Which Immersion Heater Do I Need? Sizes, Types and Ratings

Start with the cylinder, not the shop shelf

Almost every question about which immersion to buy is answered by the one already fitted. Before ordering anything, find out where it enters the cylinder, how long it is, and what kind of cylinder it sits in. A new element that is the wrong length either leaves half the tank cold or will not go in at all.

Look at the cylinder first. If the immersion cap is on the domed top, it is a top entry element and it will be long, reaching down most of the height of the tank. If it is on the side, it is a side entry element and it will be short, sticking straight in near the bottom or part way up. Some cylinders have two side bosses, one low and one high, which is the usual layout on cylinders built for off peak electricity.

Then read the data label on the cylinder. It tells you whether the cylinder is vented or unvented, whether it is direct or indirect, and on an unvented unit it usually names the immersion part or at least the rating. If there is no label and you are not sure what you have, the difference between an unvented vs vented cylinder is worth settling before anything else, because it decides whether you are choosing a part or ordering one.

Immersion heater sizes: the lengths and what they fit

Element length is quoted in inches in the UK trade even when the box also shows millimetres. The length is the part that sits inside the water, measured from the underside of the head. The common sizes each have a typical job.

The 27 inch element is the standard long top entry heater. It hangs down through most of a normal height cylinder so the whole volume is heated. Longer 30 and 36 inch versions exist for taller tanks. Fitting a 27 inch element in a cylinder that needs a 36 will heat only the top two thirds, which then gets described as the hot water running out.

The 11 inch and 14 inch elements are the usual side entry sizes. They go in horizontally, so the length is limited by the diameter of the cylinder. A side element too long for the tank will not go in; one too short heats less water than it should.

Dual or twin element heaters put a short and a long element in one head on a top boss, each with its own thermostat and its own supply. They were fitted to run the long element on off peak electricity and the short one as a daytime boost. If yours has two sets of terminals under the cap, you have one of these, and our page on the dual immersion heater covers how they are wired and switched.

• 27 inch: standard top entry, heats most of a normal height cylinder
• 30 or 36 inch: top entry for taller cylinders
• 11 or 14 inch: side entry, low or high on the tank
• Dual element: a long and a short element in one top entry head
• Unvented cylinders: the maker's specified element, often on a smaller boss

The boss thread, and why unvented cylinders are different

On most vented cylinders the immersion screws into a standard 2 1/4 inch boss, which is why a vented cylinder element from one maker will generally fit a cylinder from another. It seals on a fibre or rubber washer under the head, and that washer should always be new when the element is changed.

Unvented cylinders are designed around their safety controls. Many take a smaller boss and an element built with an extra high limit cut-out that shuts off the power if the thermostat fails, as part of the protection that stops a sealed tank overheating. A standard vented element can look similar and even screw in, but it does not carry the same protection and it is not what the cylinder was approved with. On an unvented cylinder, use the part the manufacturer lists for that model and nothing else.

That is also why changing an element on an unvented cylinder is work for someone qualified to work on them, not a general replacement. The cylinder has to be isolated and drained down properly, and the safety controls should be checked while it is open.

If the only immersion you can find for an unvented cylinder is a generic one, stop there. The absence of the right part is a question to put to an engineer, not a gap to fill with whatever fits the thread.

Copper, Incoloy or titanium: which element material

This is the choice that matters most in London, because it decides how long the element survives the water.

Copper sheathed elements are the cheapest and the most common in older vented cylinders. They work well in soft water. In the hard water that most of London gets, scale builds on the hot element surface, the element runs hotter underneath it, and it burns out sooner. They are also a poor match for stainless steel cylinders, where makers generally specify a different sheath metal.

Incoloy is a nickel based alloy that handles heat and scale better than copper. Elements sheathed in it cost more and usually last noticeably longer in hard water, and they are the normal choice for stainless cylinders. For most London homes replacing a failed copper element, an Incoloy one is the sensible upgrade.

Titanium sheathed elements are sold as the long life option for very hard water and are often marketed with a longer guarantee. Whether the extra cost pays back depends on how hard your water is and how long you expect to stay. What none of them does is stop scale forming in the tank. Our guide on hard water and hot water cylinders in London explains where the scale actually goes and what reduces it.

• Copper: cheapest, fine in soft water, shortest life in hard water
• Incoloy: the usual upgrade for hard water and for stainless cylinders
• Titanium: premium long life option for the hardest water

Does it have to be 3kW?

Almost every domestic immersion heater in the UK is rated at 3kW, and the circuit, the fused switch and the cable feeding it are sized for that load. A higher rated element is not a safe way to heat the water faster, because the wiring was not designed for it. A lower rated one exists for special cases but heats more slowly.

A 3kW element uses 3kWh for every hour it is actually heating. How long it runs depends on the size of the cylinder and how cold the water starts, not on which brand of element you choose. If your cost worry is the running cost rather than the part, the immersion heater running cost guide works through it with your own tariff.

The thermostat is a separate part

The element and the thermostat that controls it are two parts. On a vented cylinder the thermostat is a rod that slides into a tube in the centre of the element head, and its length has to suit the element: a short rod for a side element, a longer one for a top entry element so it senses the right part of the tank.

A lot of immersions that are sold as dead are actually a failed thermostat on a good element, or a tripped cut-out. It is worth confirming which it is before buying anything. If the water has gone cold, the checks in immersion heater not working, and the guide on how to reset an immersion heater, will usually tell you whether the element itself has gone.

When a like for like replacement is the wrong call

Sometimes the right answer is not a better element. If the cylinder is on its second or third element in a short time, the scale and sediment that killed the last one are still in the tank. If the boss is corroded, crusted or weeping around the flange, a new element will not stop the leak for long. And on a cylinder that is already showing its age, spending on a premium element can be money put into a tank that is close to the end.

Those are the cases where the real question is a different one: is it worth repairing an old cylinder at all? Our guide on how long do immersion heaters last covers the pattern of repeat failures and what it usually means.

Getting the right element fitted

Tell us the make and model from the cylinder label, where the current immersion enters, and whether the cylinder is vented or unvented. With that we can bring the right element and thermostat rather than diagnosing first and ordering later. An immersion heater repair visit tests the element, the thermostat and the cut-out on a meter before anything is replaced, so you only pay for the part that has actually failed.

Call 020 4652 5111 to book. Our rates are on the pricing page and are agreed before we attend.