This is probably the second most asked question and one of the biggest things when done improperly can make the unit a big headache.
If you follow this video exactly as I have laid it out, you will not have a problem with the unit after flushing.
The biggest thing people don't do it to remove the inlet water filter and flush the unit with it out.
Please have a spare inlet water filter on hand (contractors should have at least 6 on hand) before you attempt to descale the unit.
Remember on the older unites the "R" series you need to fake out the unit buy letting it fire then kill the power to it in order to make the water servo valve open.
Flow-Aid descale kit $138.74
32oz flow-aid $18.95
You can use 4 gallons of distilled white vinegar instead of the Flow-Aid solution.
If you like you can buy any kit you want, I am just use to the Flow-air system, JUST MAKE SURE THE PUMP IS DESIGNED FOR DESCALING TANKLESS HEATER.
If you would like a filter removal tool, please e mail me your COMPLETE NAME AND ADDRESS and I will get back to you with a way you can pay.
If you live in the continual U.S., the cost will be #30.00 complete.
[email protected]
If you have a question, please e mail me at the same address.
I hope you enjoy this video,
Mike

Please use a licensed plumber trained in Rinnai level 3 training and to follow all local and national plumbing and gas codes.
File a permit and obtain an inspection from your local building department.