To use a valve with both flow and return microbore pipes connected to it,
the radiator has to be a type where the insertion tube can be placed within
the bottom horizontal channel. The reason for fitting the insertion tube is
to provide separation between water flowing into the radiator and that
flowing out of it. If this tube is not fitted, water would not enter the
radiator/very poor flow into the radiator, since the valve inlet and outlet
are concentric within the union that screws into the radiator.
On some radiators, the valve connection is in form of a “balled” connection
welded onto the rear of the panel. This type of radiator does not allow
insertion tube to be fitted, therefore it is only suited for single
connections be they microbore or small bore. Insertion tube can be an 8 mm
plastic or copper tube. (for you the size may differ).
I hope above will help
Thankfully we REAL plumbers do know heating LOL