Also, from what Rich (sudsmaster) and Launderess posted, it doesn't seem so unlikely that filter-clogging hard water deposits could develop. I suspect my friends do a lot of washing in cold water, though I haven't interrogated them about it. They've become quite the greenies in the past few years, and their towels always look dingy, as catty as it sounds to say that. So it's possible the washing soda isn't getting dissolved in the first place.