How They Work: Heaters basically resist current flow and doing so heat. Most common rating of heaters is 5.5k although some models have 4.0kw. A 5.5kw heater operating at 230vac you should see a 5-6 degree per hour water temperature increase. 

How do they Fail? The "official" statement from those who make heaters is that if it doesn't fail instantly, the only reason afterward is enduser water quality. We did service on one brand that only warrantee their heaters up to the first use. After that, nothing. Again, the belief is that enduser water quality or lack thereof causes heaters to fail. We no longer service those spas.

But in the field, its hard to determine. Yes, likely the most common reason the heater fails is water quality but there is only so much an enduser can do. Some ask why don't they make elements that last but my answer has always been the same; would you pay 3x what you pay now for a "better quality" heater? In reality, most of us would not. So we try to maintain heaters and see what happens. I have seen the same style heater last less than a year and more then 10. So maybe there is something to the industry's blame.

We now offer "treated" heater elements. They have a coating to extend their life.

In most cases, you can see corrosion on the element and that explains the failure. BUT.. elements can have pinholes and that pinhole will leak current and the gfci senses and POP. So I have seen heaters over the years that looked fine yet the element somewhere is shorting out and the GFCI pops. Solution is replacement.