So where are you dumping the heat to if there is no DHW flow ?

The cylinder itself. Although already at a high temperature (i.e. 60C), it
still acts as a nice energy dump, especially as the flow temperature rises.
Even if the flow temperature rises to 90C (unlikely), thats a nice 30C
difference across the heat exchanger and the cylinder won't get appreciably
hotter because of 30 seconds of overrun with the burners turned off.

Christian.