In a hot water heater, the dip tube is a pipe in the heater's water tank that is connected to the cold water input source. The function of this tube is to provide a path directly to the bottom of the tank for cold water coming into the heater.
A properly functioning dip tube keeps cold water from mixing with hot water inside this tank. A malfunctioning or broken dip tube can cause both types of water to mix, causing the water temperature to cool. A dip tube can also deteriorate over age and disintegrate into small white particles that can exit the heater through a tap water connection.