Hydronic Efficiency to the Max™Amtrol® indirect-fired water heaters use the existing energy in a boiler to minimize fuel use, save energy and maximize hot water output. BoilerMate® was the original Amtrol® water heater and is still a great value. HydroMax® represents the next generation featuring additional functionality and an appliance-like appearance. Its Enter-G-Net™ control (optional) provides built-in diagnostics, energy saving user modes and easy programming.