Your boiler is having trouble heating your hot water cylinder because a sensor has detected an unusually high temperature or a fault during the heating process.

ON during DHW loading to set value 1

What causes this fault?

This fault occurs when the temperature sensor in your hot water cylinder detects a reading that is significantly higher than expected or rises too rapidly. This is often caused by a faulty sensor sending incorrect data to the boiler, or a mechanical failure like a stuck diverter valve preventing heat from moving correctly to your water tank. In some cases, it can also indicate a blockage in the heat exchanger that is causing the system to overheat locally.