I found this online:
http://www.philips.co.uk/c/irons/iro...-gc024_10/prd/
Rowenta and other irons come with a warning not to use distilled water in them. Not sure why distilled water is supposed to be bad for them, although someone posted that distilled water still has salt in it (and salt is corrosive).
I think the important thing if you have hard tap water (as we do) is to remove the calcium from the water. I have cleaned the calcium from my Rowenta (and voided the warranty) by using Iron Out and even then I had to use a pin to scrape off the white calcium flakes from the holes.