A Tankless water heater needs maintenance. Tankless water heater manufacturers recommend their units be flushed at least once a year to eliminate the buildup of calcium and other minerals within the unit -- even more often if you have hard water. Some units will indicate when the unit is in need of flushing. You can flush out a tankless hot water heater in about one hour.
How to Descale or Flush A Tankless Water Heater