I've always used Muriatic acid. The amount used depends on the the level of scaling and how large your HX system is. For your machine I would take a few cups of water out of your stainless waterbox and put 1.5 cups of MA in. Then start the machine, turn the pump on and let it warm up and circulate for 20 minutes or so. Then flush the entire system with fresh water. PowerClean may recommend something different though....