Why? Because water has a higher thermal mass than air, more heat is transferred through hot water than hot air from a furnace.
Water transfers heat 3500 times better than air (air is actually an insulator), consending heaters are almost more energy-efficient than forced-air furances.
Simply put, the exchanger of energy using water versus air is more efficient and a better way of heat creation and transfer.
For more information on the Phoenix with an Air Handler (as shown above), please visit our site.