I have a 94 calpa model ps4-b on 220 power
It has CalSpa controller box that also says ps-4-B
Topside control; Air Pushbutton
2 Jet Pumps (230V), one is 2 Speed, other is single speed at least I think so
Thermostat: Knob on the front of the control box
2" Stainless Flow Thru, 15" long

I drained my tub to clean it. I then replaced the filter. Then I filled it again with water. I got rid of the air lock by using the release valves. I turned off the jets and turned up the heat. When I turned up the thermastat the heater turned on the GFCI kicked off. Now when ever I turn up the thermastat the heater trips the GFCI. If I have the secondary pump running before the heater turns on then everything is fine, until I turn off the jet pump, then the heater trips the GFCI. I am not sure if this is a flow issue, but the GFCI trips before flow even starts. The GFCI trips if I have the primary pump on high and then turn up the heat. But if I have the secondary pump going then heater turns on and heats up the tub.

The problem is that it is now freezing here and I need the thermastat to regulate the heat and currently it trips the GFCI everytime it turns on. I have to go out a couple times a night to circulate the water so that the water doesn't free in the pumps.

I have Windows Media movie that is about 500k that I can send to someone who is willing to take a look.