Your boiler is struggling to start the fan when you turn on your hot water taps, meaning the system cannot safely clear exhaust gases to begin heating.

DHW mode – fan start up error

What causes this fault?

This fault occurs when the boiler's main control board sends a signal to start the fan, but it fails to rotate or reach the required speed to safely vent exhaust gases. The most common reasons are a buildup of dirt causing the fan bearings to seize or an electrical failure within the fan's motor or the wiring connecting it to the boiler.