If you put output variable systemnodemassflowrate and systemnodetemperature (check these variable names but this is the jist) then run the simulation and open the eso file. Here you can see the flow rate through those nodes and see if it is what you expect. You will also see the temperatures at these nodes and in the space. This way you can check what your system is doing. Then in the object itself you can set the design flow rate if you need to control it further. My experience with the object is that it helps to put all the values in that it is asks and leave nothing on autosize, the ceilings control very well in this case.

ZoneHVAC:LowTemperatureradiant:VaribaleFlow throttles hot or chilled water flow based on the radiant system controls defined in this object's input fields. Input Output reference describes the controls in detail. Also, there are many E+ example files which illustrate modeling of this object.