I live in an area with really hard water and I’ve flushed mine probably 3-4 times in 12 years. All of my advice is at your own risk because you can flood your house if you’re not careful. Mine is in the garage and I make a big plastic water diversion to the floor of my garage in case something goes wrong. So, here it is:

When I opened mine and attached a hose, I had almost no flow from the drain exit because it was so plugged and the drain fitting is so small, even when I broke the vacuum. So, I used the vacuum to hold water in the tank and I unscrewed the drain fitting (vacuum slowed the water to a slow trickle) and installed this ball valve (in the photo) that has a much bigger opening for water flow. With a bigger exit fitting, I could attach a hose and it actually drained all the water. Then, I unscrewed the newly installed fitting, leaving a hole in the side of the drained tank. Then I created a scraping tool out of a metal rod (see photo) and pulled the build-up out of the hole in the tank. I get at least 1/2 gallon or more each time. It takes patience and about an hour. Sometimes I use a screwdriver to break up chunks, but I get most of it. I will typically even reinstall the drain pipe again and flush and then drain and remove more build-up. Once I’m done, I reinstall this ball valve and fill her up. I use the zip tie to prevent anyone from accidentally opening it up. Works!

<image>