While some systems advocate using the water heater as a source for radiant heating, I don't think it is a great idea. In the winter, you might have an extra 1000' of piping to move the water through - you are likely to have some places to stagnate some water. In the summer, you don't want that water hot, so then what. Use a heat exchanger so you have a closed system, and a boiler. Most wh aren't designed for the continuous use you might need in the winter, and if sized properly, you then might not get enough to take a nice shower, either.
I think you'd be better off with a boiler dedicated to the task that you can turn off in the winter.