I have an older PDC spa that will randomly blow the fuses for the pumps and heater. They are 20 and 30 amp time delay fuses. When it does this, the 50 amp GFI in my house panel will also trip. By random, I mean one time it will be one day and the next time it will be three weeks. All components appear to be in good shape. All connections are tight.

More than likely, your fuse holders are getting hot and are in need of replacement. (corrosion - very common problem, you may want to remove the terminals and try brushing them with a wire brush).

I noticed corrosion on the fuse block in the past. I tried to clean it off as best as I could while it is still in the control housing. I'll look again and see if I can remove the fuse block and give it a good cleaning.

I removed the fuse block and used my Dremel with wire wheel to clean the holders. I haven't blown a fuse since. Time will tell...
Thanks!

Ok, now the 50 amp gfi in my main house panel keeps tripping. I replaced the gfci with a new unit to rule out a weak breaker. The time delay fuses remain intact. I have narrowed the occurance down to the heating cycle. I can run all of the pumps for any length of time without issue. When I turn on the heater, it will eventually trip the gfi. It will trip within 5 minutes or 5 hours. Still very random.

we had a heater that was tripping the gfi, it was a balboa inline heater where the terminals go in it was arcing across the epoxy to the base.
so its possible that you may have some leakage current getting around the heater somehow, the gfi trips when you have leakage to ground. take a look at your heater connections to see if there is any corrosion where the lines go in if theres corrosion the voltage could jump across it enough to trip it. of course the leakage could be internal too.

I just replaced the heating coil. I thought that there might be an issue there. All connections were inspected and cleaned prior to installation.

Still tripping the breaker. Grrrrr.....

I'm ready to call a repair person. Any recommendations? Located in York county, PA.