wpc
Sounds like the dip tube in the water heater. This tube is on the cold water inlet, and extends to the lower section of your water heater. There were defective dip tubes manufactured, check the brand of your heater and call a supply house that handles that brand, they may be able to lead you to a list of plumbers who are engaged in the warranty work. One usual sign of a defective dip tube is loss of hot water volume.
