Just curious what you're using to produce your heat for the hydronic radiant? I have miles of radiant heat tubes in my house and outbuildings powered by hot water heaters, munchkin boilers and soon a new high-efficiency wood powered unit.
I am intending to pick a water heater with enough capacity for both hot water and heating hot water. Generally speaking I don't think the water heater has to be bigger than what would work for hot water since the heating for the most part will not be happening at the same time as the hot water demand. I am leaning in favor of a continuous flow unit like the RV500 if I decide I can afford one.
I am intending to use a so-called "open" system where the fresh water passes through the heating coils to keep them from stagnating. If I were to run anti-freeze then I would have to either run two water heaters or use a heat exchanger that isolates the heating water from the fresh water. Take a look at the following web site for a general schematic of the approach I am thinking of taking:
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