When I first looked at the online manual for your heater I scrolled to the part for IID heaters and thought it needed line voltage to operate. According to the schematic on page 31 (A-9) there are two options for remotely switching the heater. A 2 wire on and off, or a 3 wire option for the pool/spa/off switch. In either case, they are low voltage lines that have to be isolated from line voltages. I see Pentair makes automation controls that take this into account but they are probably overkill. Or at least more than you were thinking of spending.

Sorry I can't be of any more help. Good luck.