Ok , I drained and refilled the spa this weekend to convert from bromine to chlorine. The spa worked great on the first refill. I ran the jets for about 30 minutes to flush the plumbing then drained again. I refilled the tub and tested the water , got all the chemicals added while the pump was running. Today I came home with the gfci tripped on the spa pack. I tested my main gfci and then reset the spa pack gfci and got an immediate trip. I have taken the new circ pump out of the circuit , turned the thermostat all the way down ,and unplugged the jet pump and light from the pack . Every time the gfci trips as soon as I reset it.
Makes me wish I would have left the bromine alone since the spa was working before the refil .

Do you have an Ozone generator installed?

If so, unplug it from the control box...

If not....

Sure sounds like the heater... (even though the t-stat is turned down, the neutral still carries potential current that can go to ground, thus tripping the GFCI) Try removing BOTH heater leads on the heater to test.
(bummer)

Good luck!

~Swine

I removed the top cover to my spa pack and the problem was obvious. The ground lead going to the heater was shorted to the hot lead going to the spa packs gfci. On my spa pack the gfci is directly above the heater connector prongs . It appears that the two wires were pinched when I put the cover back on after checking the fuse . I hope this was the case . If not , something caused the wires to overheat , melt the insulation , then short.
I made new leads and installed them and the spa is working correctly at the moment. Hopefully this will continue to be the case.