I'm new to pools and to spas, First off the pool has an inground spillover spa that had a heater at one time but now none of the related heating items are present. Spa is basically half of an octagon. Approximately 5/7. I'm in southern ms. During the summer the average temp here is 90 at night it's usually in the 70s my question is will a 5.5 letter heater work for this hot tub I have a system from an older above ground spa I was contemplating using but it might not be worth the trouble. Says it's a pinnacle. Anyway any info is appreciated.