Rinnai Air Thermistor for 26iV / HD50i - Genuine Replacement (P26IV-KITH) The Rinnai Air thermistor (that's the P26IV-KITH to you and me) is a genuine replacement temperature sensor designed specifically for Rinnai 26iV and HD50i continuous flow water heaters. It monitors the air temperature within the combustion system which let's the control board tick along properly, ensuring the appliance operates at top efficiency with no nasty issues with combustion. You see accurate air temperature readings are pretty crucial for safe operation and decent performance out of your water heater. If the thermistor starts playing up, the appliance might start getting wrong combustion settings, temperature will go haywire and you might get some rather confusing error codes. Swapping out a worn out or busted air thermistor will get your system back on track and keep