why does my shower go hot and cold
Your shower usually goes hot and cold because something is interrupting the balance between hot and cold water or the supply of hot water itself. Most of the time it’s a mix of pressure issues, valve problems, or a water heater/boiler that’s struggling to keep up.
Main reasons it happens
- Pressure changes in the house
When someone flushes a toilet, starts a washing machine, or opens another tap, the cold (or hot) water pressure feeding your shower changes suddenly, so the temperature swings hotter or colder.
Older homes or pipework with limited flow are especially prone to this.
- Faulty or scaled-up shower valve
Modern showers use a thermostatic or pressure‑balance valve to mix hot and cold and keep things steady.
If the cartridge inside is worn, dirty, or clogged with limescale, it can’t react correctly, so the shower cycles between hot and cold.
- Water heater/boiler can’t keep up
A small or tired water heater, combi boiler, or tankless system may not produce enough hot water when several fixtures are running.
Once the stored hot water is used (or the burner cycles incorrectly), you feel the temperature drop or fluctuate.
- Limescale and flow problems
Hard water can clog the plate heat exchanger in combi or tankless systems and also block the showerhead, restricting flow and confusing the mixing valve.
This reduced or uneven flow makes the temperature unstable, especially when the system tries to modulate heat output.
- Thermostats, sensors, or elements failing
In electric showers and some boilers, a failing heating element, thermostat, flow sensor, or switch can cause the heater to overshoot, cut out, or cycle rapidly, which you feel as sudden hot‑cold changes.
Simple checks you can try
- Run only the shower and make sure no one uses other taps or toilets; if the problem disappears, it’s likely a pressure/usage issue.
- Clean or descale the showerhead and, if accessible, the shower valve cartridge following the manufacturer’s instructions.
- Check your water heater/boiler temperature setting (commonly around 120°F/49°C) and whether you run out of hot water quickly compared with before.
When to call a pro
- The temperature swings are extreme (scalding then freezing).
- All taps in the house have fluctuating hot water, not just the shower.
- You suspect boiler/water heater issues, electrical faults in an electric shower, or you’re unsure how to safely open valves and components.
A qualified plumber or heating engineer can test the shower valve, check water pressure and flow, inspect the boiler or heater, and clear limescale or replace faulty parts so your shower stays comfortably stable again.