Your boiler is struggling to prioritise between heating your home and heating your hot water because it has detected a pump or valve communication issue.

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.