Dual Immersion Heater: Single or Dual, Which Do You Need?

How a dual immersion heater works

Every immersion heater is an electric element sitting in the water, with a thermostat that switches it off when the water reaches its setting. A dual setup simply gives you two elements that heat different amounts of the tank.

The short element, or the top one, heats roughly the upper third of the cylinder. Hot water rises, so the top of the tank is what comes out of the taps first. Heating only that part is quicker and uses less electricity, which is why it is labelled Sink: enough for washing up and a basin. The long element, or the bottom one, heats the full volume, which is why it is labelled Bath.

The point is choice. You heat the whole tank when you need it and a smaller amount when you do not, instead of paying to heat the full cylinder every time.

• Top or short element: heats the upper part of the tank, quick top-up, labelled Sink
• Bottom or long element: heats the whole tank, slower, labelled Bath
• Each element has its own thermostat, or the unit has one thermostat shared through the switch
• Off-peak systems usually run the bottom element on the night rate and the top one as a daytime boost

Two separate elements or one twin element?

There are two ways cylinders get a dual immersion, and you can usually tell from the outside.

The first is two separate immersions, each in its own boss. One enters near the top of the cylinder, or from the top pointing down a short way, and the other enters low down on the side. Each has its own cap, its own thermostat and usually its own switch or timer. This is the common arrangement on cylinders built to be heated by electricity alone, and it is the one that works properly with an off-peak tariff because both elements can run on separate controls.

The second is a single twin element immersion screwed into one boss at the top of the cylinder, with one long element and one short element inside a single head. The switch on the wall, marked Bath and Sink, decides which one runs. Only one element works at a time. This is common on older vented copper cylinders and it is a cheap way to give a single boss cylinder some choice.

If you are not sure which type of cylinder you have to start with, our guide to direct vs indirect hot water cylinder types shows how to tell from the pipework whether the immersion is your main heat source or a backup.

Which do you need?

Work it out from how the cylinder is heated day to day.

If a gas boiler heats the cylinder through its coil, you almost certainly only need a single immersion. It is there for when the boiler breaks down or is being serviced, not for daily use. Paying for a dual setup gives you very little. The explainer on what is an immersion heater covers why the two sit side by side.

If electricity is the only way the cylinder gets hot, as in many London flats with no gas, dual is the sensible choice, especially on a tariff with a cheap overnight rate. Heating the whole tank overnight on the long element and using the short one only when you run low is what keeps the running cost down. A single element on a direct cylinder means heating the whole tank every time, often at the day rate.

If you are on a single rate tariff with a direct cylinder, dual still helps if your use varies, because the short element lets you heat a smaller amount when that is all you need. It matters less than on an off-peak tariff.

• Gas boiler heats the cylinder: single immersion as backup is enough
• Electric only, off-peak tariff: dual, with the bottom element on the cheap rate
• Electric only, single rate tariff: dual is useful, single will work
• Unvented cylinder: use what the manufacturer specifies for that model

Why a dual immersion runs out of hot water

The most common complaint we get about dual immersions is hot water running out after one shower. On an off-peak cylinder that nearly always means the bottom element has failed. The night charge never heats the whole tank, so you wake up with only whatever the top element heated, and it goes in one shower. Because the top element still works, it looks like the cylinder is working, just not very well.

The second common cause is the switch. On a twin element unit, if the switch is left on Sink, only the top part of the tank is ever heated. On a separate element setup, the boost or timer may still be set to the old clock after a power cut, or not match the cheap hours of a new tariff.

Scale is the third. Hard London water crusts the elements, they take longer to heat and eventually burn out. Our guide on hot water runs out quickly covers the wider list, and immersion heater not working goes through the checks for an element that has stopped altogether.

Never switch an immersion on if the cylinder might be empty or part drained. The element is designed to sit in water and will burn out in seconds if it runs dry.

Can you change a single immersion to a dual?

On a vented copper cylinder with a single top boss, a twin element immersion can usually replace a single element in the same boss, with a suitable Bath and Sink switch and wiring. It is a common upgrade. The length has to suit the cylinder, and the electrics have to be right: an immersion is a high load, and the supply, the switch and the fuse need to match it.

On an unvented cylinder you cannot change the arrangement. Unvented cylinders use elements specific to the make and model, with a thermal cut-out built into the safety design, and the number of elements is set by the manufacturer. Replacement elements should be the maker's own part, and work on the sealed side needs a G3 qualified plumber.

Adding a second boss to an existing cylinder is not a realistic job. If you need a second element and the cylinder only has one boss, the honest answer is a new cylinder, which is worth thinking about properly using our guide on how to choose a hot water cylinder.

Running cost

Most domestic elements are rated at 3kW, whether single or dual, so each element uses 3kWh for every hour it is actually heating. A dual setup does not use less power per hour. It saves money by heating less water when that is all you need, and by making it easy to do the heavy heating on a cheap overnight rate. The detail and worked examples are in our guide on immersion heater running cost.

Getting a dual immersion tested or replaced

If only one element seems to work, the hot water runs out early on an off-peak tariff, or the switch or timer is behaving oddly, call 020 4652 5111. Tell us the make of the cylinder, whether you have a boiler and what the immersion is doing. An immersion heater repair visit tests each element, its thermostat and the cut-out on a meter, so you know which part has failed before anything is replaced.

Our rates are on the pricing page and agreed before we attend. The rate does not change on the day; the total depends on the parts and the time the job takes.