Fireplace, radiant heating and hot water

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First of all thanks for the answer. If I do this way the ZoneHVAC:HighTemperatureRadiant and the Water Loop won't be served from the same generator so I couldn't consider the boiler performance curve and the boiler maximum capacity properly.

About "attribute boiler heat losses to a single zone" yes you understood right. There's an option to place the hot water tank in a thermal zone a thus to consider the heat losses through its insulation: I was wondering if there's something similar for boilers so I could just use this way to simulate it.

Oh, you have a fireplace that creates hot water? If that's the case then all you care about is the radiant aspect so you just need to model radiant baseboards.

Can I deliver the water to other thermal zones?

Yes, it will be served by one plant loop.

The point is that in the real system, if any zone demands heat, the fireplace has to turn on and deliver heat also in the zone where it is placed even if the serpoint temperature is already reached. I guess I can't simulate this behaviour.