Having been involved with some electric water heater modeling in a previous life, I'd guess that if the water doesn't stay hot as long when heated by the HX that the HX coil is located high in the tank. Believe it or not, the heating element in a water heater heats only the water above the element, so when the bottom element burns out in your home water heater, it starts acting like a 5 gallon tank instead of a 50 gallon tank. Similarly, when the hot water is withdrawn from the tank from the top, a distinct thermocline moves up the tank so that when you run out of hot water, you run out all of a sudden. So I'd guess that the HX coil is heating only the top of the tank. Dave and Marie Ross Miss Jackie II Tiger 41 Sundeck Reston, Virginia Docked in Kent Narrows, Maryland