The EUS heat exchanger is protected with an exclusive coating that isolates the water from the metal.
Many manufacturers void their warranty for improper chemical balance.
All heater manufacturers seem to agree that water chemistry is the major cause of heater failures. The number one cause of failure in heaters involves corrosive chemicals. This is an un-coated heat exchanger. This is what happens when heat exchanger coils in a heat pump are exposed to corrosive chemicals of treated water in any pool or spa. This creates a very expensive repair and lengthy shutdown of the heater.