Immersion Heater Not Working: What to Test, in Order

What makes up an immersion heater?

An immersion heater is four parts in a line. Power arrives through a switch, usually a double-pole switch or fused spur on the wall nearby, then travels to a thermostat, through a high-limit cut-out, and finally into the element, a coil of resistance wire inside a metal sheath that sits in the water. Most domestic elements are rated around three kilowatts, which is why they are normally on their own circuit.

The thermostat has a rod that sits in a pocket beside the element and senses water temperature. It switches the element off when the water reaches the set temperature. The high-limit cut-out is a separate safety thermostat. If the first one fails closed and the water overheats, the cut-out opens the circuit and stays open until it is reset by hand. Some designs combine both in one unit under the cover.

The element screws into the cylinder through a threaded boss. A top-entry immersion goes in from the top and hangs down into the water, a side-entry one goes in horizontally. Because the boss is part of the cylinder wall, a fault there can mean the cylinder itself is the problem. The sections below follow the power from the wall to that boss.

What can I test myself without opening anything?

Two links in the chain are visible without taking a cover off. Start there, because the cause is often at the wall, not in the cylinder.

• Switch: check the double-pole switch is on. If it has a neon, a lit neon means power is reaching the switch.
• Spur fuse: if the switch is a fused spur, the fuse inside is usually rated for the element. A blown fuse is a cheap fix, but note why it blew. Replacing it with a larger fuse is never the answer.
• Breaker: look at the consumer unit for a tripped breaker on the immersion circuit. If it has tripped, do not keep resetting it: the circuit may have a fault that needs an electrician.
• Timer: if the heater runs on a timer or a boost switch, check the timer has not lost its setting.
• Heat check: after a few hours, briefly feel the cylinder side near the element height and the pipe leaving the top with the back of your hand, as the pipe can be hot enough to scald. No warmth confirms the element is not heating.

What does an engineer test, and in what order?

With the circuit isolated and proved dead, the engineer works along the same chain that the power follows. Each step rules out the previous part before moving on, so a fault is named instead of guessed at.

First is the supply: whether power reaches the immersion cover at all, which separates a wall-end problem from a cylinder-end one. Second is the thermostat. With the water cold, its contacts should be closed. Open contacts on a cold cylinder mean the thermostat has failed, or the dial is set too low to call.

Third is the high-limit cut-out. This is the part people mean when they ask how to reset an immersion heater. On many units it has a small button which pops out when it trips. It trips because the water overheated, or because the thermostat failed to switch the element off. The unit is designed to be reset by hand, but on most heaters the button sits under the cover, which makes it an electrician job. Only a button that can be reached without removing any cover should be pressed, once, with the circuit isolated at the consumer unit first. If it trips again, the thermostat is the likely fault and further pressing hides the cause. Our reset guide explains the one safe press and why repeat trips mean a fault.

Fourth is the element. With power off and the element disconnected from the circuit, the engineer reads its resistance. A healthy three-kilowatt element on a 230 volt supply should read in the region of 17 or 18 ohms, which comes from dividing the voltage squared by the power. An open circuit means a burnt-out element. A reading to earth means the sheath has failed and water is reaching the wire, which will trip the breaker.

When is a failed element really a cylinder problem?

Most failures end at the element or its thermostat, and replacing them leaves the cylinder alone. Our immersion heater replacement page covers what that involves, including draining down and matching the element to the boss.

Three results point past the element. The first is a boss that has corroded or cross-threaded, so a new element will not seat or seep water. The second is heavy limescale that has coated the element and buried the thermostat pocket, which can happen on cylinders in hard-water areas and shortens element life. The third is an element that fails repeatedly within a short period, which suggests a problem with the water side, such as scale or aggressive water, or a thermostat that keeps letting it overheat.

A cylinder with a leaking boss or a visibly corroded shell around the element is a replacement conversation. The decision turns on the age of the cylinder and on whether the shell is sound, which an engineer judges after draining and inspecting.

How long does an immersion heater last?

There is no fixed lifespan. Elements in soft-water areas can run for many years, while elements in hard-water areas can scale up and fail much earlier. The dependable signal is behaviour, not age: a heater that takes noticeably longer than it used to, a kettling or rumbling noise as scale boils on the sheath (see the noisy cylinder guide for telling it from other noises), or a breaker that trips only when the heater runs.

Replacing an element before it fails is rarely worth it, but replacing the thermostat or cut-out at the same time can be, because they share the cover and the labour is identical. Do not choose a replacement by looks alone: the length, the boss thread, the wattage and the type of thermostat must match the cylinder.

If hot water is still missing after the immersion has been proved good, the fault is somewhere else. Go back to what to check when hot water is not working, because the immersion may never have been the problem.