Gfci
A ground fault circuit interupter is the only protection that is provided to protect living things, All other trips are for protection of the electrical circuit. The ground fault circuit interupter trips when it senses a fault to ground at a very low level and is very quick when a fault occurs. The breakers in your electrical panel are there to protect the circuits from shorts, ungrounded to grounded conductors and most breakers are inverse time delayed in that the longer the short, the hotter the breaker, the higher the magnetic field and then the breaker trips. Ive seen a wire short and burn on the out side of a building and never trip the breaker until within 2 feet of the sub-panel as each time it shorted it burned apart and took a long time to get the breaker to its trip point. as you can imagine the building caught on fire. one new personel protector that is showing up is the AFCI (Arc Fault Circuit Interupter). This is a special breaker in the subpanel that protects the entire circuit and is also a people protector. GFCI and AFCI are suseptical to failure as is any device made by man, however the protection provided far outweighs the failures which are few. I run a ground probe in each sump, main, and body of water that is seperate except for plumbing. all outlets are GFCI protected and all equipment is plugged into a surge protector. hope something here helps. just my 3 cents worth. what was the make of heater, how old, size, etc. particulars please. maybe we should start an equipement failure forum where we could post problems with equipement. Glad you found the problem.