How to Test an Immersion Heater Element

Before you test: rule out the supply and the thermostat

Most immersions that stop heating have not lost their element. Before you take a meter to anything, confirm power is reaching the heater and the thermostat is letting it through. Doing it in this order saves you draining a cylinder for a part that was never broken.

• Wall switch on and its neon lit, if it has one.
• Timer or programmer calling for heat, and on Economy 7 the off-peak period actually running.
• No tripped breaker or RCD at the consumer unit.
• The thermostat safety cut-out not tripped. Our guide on how to reset an immersion heater shows where the button is.

An immersion that trips the RCD every time it is switched on almost always has an element leaking to earth. Leave it off until it has been tested. Repeatedly resetting an RCD onto an earth fault defeats the protection it gives you.

What you need, and when to leave it to someone else

You need a digital multimeter with a resistance setting, a two-pole voltage tester to prove the circuit dead, and a screwdriver for the cover and terminals. A plain multimeter can confirm a burnt-out element, but it cannot properly test insulation. That needs an insulation resistance tester, which applies 500 volts and is what an electrician or engineer carries.

This is live electrical work up to the point you prove it dead. If you do not own a proper voltage tester, or have not isolated a circuit before, the test is a short job for an engineer and not worth the risk.

How to test the element, step by step

The test is done with the element still in the cylinder, where an immersion heater sits and heats the stored water. You do not need to drain anything to find out whether it is working.

• Switch off the wall switch and the immersion breaker at the consumer unit.
• Remove the immersion cover. Use the voltage tester on the terminals to prove there is no voltage between live and neutral, live and earth, and neutral and earth. Check the tester on a known live source before and after.
• Take a photo of the wiring, then disconnect the two supply wires from the thermostat and disconnect the thermostat from the element terminals. You want the element on its own, with nothing else in the circuit.
• Set the meter to its lowest ohms range and touch the probes to the two element terminals.
• Then test from each terminal in turn to the metal element head or boss, which is earthed.
• For a twin or dual element head, the kind with a long and a short element on one boss for Economy 7, test each element separately across its own pair of terminals.

What the readings mean

The resistance of a healthy element follows from its power. Resistance equals voltage squared divided by power, so a 3 kW element on 230 volts works out at about 17.6 ohms, and at 240 volts about 19.2. Anything from roughly 17 to 20 ohms is normal for a 3 kW element. A smaller element reads higher: a 2 kW one lands around 26 to 29 ohms.

• About 17 to 20 ohms across the terminals, and nothing to earth: the element is sound. Look at the thermostat or the supply instead.
• Open circuit (OL or 1 on most meters) across the terminals: the element wire has burnt through. It needs replacing.
• Near zero across the terminals: a short. Rare, but the element needs replacing.
• A reading in ohms or kilohms from either terminal to the earth or boss: the sheath has cracked and water has got in. This is the fault that trips RCDs. Replace it even if it still heats.
• A reading in megohms to earth: a multimeter cannot tell you whether that is acceptable. It needs an insulation resistance test.

A multimeter only sees gross earth faults. A slight leak can read open on a meter and still trip an RCD. An insulation resistance test at 500 volts should show well above 1 megohm; BS 7671 treats 1 megohm as the minimum for a 230 volt circuit.

Testing the thermostat and cut-out

With the thermostat disconnected and still cool, set the meter to continuity and test across each pair of its switched terminals. At room temperature the thermostat should be closed, reading close to zero ohms, because it is calling for heat. If it reads open with the dial turned up and the cut-out pressed in, the thermostat has failed.

The cut-out can also fail in a way that looks like a trip that will not reset. If the reset button clicks in but the thermostat still reads open, replace the thermostat. Thermostats are cheap, so many engineers fit a new one alongside a new element anyway, and a failing thermostat is the usual cause of repeated cut-out trips.

While the cover is off, check the terminals and cable for heat damage. Browned plastic, a melted terminal block or a stiff, discoloured flex means the connection has been running hot. That needs fixing whatever the element reads, and the flex should be the heat-resistant type rated for immersion heaters, not ordinary PVC cable.

If the element has failed: what replacing it involves

A failed element means draining the cylinder to below the boss, unscrewing the element with a large immersion box spanner, and fitting a new one with a fresh fibre washer. A top-entry element in a vented cylinder only needs the water level dropped a little. A side-entry one near the base means draining most of the cylinder. Our guide on how to drain a hot water tank covers that part.

Old elements on London’s hard water are often heavily scaled and can seize in the boss, and the copper cylinder wall can distort or split if too much force goes on the spanner. That is the point where most people hand it over. If the immersion is on an unvented cylinder, the replacement must be the manufacturer’s specified part, and the work falls under Building Regulations Part G section G3.

Our immersion heater repair service covers testing, element and thermostat replacement and the drain-down. It starts with the £60 call-out at any hour, and the repair price is agreed with you on site before any work starts. For market costs, see immersion heater replacement cost.