Question-and-Answer Resource for the Building Energy Modeling Community
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The ZoneHVAC:LowTemperatureRadiant:VariableFlow object represents a radiant heating system where water is used to supply/remove energy to/from a building surface (e.g. radiant concrete slab or radiant ceiling panels). To model a typical hydronic radiator I would suggest using ZoneHVAC:Baseboard:Convective:Water.
As far as the air loop, you don't necessarily need an air loop if the zones are only conditioned by hydronic radiators and there are no ventilation requirements. But to answer your question, yes, you can serve all of the zones with a single multi-zone air handler, which can either utilize a water heating coil (from either the same boiler plant serving the baseboards, or a different boiler plant) or a standalone gas furnace (Coil:Heating:Fuel).
