So to sum up, if I may, (so I can pass this on to trainees).
1. With this boiler & I assume others with the same kind of HeatX & builtin bypass the likely hood is it will be able to deal with the heat dissipation required if the system shuts down suddenly on the programmer or stats on systems without a 3 port (mid or diverter) valve or hydraulic separation.
2. The boiler manufacturers instructions state minimum flow rates that are required to allow correct modulation & pump speed control. (In this case 1290 l/h) If the system deign is such that this can't be guaranteed (due to valves closing such as TRVs & power heads on U/F) then we would install a automatic by-pass set up to open when the pump pressure reached this point.
I normally give some rough guidelines like, on a small 7-8 rad system we would expect the hall to not be fitted with a TRV so I would balance the system, wind the by-pass fully closed, shut down all the TRVs except one (plus the hall), then slowly open the by-pass feeling the pipe until it goes hot (indicating valve open) then half a turn back so hopefully any less than two full open rads or the equivalent in the system flow would open it. (3 rads for larger system.
As has been sated before the rush of hot water from the flow to the return should quickly cause the boiler to turn off on the boiler stat & enter anti-cycling mode.