I suspect that for many installations it doesn’t make much difference. If the water circulation rate is more than ~15lpm, that’s only about 3mins residence time in a 50litre volumiser - not long compared with a typical defrost cycle of 10mins or more. So the whole emitter circuit will cool during defrost, irrespective of volumiser location.
I’m happy to be corrected on this - has anyone actually measured the heat consumed by the heat pump condenser during a typical defrost cycle?
Edit: @glyn.hudson’s post Samsung MIM-C02N S-NET Pro Dongle - #11 by glyn.hudson has some useful data on this last point, and it looks like 5mins is a better estimate of defrost cycle time than the 10mins I suggested above (though I think my point about circuit cool-down still stands).