Your hot water tank will not fill automatically. You can fill it from pressurized city water or from your holding tank. Either way make sure your hot water tank is in normal mode (not in bypass mode) Then open a hot water faucet and wait for the water to run in a steady stream. Before you turn on your water heater it's a good idea to lift the pressure relief valve at the top of the tank to make sure some water sprays out. (stand aside or you may get very wet) Then you know absolutely for sure that your tank is full and it's safe to turn it on. You should periodically check that the valve is operating anyway so this actually accomplishes two things at once.