My 6.5 year-old gas hot water heater (50 gallon A.O. Smith) has started running out of hot water after 15 minutes or so. A plumber told me it was probably a bad dip tube and it would be better to replace the whole hot water heater, rather than repair it.
Does this sound like an accurate diagnosis? What can I do to confirm the problem? And do you really replace rather than repair?