Check that you have the 325W high power model heater, WB6405. In some colder areas a single 325 watt waterbed heater may not be able to maintain a temperature above 90 degrees in some waterbeds. This effect is most noticeable in king size full motion or deep fill waterbed mattresses and mattresses with only a few fiber layers. If you need a heat setting over 90 degrees two heaters are recommended for the above criteria.