What would cause an electric heating element to short out with the water and trip the GFCI?
More than likely what you are experiencing is a pinhole failure of the element sheath. This can occur in one of a couple of circumstances. If there was any trapped air in the manifold when the heater was on (I’m talking about the size of the end of your pinky) that area of the sheath can super heat and a fracture can develop in that location. This can also occur if the water does not move through the manifold at the correct speed. Low flow condition (closed valve, dirty filter) does not need air to cause an over heat as the water in the manifold boils and creates its own bubble from steam. Another is chemical damage. Low PH indicates that the water is acid and sooner or later it will attack the sheath and cause a pinhole failure. Scaling from total alkalinity being out of tolerance is another way to cause a “hot spot” fracture due to the deposits being left on the element isolating it from the water so heat transfer cannot happen correctly.
