Immersion heater not working? What to check first
Immersion heater not working? Check the switch, timer, fuse and RCD first, then the thermostat and element. Safe checks and when to call an engineer.
If your immersion heater is not working, start with the simple things: the switch on the wall, the timer, and the fuse or RCD in your consumer unit. A surprising number of “broken” immersions turn out to be a timer that lost its settings in a power cut or a switch someone knocked off. If those are fine, the fault is usually the thermostat or the heating element itself, and that is a job for a professional.
An immersion heater is an electric element inside your hot water cylinder, a bit like the element in a kettle. Some homes use it as the main way of heating water, others only as a backup. The checks are the same either way.
First, a word on safety
An immersion heater is wired to the mains and sits in a tank of hot water. Everything in this guide is a check you can do from outside the cylinder, without taking any covers off. Do not remove the cap on top of the immersion element, do not touch bare wires, and do not try to drain or open the cylinder yourself. If you have an unvented cylinder (a sealed tank with no cold water tank in the loft), it also has safety valves that should only be worked on by a qualified installer. Our guide to unvented cylinders explains how to tell which type you have.
Why is my immersion heater not working?
Work through these in order. Each one takes a minute or two.
1. Check the switch
Most immersions have a red switch on the wall nearby, often in the airing cupboard or on the landing. Some have two switches, or a single switch marked “bath” and “sink”, for homes with two elements. Make sure the switch is on and that the neon light, if there is one, is lit.
2. Check the timer
If the immersion runs off a timer, look at the display. A power cut, a flat backup battery or a change to or from British Summer Time can leave it on the wrong time or with no programme at all. Use the boost or advance button if there is one: if the water heats on boost but not on the timer, the timer settings are the problem, not the immersion.
3. Check the fuse and RCD
Go to your consumer unit (fuse box). Look for a tripped circuit breaker or RCD, usually a switch that has dropped to the off position. You can reset it once.
- Turn the immersion switch off at the wall.
- Push the tripped breaker or RCD back to on.
- Turn the immersion switch back on.
If it trips again straight away or within a few hours, leave it off. Repeated tripping usually means the element has failed and is leaking current to earth, or there is a wiring fault. Do not keep resetting it.
Older homes may have a fused spur with a cartridge fuse instead. Replacing that fuse with one of the same rating is fine, but if the new one blows too, stop there.
4. Give it time
A cylinder full of cold water takes a while to heat on an immersion, often a couple of hours. If you only switched it on recently, wait before deciding it is faulty.
Water is warm but not hot: the thermostat
If you get some warm water but never properly hot, or the water is scalding, the thermostat is the likely culprit. It sits under the cap on top of the element and switches the heat off when the water reaches the set temperature. Stored hot water is generally kept at around 60°C to control bacteria such as legionella; see the HSE guidance for the current advice.
Thermostats wear out, and they can stick on or off. Adjusting or replacing one means taking the cap off live electrical parts, so this is not a DIY job. An engineer can test it and swap it if needed, and the part itself is not usually expensive.
No hot water at all: the element
If the switch, timer and fuse are all fine and the cylinder stays stone cold, the element has probably burned out. London has hard water, and limescale builds up on immersion elements over the years. It insulates the element, makes it run hotter and shortens its life. A crust of scale is one of the most common reasons we replace them.
Replacing an element means isolating the electrics, draining the cylinder and fitting a new element with a fresh seal. Done badly it leaves a leak on top of a hot cylinder, so use a professional. On an unvented cylinder it must be done by someone qualified to work on those systems.
Common symptoms at a glance
| What you notice | Most likely cause |
|---|---|
| No hot water, switch light off | Switch, fuse or tripped breaker |
| Works on boost, not on timer | Timer settings or a faulty timer |
| Breaker trips when the immersion is on | Failed element or wiring fault |
| Water warm, never hot | Thermostat or partly failed element |
| Water far too hot | Thermostat stuck on |
| Hissing or crackling from the cylinder | Limescale on the element |
Could it be the boiler instead?
If your cylinder is normally heated by a gas boiler and the immersion is only the backup, the real question may be why the boiler is not heating the water. Our guide to no hot water but the heating works covers the motorised valve and programmer faults that cause that.
Engineer or electrician?
Both can help, and it depends on where the fault is.
- Wiring, the switch, the spur or the circuit: an electrician.
- The element, thermostat, cylinder or anything that needs draining down: a plumbing and heating engineer.
- Not sure: call us first. If it is an electrical supply fault, Lee will tell you so you do not pay for the wrong trade.
When to call an engineer
- The breaker or RCD trips again after one reset
- You can see water leaking from around the element or cylinder
- There is a burning smell, scorch marks or a hot, discoloured switch. Turn it off at the consumer unit
- Water is coming out of the tundish or discharge pipe on an unvented cylinder
- The cylinder is old, badly scaled or leaking, and it may be time to think about a replacement
We repair and replace immersion heaters and hot water cylinders across South and South West London and north Surrey. Our labour rates are on the prices page: £120 an hour including VAT on weekdays from 8am to 5pm.
If you have run through the checks and still have no hot water, call Lee on 020 8050 5913. He will happily talk it through with you for free, and if it needs a visit, he can book an engineer in.
Stuck? Call Lee.
If you are not sure it is safe, stop and call. Advice by phone is free.
020 8050 5913
