Here is a nice presentation done by John Siegenthaler (THE hydronics guy, at least in the US). It goes over the basics of a low energy house, passive house is mentioned, then he gets into hydronics, more specifically low temp heated floors and low mass heaters, and much more. He includes a link to a site that will help you size a flat plate heat exchanger (
Flatplateselect.com). There are TONS of pictures and info in this pdf. Great for getting ideas and learning the basics of hydronic heating.
Hydronic Heating for Low Energy Houses by John Siegenthaler