Some domestic water heaters manufactured between 1994 and 1997 were equipped with a plasic dip tube which will disintegrate over time. Symptoms include white, chalky debris in faucet aerators and running out of hot water. Some of the manufacturers (A.O Smith, Rheem, Ruud, Richmond, American under all of their various brands i.e: Mor-Flo, Craftmaster etc., and Bradford-White and perhaps others) will have an authorized repair company replace the bad part and flush the resulting debris from the tank and hot water lines, but they are not forthcoming with this information, probably because of the cost, upwards of $125 per tank or more if the job gets complicated.
Most have toll free telephone numbers. Give them a try, all they can say is no.