer?

Start with how the cylinder is heated on a normal day, because that decides whether a timer earns its place.

If a gas boiler heats the cylinder through its coil, the boiler programmer already controls when the hot water is made. The immersion only comes on when the boiler is broken or being serviced. A seven day timer on that immersion is mostly unused. What does go wrong in these homes is the immersion being switched on during a boiler breakdown and then forgotten, heating the tank electrically for months. A countdown boost switch, which runs for a set time and then turns itself off, fixes exactly that.

If the immersion is the main heat source, as it is in many London flats with no gas, the timer is what controls your bill. Without one the element keeps the whole tank hot around the clock, topping up heat lost through the insulation whether anyone needs the water or not. With one, you heat the water once or twice a day, shortly before you use it.

If you are on an off-peak tariff, the timer is the tariff. The cheap hours only save money if the heating happens inside them, and a dial that has slipped by two hours can turn a cheap night rate into an expensive day rate without anyone noticing until the bill arrives.

• Gas boiler heats the cylinder, immersion is backup: a countdown boost switch is enough
• Electric only, single rate tariff: a programmable timer is worth fitting
• Electric only, Economy 7 or similar: a timer is essential, ideally one channel per element
• Solar panels with a diverter: keep a timer or boost for the dark months

The types of immersion timer and which suits which home

All of these replace the double pole switch that already feeds the immersion, usually on the wall beside the airing cupboard. None of them plugs into a socket. An immersion element is a heavy load on its own circuit, and anything controlling it has to be rated for that load continuously, not just for a few minutes.

A mechanical timer has a 24 hour dial with small tabs you push in for each block of time you want the element on. It is cheap and simple. After a power cut it stops and restarts at the wrong time, which is the most common reason a mechanical timer quietly heats the water at the wrong hours.

A digital timer lets you set different times for different days, usually with a battery or capacitor backup so it keeps the clock through a power cut, and a boost button for an extra hour when you run short. For an electric only flat on a single rate tariff this is the usual choice.

A dual channel controller runs two elements on separate programmes. That is the right answer on an off-peak cylinder with a top and bottom element: the bottom element heats the whole tank in the cheap window overnight and the top element is left for a short daytime boost. If you are not sure whether your cylinder has one element or two, our guide to the dual immersion heater shows how to tell from the outside.

A smart or app controlled timer does the same job from your phone, and some show how much power the element has used. Useful if your hours change often, but no cheaper to run than a well set digital timer.

If you have solar panels, a solar diverter sends spare daytime generation to the immersion instead of exporting it. It does not replace the timer: in winter there is little spare power, and the timer or boost still has to make sure the water gets properly hot.

A countdown boost switch has no programme at all. You press it, it runs for 30, 60 or 120 minutes and then turns off. For a home with a working gas boiler it is the most sensible control the immersion can have.

On an unvented cylinder the immersion has a thermal cut-out built into its safety design. A timer switches the supply to the immersion and nothing else. It must never be wired to bypass or replace the thermostat or the cut-out.

How to set an immersion heater timer

The aim is to have the water hot shortly before you need it and not to keep it hot when nobody is using it. How long the element needs depends on the size of the cylinder and how cold it starts. As a rough guide, a single shower worth at the top of the tank takes well under an hour from cold, and a full tank takes a couple of hours or more. The detail by cylinder size is in our guide on how long does hot water take to heat up.

Once the element has brought the water to temperature, its thermostat switches it off, so a slightly generous window costs little. What costs money is leaving the timer on all day on a single rate tariff, because the element keeps cycling on to replace heat the tank loses through its insulation.

On Economy 7 or a similar tariff, set the bottom element to run through the cheap window, and leave the top element on boost only. Check the actual hours with your supplier rather than assuming midnight to seven. Many off-peak meters keep to GMT all year, so in summer the cheap window starts and ends an hour later by the clock on the wall, and a timer set in winter can end up heating the last hour at the day rate.

Do not use the timer to keep the water cool. The stored water should reach 60 degrees regularly so that Legionella cannot grow in a tank of lukewarm water, which is why the thermostat is set there. Our article on what temperature should a hot water cylinder be set at explains the numbers.

• Single rate, electric only: one or two short windows before the busiest times
• Off-peak with two elements: bottom element through the whole cheap window, top element on boost
• Off-peak with one element: run it through the cheap window and accept that a large day time top-up costs day rate
• Gas boiler home: immersion off, boost only when the boiler is out of action

Does a timer actually save money?

It saves money by stopping heat being wasted, not by making the element more efficient. An element rated at 3kW uses 3kWh for every hour it is actually heating, whatever controls it. The saving comes from not heating water nobody uses, from not reheating a tank all day to cover standing loss, and on an off-peak tariff from moving the heating onto the cheap rate.

How much that is worth depends on the cylinder and the tariff. On a modern, factory insulated cylinder the difference between left on and timed is smaller than people expect, because the tank loses heat slowly. On an old copper cylinder with a thin jacket it is much larger, and fitting a proper hot water cylinder jacket or replacing the cylinder may save more than the timer does. Our guide to immersion heater running cost works through the arithmetic with your own tariff.

Timer on but no hot water

Check the simple things first: the clock is showing the right time, the programme has not been reset by a power cut, the tabs on a mechanical dial have not been pushed back out, and the backup battery on a digital timer has not run flat.

If the timer shows it is on and a neon or light confirms power is reaching the immersion, but the water is still cold, the problem is further along: a failed element, a thermostat stuck open or a cut-out that has tripped. Those are covered step by step in immersion heater not working, including which checks are safe to do yourself.

Getting a timer fitted or checked

Fitting a timer means working on the fixed wiring of a high load circuit, and on an unvented cylinder it sits next to safety controls that must not be disturbed. It is a job for someone competent with both. The switch, supply and cable all need to suit the element.

If you want a timer fitted, or the hot water is not coming on when the timer says it should, call 020 4652 5111. We will look at the switch, the timer and the element together, because a timer fitted to a failing element only programmes the failure. An immersion heater repair visit tests the element, its thermostat and the cut-out on a meter before anything is replaced. Our rates are on the pricing page and agreed before we attend.