Heater issues are one of the easiest to diagnose. Most problems occur wherever water makes contact, such as at the heat exchange / tube bundle (gas heaters) and heating elements (electric heaters). Water breaks down everything given enough time. Poor water chemistry accelerates the process. As a result, heat exchangers leak and kill your pilot light and heater elements break down causing a short (circuit breaker tripping).

Of course, thermo couplers, pressure switches and all sensors and relays can cause a heater failure. Gas valve failures are rare. However, like everything else, they don't last forever.