Immersion heaters

What it is and what it is for

An immersion heater is an electric heating element screwed into the side or top of a hot water cylinder, with a thermostat in a pocket down its centre. It exists as a backup to the boiler, as a summer heat source so the boiler does not have to run for hot water alone, and in some properties as the primary heat source on an off-peak tariff.

Many cylinders have two: a short one near the top for a quick partial heat, and a longer one lower down that heats the whole cylinder. The top one is the economical choice for a single shower, the bottom one for a full tank – which is worth knowing, because a lot of people run the bottom element to get one bath.

Why they fail here in particular

Hard water is the reason. Every time an element heats water, calcium comes out of solution and deposits on the hottest surface, which is the element itself. Scale is a thermal insulator, so as it builds the element has to run hotter to push the same heat into the water. Hotter running deposits more scale. Eventually the element overheats inside its own insulating shell and burns out.

The visible symptoms in order of progression: hot water takes longer than it used to; the element makes a rumbling or crackling noise as it heats; then nothing at all, or the RCD trips when it is switched on.

The RCD tripping is worth understanding. A failing element develops a path to earth through its degraded insulation, which is exactly what an RCD is designed to detect. A tripping RCD when the immersion is switched on is a fairly reliable diagnosis.

Why replacement is not always a ten-minute job

In soft water it usually is. In Norfolk it frequently is not, because the threaded boss and the element have been slowly cemented together by scale for a decade.

• It needs the correct spanner. An immersion element takes a large box spanner of a specific size. Using anything else – a pipe wrench, adjustable grips – distorts the element head and can crack the cylinder boss, which turns an element replacement into a cylinder replacement.
• The cylinder has to be drained to below the element, which on a lower element means draining the whole thing.
• The boss can be seized. Heat, penetrating oil and patience. Force is what breaks cylinders.
• Access is often terrible. Immersions are typically in a cupboard with a shelf built over them, and getting a long box spanner and a long element in and out sometimes means removing joinery.
• The element length matters. Too long for the cylinder and it will not fit or will foul the coil; too short and it heats only the top.
• The electrical side must be correct – the right flex, the right connection, and the circuit properly protected. An immersion draws a substantial current continuously.

This is the one hot water job where attempting it yourself has a genuine chance of converting a modest repair into a cylinder replacement.

Thermostats, and the fault people misdiagnose

An immersion has its own thermostat in a pocket running down the middle of the element, with a separate high-limit cut-out. A very common scenario is an immersion that has ‘failed’ when in fact the thermostat has tripped its cut-out or drifted out of calibration.

Symptoms that point at the thermostat rather than the element: water that is far too hot, water that is lukewarm and never gets hotter, or an immersion that works intermittently. Thermostats are cheap, and it is worth testing before condemning the element.

Temperature setting matters too. Storing hot water below about 60 degrees allows bacterial growth in a stored vessel; storing it much above that accelerates scale formation dramatically in hard water. The correct answer is to store at the recommended temperature and control the delivery temperature at the outlet with a blending valve, rather than turning the cylinder down.

Worth doing at the same time

• Fit a replaceable-element type where the cylinder allows it, so the next failure does not need the whole assembly out.
• Check the cylinder insulation. If the jacket is thin or missing, the element is fighting standing losses continuously.
• Consider whether the water needs softening. If elements are failing every few years, that is the water telling you something – see water softeners.
• Check the thermostat setting and consider a thermostatic mixing valve at the outlets rather than running the cylinder cool.
• Check the wiring and the switch. Immersion switches get hot and the contacts degrade; a discoloured or warm switch plate needs replacing.