How do you know its saving 15%? Did you turn your room thermostat down or the water temperature as they suggest? That'll be why you're making a saving, not because of a magnet!
Call me a cynic, but I reckon the Questioner and Answer 2 are one and the same person. Its a blatent attempt at advertising a product on AB.
The Questioner asserts that it saves 15% and he would have checked it how it worked before buying.