Electric shower not hot enough

What it usually means

There is one equation behind almost every complaint of this kind, and it is worth seeing because it explains things that otherwise look like faults. The heat a shower can add is fixed by its power rating, so the temperature rise it achieves depends entirely on how fast water is going through it. Roughly, kilowatts equal litres per minute multiplied by the temperature rise, divided by fourteen. An 8.5 kW shower pushing 4 litres a minute lifts the water about 30 degrees. The same shower at 6 litres a minute lifts it about 20.

Now add the season. Mains water arrives at around 17 or 18 degrees in late summer and can fall to 5 or 6 degrees in February. Nothing about the appliance has changed, but that same 30 degree rise produces a shower at 47 degrees in September and 35 degrees in January — and 35 degrees is below skin temperature, which is precisely what "gone lukewarm" feels like. This is why almost every electric shower has a winter and summer setting, or a variable flow control. It is not a comfort preference. It is a control on flow so that the fixed power available can be concentrated into less water when the incoming water is colder.

Understanding that also fixes the intuition most people have backwards. Reducing the flow makes the shower hotter, not cooler. So a shower that is too cool is usually passing too much water, and a shower that has recently become scaldingly hot is usually passing too little. It also explains the puzzle where somebody turns the temperature dial up and the shower gets colder in bursts: on a shower with a simple thermal cut-out, demanding more heat than the appliance can deliver at that flow makes the cut-out operate, drop the elements out and let them cool, which produces a cycle of hot and cool rather than steadily hotter water.

Genuine faults do exist behind this symptom and they have their own signatures. Many showers have two elements or two heat settings; losing one halves the power available, which produces a shower that is warm but never hot at any setting, rather than one that is stone cold. Scale reduces how efficiently an element gets heat into the water, so a gradual decline over a couple of years in a hard water area is real. On thermostatic showers there is a sensor watching the outlet temperature, and a sensor drifting or failing makes the shower back off for reasons the user cannot see. And a shower shared with the rest of the house will vary as other taps are used, because every other draw-off changes the flow through it.

The last question is whether the shower was ever adequate. A 7.5 or 8.5 kW unit fitted twenty years ago is a modest amount of power by current standards, and if it has always been disappointing in winter then no repair will change that — the answer is a higher rating, which is not a straight swap, because the cable and the breaker were sized for the original.