[Leif]

I just changed the water in my hot tub, and now when I power it up to heat the water it runs for a few seconds the GFCI trips and power is lost. This has happened before and seemed to resolve itself after a few tries, but not this time. The hot tub had been operating perfectly well before I changed the water.

[ballengerb1]

Check all of you wiring connections for moisture or loose wire nuts. It could be that the GCFI is doing its job. If you can get your hands on a meter it would be helpful to measure the load being pulled by the pump and the heater.

[Dr_Spa]

How old is the spa? It's not unusual on an older spa, with a weak heater, for the heater to go out upon a water change due to the "shock" of either the cold water, or the initial start up of getting any air out of the lines.