For years, I've used vinegar. It works fine I suppose, but at $3-$3.50/gal. (and I need 5 or 6 gallons for a 50/50 mix) it kills a $20 dollar bill ever year. So, I started looking for an alternative. I immediately started thinking about de-scaling my coffee pot. Commercial cleaners are usually citric acid based. Same stuff that makes all sour candies "sour". Non-toxic and pretty safe to handle. You can buy it at any candy making supple store, like a bigger kitchen store.
However, I had some handy right under my sink! It is the product I use to clean my dishwasher and keep water spots away. It is called "Lemi Shine".
I'm going to mix a bottle of it (It is a powder) into 5 gals. of hot water and use the winterize port to suck it into the water heater. I'll top it off with water and let it sit over night. Drain, flush and done! Should work every bit as well as vinegar, if not a lot better, and total cost will be about $3.50
It touts being derived from "citrus" and of course lemons are full of citric acid. I know it keeps the inside of my stainless dishwasher absolutely spotless!
Here's a link that shows the product:
https://farm4.staticflickr.com/3956/...290ce4e1_z.jpg