Hello all - - my first post. I have a Sta Rite SR400LP pool/ spa heater. Approx 4 1/2 years old. The heater is only used 20-30 times a year to heat a hot tub attached to the pool. When I turned it on yesterday, all indications were initially normal. The blower activated, water temperature illuminated on led, and approx 20 seconds later heard a "click" followed by the propane burner igniting. Then the PROBLEM: Unit began to vibrate with accompanying "banging" sound. The water temperature display then began to climb rapidly and the burner shut down. The blower remained on (normal). There were no codes displayed on the exterior LED screen. I assume the banging was caused by water in coil assembly overheating and becoming steam. I removed the thermal regulator from beside the intake/ exhaust water connections. It is clean and doesn't appear damaged. But, I don't know how to check it operationally. Any ideas on how to proceed?