This is a complete flushing kit (except for the vinegar you'll need), ready to go. Works great and frankly, considering the high quality of the included pump & the amount a service company would have charged to come do the job, this is very reasonably priced. I highly recommend it.
This is a convenient collection of off the shelf parts. It consists of a bucket (like the orange ones Home Depot sells) a Franklin UP16M 1/6 HP submersible pump and two washing machine hoses.
Add two gallons of white vinegar. Connect one hose from the pump to the cold service valve on your tankless heater and the other one from the hot service valve to drain back into the bucket. Set the valves and run for 45 min to an hour. That's it.
To purge the vinegar, relocate the "hot" hose to a drain and restore just the cold service valve to normal. Let it run for five minutes. Only then restore the hot valve to normal.