Hot water heater not making hot water. Diagnose and find rust and corrosion on water heater. Hot water heater should be replaced. 20 year old Navien water heater. Recommend replacement Rheem tank water heater, or Navien tankless water heater. Average water heater life span is 8-12 years old. Install new Navien tankless water heater with new pan and test for functionality. Recommend Gemline water softener to protect new water heater from hard water damage.