Rinnai P70EV-713 HD70E Overheat Switch - Genuine Spare Part (P70EV-713) The overheat switch sits directly on the HD70E's heat exchanger, monitoring temperature and cutting the gas supply the moment it runs too hot — preventing heat exchanger damage and runaway overheating before either becomes a real problem. It's a bi-metal switch, so it will usually reset itself once temperature drops back down, but if the underlying cause hasn't been addressed, it will trip again the next time conditions push temperature out of range. That cycling behaviour is worth reading correctly: the switch doing its job is also a clear signal that something upstream needs investigating. Common causes include restricted water flow, a scaled or partially blocked heat exchanger, airflow problems at ignition, or incorrect gas pressure. Replacing the switch without identifying