Immersion Not Heating The Water? How To Tell What Has Actually Failed

It is the most Irish fault there is. Somebody goes for a shower, there is no hot water, and the immersion gets the blame before anybody has looked at anything.

Sometimes fairly. An immersion is a heating element sitting in a tank of water, switched by a thermostat, and all three of those parts fail eventually. The useful thing is that they fail in different ways, so you can usually work out which one it is before anybody opens anything.

First, is it actually switched to the right thing?

Worth clearing up, because it catches people out constantly. Many Irish cylinders have two elements and a switch marked sink and bath.

Sink uses the upper element, heating only the top portion of the tank. It is quick and it is cheap, and it gives you enough for washing up or a quick wash.

Bath uses the lower element, heating the full tank. It takes considerably longer and uses far more electricity, which is exactly why leaving it on is such a household offence.

So if you have lukewarm water at the taps but nothing like enough for a shower, the system may be working perfectly and simply be on sink. And if bath gives you nothing while sink works, you have not got a dead immersion, you have got one dead element out of two, which is genuinely useful information.

How do you tell an element from a thermostat?

The symptoms separate reasonably cleanly.

A failed element usually gives you no heat at all on that setting, no matter how long it is left. If the element has broken down internally and is leaking current to earth, switching the immersion on will trip the RCD in your board, either straight away or after a few minutes as it heats. That combination, no hot water plus a trip when you switch it on, is about as clear a sign as you get. Leave it off.

A failed thermostat gives you stranger behaviour. Water that gets warm but never hot. Water that is scalding hot regardless of the setting, because the thermostat has stuck closed and the element never switches off. Or heating that works one day and not the next. A thermostat that has stuck closed is a real risk rather than an inconvenience, because the tank keeps heating with nothing telling it to stop.

A failed switch or timer gives you nothing at all, often with no neon light on the switch when you turn it on. If the switch has a neon and the neon is dead, the fault may be before the immersion entirely, which is a different job to a tank problem and falls under ordinary fault finding.

Could it be the wiring rather than the immersion?

It can, and it is worth ruling in or out early, because replacing an element on a perfectly good circuit fixes nothing if the supply is the problem.

Immersions are usually on their own circuit with their own protective device, and they draw a serious current for hours at a time. That is hard on connections. A loose terminal at the switch or at the tank generates heat, and heat at a joint is how a nuisance turns into something worse. If the immersion switch plate is discoloured, warm to the touch, or smells at all of hot plastic, switch it off at the board and leave it off.

Bear in mind too that the immersion is often the single biggest electrical load in a house. SEAI point out that the price of electricity is about three times the price of gas or oil, so where a home has a working boiler it is generally much cheaper to heat water with the central heating and keep the immersion for summer or for a top up.

What should the thermostat be set to?

SEAI advise checking that a hot water cylinder thermostat is set no higher than 60 degrees Celsius, and they are equally clear that cylinders need to be kept at 60 degrees to avoid the dangers of legionella bacteria in stored water. So it is not a case of turning it down as far as it will go. Sixty is the number.

Two other things SEAI recommend that cost very little. If the cylinder is old and uninsulated, fit a lagging jacket, which should pay for itself in a few months. And if you are heating water on an immersion with no timer, fit one, so the water is heated when it is needed rather than continuously. Their advice is not to leave an immersion running all the time, because the tank loses heat regardless and the element simply keeps coming back on, like a kettle reboiling over and over.

What is safe to do yourself?

Checking the switch setting, checking whether the neon lights, checking the board for a tripped device, and noticing whether sink works while bath does not. All of that is useful and none of it involves touching anything.

What is not a do it yourself job is the element itself. It is wired, it is immersed in water, and getting it wrong has consequences in two directions at once. The same goes for the switch and its supply if anything is warm or discoloured, which is the point to get somebody to look at the switch and its connections properly.

Common questions

Why does my immersion trip the electrics when I turn it on?

Almost always a failed element. When the element breaks down internally, current leaks to earth through the water, and the RCD detects that leak and cuts the power. It is the protection working exactly as designed. Leave the immersion switched off until it is looked at.

How long should an immersion take to heat a tank?

It depends on the size of the cylinder, how well insulated it is and how cold the water started. SEAI suggest an hour in the morning and an hour in the evening as a starting point and reducing it if you are not running out. If yours never gets there no matter how long it is on, that is a fault rather than a setting.

Is it cheaper to leave the immersion on all day?

No, and it is one of the more expensive myths going. A tank loses heat constantly, so the element keeps switching back on to hold the temperature. SEAI advise against leaving it running and recommend heating the water only when it will actually be used.

Do I need an electrician or a plumber for this?

If the fault is the element, the thermostat, the switch or the circuit, it is electrical. If the fault is the tank itself, the pipework or a leak, it is plumbing. No hot water with a tripping board is electrical. No hot water with water visibly escaping somewhere is not.

Before the cold really lands

An immersion is the thing people fall back on when the heating is struggling, which means it tends to get discovered as broken on exactly the coldest, most inconvenient week of the year. If yours has been behaving oddly, late October is a much better time to find out why than late December.