This is one of the most common questions about immersion heaters, and it comes with a persistent myth attached. Many homeowners believe it is cheaper to leave the immersion heater on all the time than to switch it on and off. It is not. This guide explains why the myth exists, what actually happens when you leave it on continuously, what the right approach is, and why repeated switching cycles on a heavy electrical load have safety implications that are worth understanding.
The Myth: It Is Cheaper to Leave It On
The belief that leaving an immersion heater on constantly saves money almost certainly originates from advice that was sometimes given about older gas central heating systems. There is a genuine argument, in certain circumstances, that keeping a well-insulated wet central heating system at a low background temperature can be marginally more efficient than repeatedly heating it from cold in very cold weather. That argument does not transfer to an immersion heater.
An immersion heater left on all the time is not simply sitting at temperature doing nothing. The hot water cylinder is continuously losing heat to the surrounding air in the airing cupboard, to the cold pipework connected to it, and through any weak points in its insulation. The thermostat responds to this heat loss by switching the element back on to restore the set temperature, then switching it off again when that temperature is reached. The element cycles on and off continuously throughout the day and night, consuming electricity each time.
The energy cost of this continuous cycling is the energy needed to replace every unit of heat that leaks out of the cylinder over the course of the day. In a well-insulated modern cylinder this standing heat loss might be 1.5 to 2 kWh per day. In an older, poorly insulated cylinder it can be 3 to 5 kWh per day. At the current electricity unit rate of approximately 24p per kWh, that represents between 36p and £1.20 per day in heat loss alone, before accounting for the energy used to heat water that is actually drawn off and used.
A timed immersion heater, set to heat the cylinder once in the morning for two hours, uses energy only during that heating period. Once the element switches off, the thermostat is no longer cycling, no electricity is being consumed, and the well-insulated cylinder retains enough heat to serve the household’s needs for several hours without any further input. The saving compared to continuous operation is real, measurable, and in most households significant.
What Actually Happens When You Leave It On
When the immersion heater is left on continuously, the thermostat is the primary control keeping the cylinder at its set temperature. As the water cools below the lower threshold of the thermostat, the element switches on. As the water returns to the set temperature, the element switches off. This cycle repeats throughout every hour of every day.
Each switching event is a thermal and mechanical event for every component in the circuit. The MCB or RCBO at the consumer unit sees a current inrush as the element starts. The isolator switch carries the full 13-amp load as the element runs. The thermostat contacts open and close. The flex and the connections at the heater head heat up and cool down with each cycle.
For a timed immersion heater that runs for two hours in the morning and perhaps an hour in the evening, this amounts to a handful of switching events per day. For an immersion heater left on permanently, the number of switching events is considerably higher, occurring through the night and throughout the day regardless of whether any hot water is being used. Over months and years, the cumulative effect on the thermostat contacts, the thermal cut-out, and the connections is measurably greater than for a timed installation.
The Right Approach: Timed Heating
The correct way to use an immersion heater is to heat the cylinder to temperature during a defined period timed to finish shortly before hot water is needed, then allow the thermostat to maintain that temperature passively through the cylinder’s insulation until the next heating period.
For most households, a morning heating period timed to finish around 30 minutes before the first shower works well. If the household also uses hot water in the evening for washing up or bathing, a second heating period in the late afternoon is appropriate. Between these periods the element is completely off and the cylinder is retaining heat from the last heating session.
A well-insulated modern cylinder will retain a usable temperature for six to eight hours after heating. An older cylinder with a thin or degraded jacket may cool more quickly, and the timer programme may need to be set accordingly. If the household finds the water is not hot enough by early afternoon, the morning heating period can be extended, a second period can be added, or the cylinder jacket can be improved.
A practical starting programme for a family of four with a 150-litre cylinder might be two hours of heating finishing at 7am, and one hour finishing at 6pm. This covers the morning shower routine and the evening washing up and bath, with the cylinder retaining adequate heat through both periods.
Setting Up a Timer
A 24-hour mechanical or electronic timer on the immersion heater is one of the cheapest and most effective improvements available to a household running on immersion heating. Mechanical timers use a rotating dial with push-in pegs that set the on and off times. Electronic timers have a digital display and buttons for programming. Both types are reliable and inexpensive.
When setting the timer, remember that the immersion heater heats water relatively slowly compared to a gas combi boiler. A 3kW element heating a 150-litre cylinder from cold takes approximately two to two and a half hours to reach 60 degrees Celsius. Allow enough time in the heating programme for the full cylinder to reach temperature, not just the upper portion. The thermostat will cut the element off automatically when the temperature is reached, so erring on the side of a slightly longer heating period wastes very little electricity.
If the timer is electronic and has lost its programme after a power cut, it will need reprogramming before it will operate the heater correctly. A timer showing the wrong time, or flashing 00:00, is not controlling the heater. Check the timer display before concluding there is a fault with the heater itself.
Economy 7 and Off-Peak Timing
For properties on Economy 7 or a similar off-peak tariff, the immersion heater timer should be set to run during the cheap overnight period, typically between midnight and 7am. A 3kW element running for two hours during the overnight cheap period at approximately 11p per kWh costs around 66p, compared to approximately £1.44 at the standard 24p daytime rate for the same heating period. The overnight charge fills the cylinder for the morning, and the cylinder retains heat through the day.
