My new humidifier warns that if you have HARD water, you should use distilled water instead of tap water. I think my old humidifier died because of hard water (maybe), but I'm not 100% sure. The sensing element in the old one was covered in white powder, which is obviously mineral or salt buildup from the distillation process. I cleaned that element as best I could, but it still wouldn't work. I am on "city water", so it should be relatively clean, but also might be injected with things like fluorides.