Your boiler is trying to run the heating even though you have turned the control knob to the off position, which usually indicates an internal electronic fault or a stuck component.

Light on constant (CH knob in OFF position) - likely caused by Autofrost stat mode or wet board

What causes this fault?

This fault occurs when the boiler's internal computer wrongly detects a demand for central heating even though you have turned the dial to the off position. The most common reason is an electrical short or failure within the main control board, which causes the system to run uncontrollably. In some cases, a mechanical part like the diverter valve can get stuck, sending confusing signals to the electronics.