Immersion Heater Repair & Replacement

Why has my immersion heater stopped working?

Check the simple things before assuming the element has gone. Work through them in this order, because it runs from the most common and cheapest fault to the least.

• The fused switch. Most have a neon light that shows when the switch is on. No light usually means no power reaching the heater.
• The timer or off-peak supply. On Economy 7 homes the lower element often only runs overnight, so a cold cylinder in the afternoon can be normal. Check the timer has not lost its settings after a power cut.
• The consumer unit. An immersion that trips the RCD every time it is switched on usually has an element leaking current to earth; one that trips its own breaker may have a short. Either way, do not keep resetting it.
• The thermostat cut-out. Modern immersion thermostats have a second, manual-reset safety cut-out that trips if the water overheats. It sits under the cover with live terminals, so it is a job for an engineer, and if it trips again the cause has to be found, usually a faulty thermostat but sometimes the boiler overheating the cylinder.
• The element. If power reaches the heater and the thermostat is fine, the element has usually burnt out.

Never take the cover off an immersion heater with the fused switch on. If you smell burning, see scorching on the switch or the cable feels hot, switch off at the consumer unit and leave it off.

What we do on the visit

With the supply isolated, the engineer checks the thermostat for continuity and then measures the element. A healthy 3 kW element reads roughly 17 to 20 ohms across its terminals. An open circuit means it has burnt through. A low insulation reading between either terminal and the earth or the boss means it is leaking to earth, which is what trips the RCD. Our guide on how to test an immersion heater explains those readings in more detail.

If the element has failed we drain the cylinder to below the boss. A top-entry element in a vented cylinder only needs a little water taken out; a side-entry one near the bottom means draining most of the cylinder. The old element comes out with a large box spanner, the boss thread and seal face are cleaned, and the new element goes in with a fresh washer. We refill, bleed the air out of the taps, then power it up and watch the boss while it heats.

London water is hard, and scale on the element is a common reason they fail. Where that is the cause we suggest an element with a scale-resistant sheath, such as Incoloy, rather than plain copper. Unvented cylinders usually need the maker’s specified element with its own thermal cut-out rather than a generic one, and the safety valves are checked while the system is open. If the cylinder itself is weeping rather than the element, see a hot water tank that is dripping.

What does immersion heater replacement cost?

Getting an engineer to you is the £60 call-out, day or night, and the repair price is agreed with you on site before any work starts. Our pricing page explains how that is worked out.

As a market guide, the parts are cheap: a standard element or rod thermostat is usually tens of pounds. The labour is mostly the drain-down and refill. Across London a straightforward element and thermostat replacement on a vented cylinder typically comes to somewhere between about £150 and £300 fitted, more for a long side-entry element, a seized element or an unvented cylinder. There is a fuller breakdown in immersion heater replacement cost.

When an immersion fault cannot wait

If the immersion is your only source of hot water, as in many all-electric flats, a failed element means no hot water at all, which for young children, older people or anyone unwell is urgent. The same goes for anything that trips the electrics repeatedly, smells of burning, or leaks at the boss. If the boiler also heats the cylinder, you can usually run it on the boiler and book the immersion repair for a convenient time.