Your boiler is running, but it isn't getting enough water flowing through its hot water side, so it can't heat your tap water properly, and a qualified engineer should look at the cause.

No DHW heating. Cause: Minimum flow rate not achieved during DHW heating. Possible causes include shut-off or too much constriction, scaling or sludge, incorrect hydraulic configuration, faulty circulation pump or air in the heating circuit, and unstable or excessively low system pressure.