I just purchased a used 95 calspa (220v). When I filled it with water I noticed one of the couplers was leaking so I replaced it but when I was removing it I accidentally broke the thermostat. I put a new Thermostat on it but when I turn the tub on the jets and the light works fine but if i turn the thermostat to heat it trips the breaker (first it trips the gfci on the tub itself and then the breaker itself on the shutoff trips). The heater did run the first time I turned it on for about half an hour then this problem started. I never turned the tub on with the old thermostat.

I would disconnect both wires that go to the heater element, and try the spa. If the breaker holds (I would suspect it will) then you would be needing to replace your heater element.