[Trainer]

Dario, Your Aqua Hot heater is of the older design in which the fresh water coils are not inside the coolant tank allowing one to use engine antifreeze, what ever you use in the engine and generator.

With the heater cooled down, remove the radiator cap and fill the coolant tank lipping full of 50/50 engine antifreeze. Check radiator cap for good gasket. If you need one, it is a 13 lb cap at all auto part stores.

Replace the cap and start the diesel burner. When it cycles off, fill the overflow tank to the hot line. As the tank cools, the overflow tank level should go down some. If it does not go down, the overflow hose and/or the radiator cap is not air tight. That needs attention.