Hot Water Not Hot Enough or Running Cold

Cylinder thermostat set too low

The cylinder thermostat controls the target temperature. If set below 55°C, water won't reach a proper hot temperature and may feel lukewarm. Should be set to 60°C minimum.

Faulty or over-cautious thermostatic mixing valve (TMV)

TMVs mix cold into the hot supply for scalding prevention. When they fail or are set too conservatively, the blended temperature is too low.

Short cycling (combination boilers)

Some combi boilers cut off before fully heating water at the tap, especially with high flow rates. This can make water feel hot at first then cool.

Failing immersion heater element

If heating by immersion only, a partial element failure can mean the full volume of water isn't reaching temperature.

Practical checks and next steps

Check the cylinder thermostat is set to at least 60°C. If it is and water still isn't hot, the fault is likely a thermostatic mixing valve or a failing heating element — both need a plumber.

• Check the cylinder thermostat — it's on the side of the hot water cylinder. Set to 60°C if lower.
• Run the hot tap for 30 seconds — does it get hotter after the initial flow?
• Check the boiler temperature setting (combi boilers) — domestic hot water should be at least 60°C.
• If you have a TMV on a bath or basin, it may be adjusted or replaced.
• Call a plumber if the cylinder reaches temperature but mixed temperature is still low.