Where this starts
A shower is a balance between two supplies and a demand, and there are only three ways it fails: something takes flow away, the hot runs out, or the thing controlling the mix stops controlling it. Which one it is can nearly always be established without tools.
THE TIMING IS THE DIAGNOSIS. Stand in it and watch the clock. Temperature dropping the instant somebody opens a tap, flushes a WC or starts a machine is another demand competing for the same supply. Temperature holding for several minutes and then falling steadily is a stored volume running out, or an instantaneous appliance being asked for more than it can deliver. Temperature swinging up and down every few seconds with nothing else in the house running is the control.
WHICH SYSTEM IT IS narrows it further and is worth establishing first. A COMBI heats water as it flows, so it has no reservoir and its limit is a flow rate at a temperature rise — two outlets at once halves what each gets. A STORED system has a cylinder, so it will hold temperature at full flow until the cylinder is exhausted and then go cold over a minute or two. An ELECTRIC shower heats its own water and is independent of the hot supply entirely, so its symptoms point at the cold supply and the unit.
THE SEASON MATTERS MORE THAN PEOPLE EXPECT on an instantaneous system. Incoming mains water is perhaps fifteen degrees colder in February than in September, and the appliance has to make up the whole difference — so a combi or an electric shower that was adequate in autumn genuinely cannot deliver the same shower in midwinter. That is physics rather than a fault, and it is why the complaint is seasonal in a great many houses.
One thing worth ruling out early because it is cheap and common: a blocked shower head reduces flow, and on a thermostatic valve reduced flow changes how the valve behaves. Descaling a head has fixed more showers than any other single action on this page.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Another outlet taking flow
Most commonTemperature or pressure changing at the instant a tap is opened, a WC is flushed or an appliance fills, and recovering when that stops.
How to check: Have somebody open a tap while you watch. An immediate change is competition for flow, not a fault in the shower.
An instantaneous appliance at its limit
Most commonA combi or electric shower delivering less heat in winter than in summer, or going cool when the flow is turned up, with nothing else running.
How to check: Whether turning the flow DOWN restores the heat. An instantaneous heater trades flow against temperature rise.
Stored hot water running out
CommonFull temperature for several minutes then a steady fall to cold, on a system with a cylinder, and recovering an hour or two later.
How to check: How long it lasts and whether it comes back later. A reservoir emptying behaves completely differently from a heater struggling.
A failing thermostatic cartridge
CommonTemperature swinging in bursts with nothing else in the house running, sometimes with the control feeling gritty or the safe-stop button no longer holding.
How to check: Whether it swings with nothing changing. A valve losing its reference does that; a supply problem needs a trigger.
A scaled shower head or a blocked filter
CommonReduced flow as well as temperature trouble, visible deposit on the head, and often worse over months in a hard-water area.
How to check: Take the head off and run it without. Better flow means the head, and the valve was reacting to what the head was doing.
Hot and cold supplies at different pressures
Less commonA mixer that will not hold a setting at all, commonly where a mains-pressure cold meets a gravity-fed hot, or after part of a system has been changed.
How to check: What feeds each side. A mixer balancing two very different pressures moves whenever either one changes.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
Another outlet, nothing at all, or a steady fall after minutes — the three answers point at flow, control and capacity, and separate the whole problem in one shower.
- 2
Combi, stored cylinder and electric shower fail in three different ways, and a symptom that is normal on one is a fault on another.
- 3
It identifies an instantaneous appliance at its limit immediately, and that is a sizing answer rather than a repair.
- 4
Scale in a head reduces flow and changes how a thermostatic valve behaves, and descaling one has fixed more showers than anything else here.
- 5
An instantaneous system has to make up the whole difference between incoming water and shower temperature, so a seasonal complaint on a combi or electric shower is physics rather than a fault.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where the water can run scalding as well as cold, since a failed thermostatic valve is a burn risk and is the reason those valves exist.
- Where the appliance is gas — a combi or a boiler — because that is a controlled installation and only a registered engineer may work on it.
- Where the system is unvented, which carries stored energy and has safety components that must not be disturbed by anyone not qualified on them.
- Where an electric shower trips the RCD, which is a fault in the unit or its circuit rather than a temperature problem.
- Where several outlets in the house have poor flow, since the question is then the incoming supply rather than the shower.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.