I confess that I never ran a descaling operation on my tankless water heater in the 12 years that it's been on my house. I decided I really should do something about this problem, and here is my video showing how I did it.
I have a Noritz NR981-OD, which is rated at 199,900 BTU, and 7.5 GPM. I believe the current equivalent model is the NR98-OD, and it's available in either Natural Gas or Liquid Propane gas. It also has a higher flow rate of 9.8 GPM. Here's a link if you're interested in looking at one for your house:
ebay.us/7DPioM
"OD" means it's made for outdoor installation and doesn't require ducts - if you need to install it indoors, there are different configurations that you should consider. I strongly recommend hiring a professional, licensed plumber to install one if you want to do this.
The descaling/flush kit that I used is from Vevor, and can be purchased here: ebay.us/LALoLv
The Calci-Free from Rectorseal: amzn.to/4fabaNF
Mesh Paint Strainer: amzn.to/3XZXONO (set of 10 pcs)