Tankless systems and boilers designed for heating are similar but not exactly the same. Personally, I think you'd get better reliability and easier installation using a boiler for your heating both the house and the hot water. Hooked to an indirect tank, a modern high-efficiency boiler makes great use of the energy for both purposes...a tankless may not. Some tankless systems void their warranty when used for space heating. Again, personal opinion, while some people think they work great, an open system has some pitfalls. One, especially when used with cast iron radiators is that you'd rust them out. An open system is constantly adding oxygen to the system in the incoming water supply - guess what, that will rust out your radiators. A closed system gets (ideally) filled once, and shortly, all the oxygen is used up, then, the water becomes relatively inert.