Electric shower not working

What it usually means

An electric shower is a simple machine doing a hard job. Cold mains water enters, passes over one or more heating elements, and leaves hot, all in the length of the unit and in real time. There is no tank and no stored hot water, which is why it works during a boiler breakdown and why it needs an enormous electrical supply: a 9.5 kW unit draws around 40 amps continuously, making it the biggest single load in most houses. It has no plug. Its supply comes on its own heavy circuit through a ceiling pull cord or a wall switch, and that is the only isolation you have.

Because the water side and the electrical side are independent, the useful diagnosis is to work out which one has stopped. Pull the cord and press start. If the indicator neon lights, the supply is arriving and the unit is alive, and the fault is downstream. If nothing lights at all, the fault is upstream of the shower: the pull cord itself, which switches 40 amps in a damp ceiling and is a genuine wearing part, the 40 or 45 amp breaker in the consumer unit, or a residual current device that has tripped.

Water not flowing with the neon lit points at the plumbing side, and there is one design detail behind most of these. A shower will not energise its elements until it can prove water is moving through them, using a pressure or flow switch, because heating a dry element destroys it in seconds. So a shower short of flow does not run cool — it does nothing at all, which reads exactly like an electrical failure. Behind the unit or above it there is usually an isolating valve, often with a screwdriver slot, and inside the inlet there is a filter that collects grit and scale. Both are common, and both are ahead of any electrical component in the queue.

Water flowing but never warming is the third pattern, and it means the elements are not being energised or have failed. In hard water areas that is the ordinary end of a shower's life: scale builds on the element until it overheats locally and either the element fails or a thermal cut-out takes it out of circuit permanently. Which brings up the honest economics. Electric showers are consumable appliances, and once the element or the control has gone on a unit that is eight or ten years old, replacement is usually the better spend. One caution if you replace it: a higher kW model is not a like-for-like swap, because the existing cable and breaker were sized for the old one.

The remaining two patterns are the ones to stop for rather than diagnose. A shower that trips the breaker or residual current device is reporting current leaking to earth, and the usual path is through the water inside a failing element. A shower leaking from the casing rather than from a hose or the head has mains water loose inside an enclosure that contains the terminal block. Neither is a "keep using it for now" situation.