For households with a dual-element cylinder, the upper boost element can be used for a midday top-up if needed. The boost element heats only the upper portion of the cylinder, typically 50 to 80 litres, in 30 to 45 minutes, at the daytime rate. Using the boost element sparingly, only when the overnight charge has been depleted and hot water is genuinely needed, keeps the daytime electricity cost manageable. Using the boost as a substitute for a properly timed overnight charge is an expensive habit.
The Safety Dimension
Beyond the running cost argument, there is a genuine safety case for timed rather than continuous immersion heater use, and it relates to the nature of the load.
After the electric shower and the electric cooker, the immersion heater is typically the third-heaviest regular electrical load in an all-electric home. A 3kW element drawing 13 amps continuously is a sustained and significant load on the circuit. When that load cycles on and off repeatedly throughout the day and night for months and years, every component in the circuit experiences repeated thermal stress.
The thermostat contacts open and close with each switching event. Over time, thermostat contacts wear and the thermostat becomes less reliable. A thermostat that begins to fail may allow the water to overheat before the cut-out operates, or may fail to switch the element on at all. Either failure mode is a problem. In a continuously running installation, the thermostat is working harder and wearing faster than in a timed one.
The thermal cut-out, the safety device that disconnects the element if the thermostat allows the water to overheat, must be in working order for the installation to be safe. Electrical Safety First is clear that the thermostat and thermal cut-out are both essential safety components on an immersion heater. An installation where the element has been running continuously for many years, and where the thermostat and cut-out have never been inspected, is an installation where the safety devices may be degraded without the homeowner being aware of it.
The isolator switch, the flex, and the connections at the heater head are all subject to the same argument. A circuit that has been in continuous operation for years without inspection may have loose connections, degraded flex insulation, or a worn isolator that would be identified during a routine electrical check. An immersion heater that has been left on permanently and never inspected is worth having checked by a qualified electrician.
Legionella: The Temperature Requirement That Cannot Be Compromised
Whatever timing arrangement is used, the hot water cylinder must reach a minimum of 60 degrees Celsius at least once a week. This is not optional. The HSE is explicit in its guidance on Legionella control that stored hot water must be maintained at 60 degrees Celsius or heated to that temperature regularly to prevent the growth of Legionella bacteria, the cause of Legionnaires’ disease.
Legionella multiplies most readily in water held between 20 and 45 degrees Celsius. A cylinder that is turned down below 60 degrees to save electricity, or one that is running on a timer that only partially heats the water to a lower temperature, can provide conditions in which the bacteria multiply. This is a particular concern in properties that have been unoccupied for a period, where the water in the system has been standing at ambient temperature.
Setting the thermostat to 60 degrees and ensuring the timer is set to run for long enough to heat the full cylinder to that temperature at least once a week is the correct approach. Do not set the thermostat lower than 60 degrees to save electricity. The saving is marginal and the risk is not.
Holidays and Extended Absence
When a property is to be unoccupied for more than a couple of days, the immersion heater should be switched off at the isolator switch. There is no benefit to leaving it on, and leaving a 13-amp load running in an unoccupied property is an unnecessary risk.
Before returning to the property after a longer absence, particularly one of several weeks or more, it is worth running all the hot taps to flush the hot water system through before using it normally. This ensures any water that has cooled below the safe temperature threshold during the absence is replaced, and that the system returns to full operating temperature before being relied upon.
For very long absences, draining the hot water cylinder avoids the question of standing water and Legionella risk entirely. This is worth discussing with a plumber before a property is left empty for an extended period.
Smart Controls
For households that want to go beyond a basic timer, smart immersion heater controllers can learn the household’s hot water demand pattern and heat the cylinder optimally without manual programming. Devices such as Mixergy, Sunamp, and similar systems monitor cylinder temperature, track usage, and schedule heating automatically. Some can integrate with smart electricity tariffs to shift the heating load to the cheapest period of the day automatically, without the user needing to adjust the timer when tariff periods change.
Where the property has solar PV installed, a solar diverter routes surplus generation directly to the immersion heater during the day. Rather than leaving the immersion heater on all day at full tariff cost, the diverter uses electricity that would otherwise be exported to the grid at a much lower export rate. The result is effectively free hot water during sunny periods, reducing the reliance on grid electricity for immersion heating significantly.
Talk to Flodman Electrical
Flodman Electrical are NAPIT-approved electricians based in Farnborough, serving households across north Hampshire and Surrey including Aldershot, Fleet, Camberley, Farnham, Guildford, Basingstoke, Hook, and Yateley.
If your immersion heater has no timer, or if you are not sure when the circuit was last inspected, we can fit a timer or smart controller, carry out an inspection of the circuit and its components, and give you a clear picture of its current condition.
Call us on 01252 751322 or use the link below to get in touch.
Contact Flodman Electrical to discuss your immersion heater
Flodman Electrical Ltd, NAPIT Approved Contractor. Serving Farnborough, Aldershot, Fleet, Camberley, Farnham, Guildford, Basingstoke, Hook, Yateley and the surrounding area.
