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You could use an electric heater with a spa controller. A Balboa VS501 pack kit comes with everything you need.https://www.spaguts.com/spaguts-brand-sgvs501z-spa-controller-kit-with-vl401-topside-5-5kw-54801-01-1669?gclid=Cj0KCQjw3JanBhCPARIsAJpXTx7-bbyV_C2Y4nadHFGguHu9ej8xUUO29fXYeujk1Si5xb6u2lz0aJAaAsL8EALw_wcB