Shower temperature keeps changing hot and cold
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Most likely cause & what to check
Temperature fluctuation is usually caused by changes in supply pressure while the shower is on. When someone elsewhere in the house uses hot or cold water, it affects the balance of pressure reaching your shower mixer.
For a manual mixer shower (no thermostatic cartridge): every flush of the toilet or use of another tap will cause a temperature change. The only proper fix is fitting a thermostatic shower valve or mixer.
For a thermostatic mixer: the thermostatic cartridge should compensate for pressure changes automatically. If it's not doing this, the cartridge is worn out or failed. Replacement cartridges (brand-specific) cost £25–£80.
Check the pressure balance: if your hot and cold supplies come from different sources (e.g. hot from a cylinder at low gravity pressure, cold at high mains pressure), a standard thermostatic cartridge may struggle. A pressure-compensating thermostatic valve solves this.
For electric showers: the heating element cycles on and off to maintain temperature, causing slight fluctuations. Rapid or extreme fluctuations suggest the element, thermostat, or PCB is failing.
A non-return valve fitted on both hot and cold supplies to the mixer prevents pressure changes in one supply from affecting the other — an inexpensive fix if cross-flow is the issue.
Check all isolating valves to the shower are fully open — a partially closed valve creates restriction that exaggerates pressure fluctuations.
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