Your boiler has detected that hot water is being drawn through the system, often serving as an advisory notification rather than a breakdown.

DHW flow rate: Hot water flow rate.

What causes this fault?

This code is triggered when the boiler's internal sensor detects water moving through the hot water pipes, even if you haven't turned on a tap. It often occurs due to a 'passing' tap that isn't fully shut off, a minor leak in the pipework, or a sensor that has become overly sensitive or faulty. Effectively, the boiler is simply notifying you that it is working to heat water because it believe there is a demand.