I have an Amish spa, about 10 yrs old.

230v w/GFCI
The heater is a 2" dia steel tube with element inside.
Hercules M/N H941008E

The pump runs on low for 1-2 hours, then trips the GFCI

It appears to run ok with the heater disconected.

Is the heater bad ? or is the GFCI bad ?

How can I test the GFCI ?
How can I confirm that the heater is bad ?

BMartin - I had a similar problem. If you have an ohm meter (to measure resistance to current flow) you may be able to test the heating element. On my tub, I had a problem with the heating element that caused the GFCI to trip. What I found with the meter was that there was a very small short in the element that apparently allowed current to flow thereby tripping the GFCI. On my element I was able to observe a lack of infinite resistance when one test lead of the ohm meter was connected to the outside part of the heating element (the part that would normally be in the water) and the other lead was conneted to where the power would normally be connected. To perform this test you would need to have the element removed from the tub and all power disconnected. What I found with the new element that there was absolutely no current flow (i.e., infinite resistance) when the meter was hooked up in the same way to the new element. Once I installed the new element in the tub, my problem was solved. Please note that I am not an electrician, and there may be other ways to test your element. This is just what I observed with mine.

Good luck!