Your boiler is currently ignoring the standard priority for hot water and is attempting to provide heating and hot water at the same time, which may result in lukewarm water or poor heating performance.

Without DHW cylinder priority applicable to heating circuit pump and mixer

What causes this fault?

This fault occurs when the boiler's control logic fails to prioritise your hot water tank over your radiators, often due to a communication error between the sensors and the main control board. It is typically caused by a faulty diverter valve sticking in a mid-position or a failing temperature sensor providing incorrect data about the hot water demand. As a result, the boiler struggles to deliver enough heat to both systems simultaneously, leading to a drop in performance.