Heater element evaluation start here
Heater elements are small, strong wires (filaments) encased in an insulating powder surrounded by an Incoloy sheath. The electrical connections are made at the terminals connected to cold pins attached to the filament. The filament super heats when power is applied which heats the powder and the sheath. Portable spa elements are made to be totally immersed in flowing water to prevent damage from overheating.
To test the integrity of a heater element, use an Ohm meter on its lowest setting and measure the resistance between the terminals. The acceptable range for most portable spa elements is 9 - 12 Ohms. However depending on the Kilowatt range of the element a reading of <25 ohms is nothing to worry about. Next test for a short to ground. On your highest Ohm setting measure between one element lead and the bulkhead, nut, or sheath of the element. You should get an infinite reading on your meter indicating no continuity to ground.
Heater elements are similar to light bulbs in that the filament cannot "sort of go bad, it either has connection or it doesn't. The difference is that an element can be capable of heating and still need replacing due to allowing current to leak into the water (short to ground). The causes for a short to ground include: pin hole failure from chemical damage, overheating due to dry fire, and overheating due to insufficient water flow.
Testing heater components
Electric heaters, whether integral or stand alone, have most of the same basic components. One of the ways to test these components is to disconnect power and check for continuity or resistance across each unit.
Remembering that since Hi-limits, Thermostats, and Pressure Switches are nothing more than a switch activated other than manually, we will be looking for a short across the terminals. The Element is considered a load, and will have resistance (usually 9-12 ohms in the case of 1- 6kw elements). None of the components or loads in a heater circuit should ever show any continuity (or resistance) to ground.
The diagram below illustrates the test points for each component :
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