My house has a tankless water heater. Good story huh?
Anyway they need to get flushed once per year. The kit is a submersible pump, cleaner and some connecting hoses.
You take this setup pour in your favorite radiator flush chemicals, I used Blue Devil. I first flushed with water, then chemicals, then water again. To reverse flush you just swap the pump and return hose. Set the bucket on an electric hot plate for heated flush.