It's an ARS 5520. I think it fixed itself though after leaving it long enough. The tubs slightly bowed upwards so that the sides are pressing on the heat panel more than the center of the tub causing it to be hotter in small spots. I left it on all weekend and checked again Monday evening and the hot spots seem to have went away. Maybe the tubs un-bowed slightly or the velcro/padding settled enough to take enough pressure off *shrug*