Electric shower [Water suddenly scalding]what to check

The physics of an electric shower makes this fault counterintuitive, and getting the direction right is most of the diagnosis. The unit puts a fixed amount of energy into whatever water passes through it, so the temperature you get is set by how fast the water is moving rather than by a thermostat deciding on a target.

Slow the flow down and the same energy goes into less water, and the water gets hotter. That is why every manufacturer in this category answers "water too hot" with instructions about flow rather than about heat: Bristan say to clean the handset and increase the flow, Triton list "Not enough water flowing through the shower" as the first cause and blocked spray plates as the second, and Redring send you to the inlet filter. It is also why this fault appears gradually in hard water areas — the showerhead scales up over months, the flow drops, and the temperature climbs to match. The other common version is seasonal and is not a fault at all. These showers heat the incoming mains, so in August the water arriving is already ten or fifteen degrees warmer than it was in February, and the same setting produces a hotter shower.

Manufacturers document both directions of that. There is one case that is genuinely different and it is the reason this page is rated the way it is rather than as an inconvenience. A thermostatic shower measures its outlet and holds a temperature, so on that kind of unit a sudden jump to scalding is a control failure rather than a flow problem, and the protection you were relying on is the thing that has stopped working. That distinction decides whether this is a descaling job or a stop-using-it job.

1. 1

   Establish whether your shower is thermostatic. If it is, and it has jumped to scalding without warning, stop using it and treat the rest of this page as background rather than as instructions.

   A thermostatic shower that has lost temperature control is a scald risk to anyone who cannot get out of the way quickly. Do not leave it in service for children or elderly users while you work out what is wrong.
2. 2

   Turn the temperature control towards cold and, if there is a power selector, drop it a setting. On most units this alone brings a summer shower back to comfortable.

3. 3

   Take the showerhead off and run the hose into the tray. If the temperature falls immediately, the head is the restriction and descaling or replacing it is the fix.

4. 4

   Descale the head, or fit a new one. Rubbing the flexible jets with a thumb clears light scale; a head that has gone hard and uneven is cheaper to replace than to rescue.

5. 5

   Have the inlet filter cleaned. On a Bristan you can do it yourself: isolate the electricity and the water and turn the filter cap anticlockwise with a large flat-blade screwdriver from outside the unit. On a Triton, a Mira or a Redring it is inside the case and the manufacturer gives the job to the installer or a competent person. It is the restriction people never look at.

   Pull the cord before touching the plumbing, and close the servicing valve on the inlet rather than the whole-house stopcock. Do not take the front cover off a Triton, Mira or Redring to reach the filter.
6. 6

   If flow is good, the head is clean and it still runs too hot at the lowest setting, the temperature control or its sensor has failed and the unit needs looking at.