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Fireplace: Use ZoneHVAC:HighTemperatureRadiant to model the fireplace, assuming it's gas fired. Hot Water Radiators: Use a PlantLoop with a Boiler:HotWater which serves either ZoneHVAC:Baseboard:RadiantConvective:Water or ZoneHVAC:Baseboad:Convective:Water depending on your situation.
Not sure what your trying to do wrt '...attribute boiler heat losses to a single zone'. The Boiler:HotWater assumes no losses to the space if that's what you're asking.