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.
"I ordered a Rheem outdoor tankless water heater last year and was so happy with the item and the shipping speed that I decided to get a point-of-use tankless this year for the bathrooms on the opposite side of the house. I did not even consider buying it from another merchant. Great prices and was delivered two days after I ordered it!" - John W.