Have to disagree, but I might have mis-understood you. It is improtant that the return from the HW coil comes before or after /all/ the radiators returns join together.
In an electrical circuit the voltage drop (equivalent to pressure diffence) along conductors is typically two magnitudes less than across the 'loads' (equivalent to radiators). In plumbing this is not true, if the return from the coil divides the return from upstairs and downstairs (say) then this happens: A small proportion of the return water from the coil will travel against the usual direction of flow into the returns from one lot of radiators and then via the point where the radiators flows are joined into the other radiators and then via their return pipework to the main return. It's only a small pressure difference but it can and does drive enough primary water into radiators to cause a problem in summer.
An indication that this is happening is that the warmest of the radiators warm up from the return end, whereas when this happens due to the zone valve letting by or thermosyphoning the radiators heat up from the flow side.