I took a chemistry class this spring, and my instructor actually talked about this topic. He said that the impurities in tap water require a higher temperature to get the water to steam and that is why many manufacturers say to only use tap water. I quit buying Rowentas, I didn't feel they were worth the money. I have a black and decker now and love it. I steam and spray when I am quilting (not at the same time, lol).