Thermistors and temperature sensors

What they do

A thermistor is a resistor whose resistance changes predictably with temperature. The board reads that resistance and converts it to a temperature. A tankless heater carries several: inlet water, outgoing water, heat exchanger, exhaust gas, and a freeze protection sensor.

They are checked the same way everywhere. Rinnai publishes the expected values for its standard sensors — roughly 11.4-14K ohms at 59°F falling to 2.2-2.7K ohms at 140°F — and a technician compares the sensor against that table before condemning it.

The SENSEI RX exception worth knowing

Rinnai’s SENSEI RX generation adds a lime detecting thermistor clamped to the hot water pipe, and it is not the same kind of sensor: its range runs around 70.6-85.0K ohms at 59°F, five times the standard sensors. It is the sensor behind the LC scale lockout, and it is why code 34 means something completely different on an RX than on Rinnai’s older units, where that same number refers to a combustion air temperature sensor.

Why they fail

Scale, mostly. Any sensor sitting in the water path collects the same mineral deposit that coats the heat exchanger, and a coated sensor reads slow and wrong. Rinnai’s step before replacing a water-side thermistor is to remove it and clean the build-up off. The rest is broken wiring, a crushed connector, or moisture in the harness.

Why this is not a DIY part

The part is trivially cheap; getting to it is not. Water-side sensors need the supply isolated and the unit drained, with an O-ring that has to be intact and lubricated on reassembly, and a leak check afterwards. Exhaust side sensors sit in the flue path of a gas appliance.

What a replacement usually costs

$20-80 for the sensor. Realistically $150-450 done, because you are paying for a visit, a drain-down, and a leak test.