Why does my shower keep going hot and cold?
A shower that suddenly turns cold, then hot again, can be uncomfortable and unsafe. The cause may be the shower itself, changing water demand elsewhere in the home, or the hot water system struggling to provide a steady supply.
Does it happen when another tap is used?
If the temperature changes when somebody runs a tap, flushes a toilet or starts an appliance, the shower may be reacting to a change in water pressure or flow. This is more noticeable in homes where several outlets share a limited supply.
A thermostatic shower is designed to reduce sudden temperature changes, but a worn cartridge, blocked filters or unsuitable plumbing can stop it responding properly.
The boiler may be cycling on and off
With a combi boiler, hot water is heated as you use it. If the shower flow is too low, or the boiler cannot maintain a steady burn, the water may cool and heat repeatedly.
Check whether the same problem happens at a basin or kitchen hot tap. If several outlets fluctuate, display a fault code or take a long time to become hot, the boiler or hot water system may need checking by a suitably qualified engineer.
The shower valve or cartridge may need attention
Inside a thermostatic mixer shower, a cartridge blends hot and cold water to hold the selected temperature. Limescale, debris or wear can make it slow to react or cause the temperature to wander.
A shower head or hose restricted by limescale can also reduce the flow enough to affect performance. Cleaning removable parts according to the manufacturer's instructions is a sensible first step, but dismantling the valve is usually a job for a plumber.
What about electric showers?
An electric shower heats cold mains water inside the unit. Its temperature can change if the incoming pressure varies, the shower head is blocked, or the unit cannot maintain the chosen flow and heat setting.
Do not remove the shower cover or attempt an electrical repair yourself. If cleaning the shower head does not help, arrange an inspection by a competent electrician or shower engineer.
Quick checks before arranging a repair
- Notice whether another tap, toilet or appliance triggers the change.
- Test whether other hot taps also run hot and cold.
- Clean a removable shower head following its care instructions.
- Check the boiler display for a warning or fault code.
- Stop using the shower if it becomes dangerously hot or cuts out repeatedly.