Many water heaters include a diagonal tube, or “dip tube”, that must be installed in the cold water inlet to direct the water into the tank. Dip tubes are usually made of flexible plastic and can be made with one or more coils to help lower their height. Dip tubes should be free of kinks and not look worn or brittle.
Some older, shorter (10-12 feet) models do not use dip tubes but instead rely on natural draft – gravity to bring the cold water from the top of the tank to about halfway up where it is heated before rising towards the top of the tank. In these cases, you will only need a coupler between your supply tube and the water heater.
If your water heater does have a dip tube, it can be removed by either removing the threaded coupling connected to the heat exchanger or by disconnecting the supply line from the top of the tank. Dip tubes should not interfere with the normal operation if left in place but may require you to raise your hot water faucet above kitchen sink height.