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Shower running hot and cold


A shower that alternates between hot and cold can quickly turn an enjoyable experience into an annoying one. But what’s causing it—your shower thermostat or your boiler? Let’s break it down.

Shower running hot and cold

1. Shower Thermostat Problems

Your shower’s thermostat is designed to regulate water temperature by blending hot and cold water. If it’s malfunctioning, it may fail to maintain a consistent temperature.

Signs the Thermostat Is the Problem:

  • The temperature fluctuates only in the shower, while taps elsewhere deliver steady hot water.

  • The issue began after recent plumbing work or changes to water pressure.

Solutions:

  • Clean the Thermostat

    : Over time, limescale or debris can clog the thermostat. Remove it and soak it in a descaling solution.

  • Replace the Thermostat

    : If cleaning doesn’t work, you may need a new thermostat cartridge.


2. Boiler-Related Issues

If the problem extends beyond the shower—affecting all hot water outlets—your boiler might be to blame.

Common Boiler Issues That Cause Temperature Fluctuations:

  • Inconsistent Boiler Output

    : If the boiler struggles to maintain a steady temperature, it could be due to a faulty heat exchanger or thermostat within the boiler.

  • Low Water Pressure

    : Boilers require consistent pressure to deliver hot water. A drop in pressure can result in fluctuating temperatures.

  • Demand vs. Capacity

    : If multiple outlets are using hot water simultaneously, your boiler may not be able to keep up.

  • Faulty Diverter Valve

    : In combi boilers, a diverter valve switches between heating and hot water. If it’s malfunctioning, you may experience temperature swings.

Solutions:

  • Check your boiler’s pressure and top it up if it’s below 1.0 bar.

  • Schedule a service to inspect the boiler’s components for wear or faults.

  • Upgrade to a larger boiler if your household demand exceeds the current boiler’s capacity.