LowTemperatureRadiant:VariableFlow is appropriate for this application. It is important that the active surface area be realistic. I would recommend using door objects to model the radiant panels. (Contrary to what the current Input Output Reference states, radiant systems may be embedded in door subsurface - this will be fixed for the next release) By using subsurfaces, you can easily apply the radiant systems to a smaller surface area.

I am puzzled by the statement " in internal source we don't really model the material of the heating panels e.g. Aluminum panels. We just consider the tubing distance and spacing etc." The internal source object requires at least two material layers, one on either side of the source. Are you saying you are including all of the material layers except the aluminum? That is probably OK for this application, a minor effect in the overall heat transfer from the fluid to the zone.

LowTemperatureRadiant:VariableFlow has fields for specifying the controls. It has several control options, including zone air temperature.