If there's one thing you'll find in the hydronic heating arena, it that there's a ton of people that, "my grandaddy has been doin this for years with 2 nash radiators and a corncobb stove, works great, and the cobbs are free!!"
I'd try calling your manufacturer and talk to them. ASk them at what ouput temp their heater is most efficient. Ask them at what input temp and flow rate it's the most efficient. The documentation may have some info somewhere.
I'll ask you later if you know where I can get free corn cobbs............