For awhile, I used only distilled water in my CPAP. There was a period when I was traveling frequently, and didn't want to be carrying DW with me, so I started using tap water instead, and I eventually started using it at home also. For several years I have continued doing that. It doesn't seem to do any damage to the unit, aside from building up some scale in the water reservoir over time. That scaling can be limited greatly by dumping out any water remaining in the reservoir the next day, rinsing it, and then refilling it. As my wife uses a lot of citric acid in our dishwasher (I buy it in bulk, 10# at a time), I put a teaspoon or so of it in the reservoir about once a month, and add some water. In an hour or so, all of the scale is dissolved. A little Vinegar would probably work just as well.
As I earlier mentioned, every two or three weeks, I disassemble the plastic mask and put it in the dishwasher. It comes out looking like new. I also soak the elastic headpieces in hot soapy water as needed. An elastic headpiece will last me at least six months before it loses its elasticity. I generally buy a couple of elastic headpieces a year off eBay for around $7 each instead of going the prescription route. Much less hassle. I also periodically flush the hose with hot water. Hoses will last indefinitely, but I have three of them.
