Hot water problems
What Does an Immersion Heater Do?
Published
An immersion heater heats the water in a hot water cylinder using electricity. It is a metal element, like a large kettle element, that sits inside the tank and is controlled by its own thermostat. In homes with a boiler it is usually a backup; in homes without gas it is often the main way water gets hot.
How an immersion heater works
The heating part is a loop of metal tube, called the element, that sits in the water inside the cylinder. Inside the tube is a resistance wire packed in insulating powder. When current flows through the wire it heats up, the tube heats the water around it, and the hot water rises to the top of the cylinder while cooler water sinks to the bottom. That natural circulation is why hot water is drawn off the top of the tank.
Beside the element, in its own tube, sits a rod thermostat. You set it with a small dial under the cover, usually to around 60°C. When the water reaches that temperature the thermostat switches the element off, and when it cools it switches it back on. Built into the same thermostat is a safety cut-out that trips if the water gets far too hot, and stays off until someone presses its reset button.
Almost every domestic immersion element in the UK is rated at 3 kW, the same as a powerful kettle, and it is wired on its own circuit from the consumer unit, through a double-pole switch near the cylinder.
What does an immersion heater look like?
You will not see the element itself, because it is inside the tank. What you see is the head where it enters the cylinder. The easiest way to find it is to follow the thick heat-resistant flex from the wall switch to the cylinder.
- The cylinder: usually in an airing cupboard, a tall copper tank sprayed in yellow or green foam insulation, or wrapped in a red or green insulating jacket on older ones. Unvented cylinders are often white or grey metal cases.
- The cover: a round plastic or metal cap, about the size of a saucer, either on the dome at the top of the cylinder (top-entry) or low down on the side (side-entry).
- Under the cover: a brass fitting screwed into the cylinder, called the boss, with two terminals for the element and the thermostat’s dial and wiring on top. The UK standard thread for this boss is 2¼ inch BSP.
- The switch: a square wall switch nearby, often labelled water heater, with a neon light. Some have two rockers marked bath and sink, which choose between a long element and a short one.
- On Economy 7 cylinders you may see two covers, one near the top and one near the bottom, or a single head with two elements of different lengths.
Leave the cover on unless the circuit is isolated at the consumer unit. The terminals underneath are at mains voltage whenever the switch is on.
Do I need an immersion heater if I have a boiler?
It depends on the kind of boiler. A combi boiler heats water instantly as you run the tap and has no hot water cylinder at all, so a home with a combi has no immersion heater. If you have a combi and no hot water, the fault is in the boiler; see combi boiler hot water not working.
A regular or system boiler heats a separate hot water cylinder through a coil of pipe inside it. That type of cylinder usually has an immersion heater as well. It is a backup for when the boiler breaks down or is being serviced, and some people use it in summer so the boiler does not have to fire just for hot water.
In flats and houses with no gas supply, the immersion is often the only way hot water gets made, typically on an Economy 7 or other off-peak tariff with a timer. In that case a failed element means no hot water at all, and it becomes an urgent repair rather than a backup.
Top-entry, side-entry and dual elements: the differences that matter
Top-entry immersions hang down from the dome of the cylinder. They come in lengths such as 11, 14, 18 and 27 inches, and the length decides how much of the tank they heat, since an element only heats the water beside it and above it. A short one gives a small amount of hot water quickly; a long one heats most of the tank.
Side-entry immersions sit near the base and heat almost the whole cylinder. On off-peak cylinders the bottom one runs overnight on the cheap rate, and a second, higher element is used for daytime top-ups.
Dual or twin elements share one head: a long element for a full tank and a short one for a quick top-up, chosen by the bath and sink switch. The layout matters when something goes wrong, because a side-entry element means draining most of the cylinder to replace it, which affects what the job costs. See immersion heater replacement cost for how.
How to tell if your immersion heater is working
Switch it on for an hour and feel the pipe leaving the top of the cylinder, carefully, since it can get very hot. A working immersion will have made it hot. If it stays cold, check in this order: the neon on the switch, the timer, the consumer unit, then the thermostat reset. Our guides on immersion heater stopped working and how to reset an immersion heater go through each check.
The clues that point to a fault rather than a setting are an RCD that trips the moment the immersion is switched on, water that only ever gets lukewarm, a reset button that trips again within days, or a burning smell at the switch. Each of those needs an engineer. What it costs to run while healthy is covered in how much an immersion heater costs to run.
Our immersion heater repair service tests the supply, thermostat and element and replaces whichever has failed. It starts with the £60 call-out at any hour, and the repair price is agreed with you on site before any work starts.
Questions people ask
Where is my immersion heater?
On the hot water cylinder, usually in an airing cupboard. Look for a round cover on the top of the cylinder or low down on its side, with a thick flex running to a switch on the wall nearby. A home with a combi boiler has no cylinder and no immersion.
Should I leave my immersion heater on all the time?
It is safe, because the thermostat switches it off at the set temperature, but it costs more than using a timer. The tank stays hot all day and you pay to replace the heat it loses. Heating once or twice a day before you need the water is cheaper.
What temperature should an immersion heater be set to?
Around 60°C. HSE guidance on controlling legionella is to store hot water at 60°C or higher. Setting it much hotter wastes energy, scales the element faster and makes the safety cut-out more likely to trip.
Is an immersion heater the same as a boiler?
No. A boiler burns gas or oil and heats both radiators and hot water. An immersion heater is an electric element that only heats the water in the cylinder it is fitted to.