The dip tube is a plastic pipe inside your water heater that directs incoming cold water to the tank bottom for efficient heating. When the dip tube breaks or deteriorates cold water mixes near the top resulting in lukewarm water output. Small white plastic particles at faucet aerators indicate dip tube disintegration. Replacement involves removing the cold water inlet fitting and pulling out the old tube. New dip tubes cost under 20 dollars and fit most standard heaters.