Our estimated load to heat the house is 106,000 btu, We have two forced air furnaces, one on each end of the house. Air to water coils est. btu. range is 75,000-100,000 each. How does one calculate gpm flow rate to each furnace coil if 1 gpm=10,000 btu. We have on demand hot water two pipe system with circulating pump from furthest fixture from hot water tank back to cold water supply input to on demand tank. We not sure if we should use a plate to plate heat exchanger on supply side of tank or a boiler mate. What is the best way to calculate gpm domestic hot water demand load for a family of four. On demand hot water tank has a 180,000 btu/hour max.. How do you calculate gpm flow rate thru plate to plate ir boiler mate from wood boiler secondary piping. Any help would be greatly appreciate. We've in the process of order the garn 2000 system.