Rinnai tankless water heater code 51: meaning and exact next steps

Model-specific

What is true of this model and not of its siblings

A sensor that sets the heat

The inlet thermistor tells the control how cold the incoming water is, so it can work out how much heat to add. A wrong reading makes the water too cold or too hot, so the control flags a fault.

Rinnai's checks

Rinnai lists checking the wiring for damage, measuring the sensor's resistance, cleaning scale from it and replacing it.

Hard water

Scale on a sensor insulates it, so hard water is a common underlying cause.

A small sensor with a big job

The inlet thermistor reads the temperature of the incoming cold water. The control subtracts it from your set temperature to work out how much heat to add. A sensor that reads wrongly sends the burner to the wrong rate, so the control flags a fault instead of guessing.

Scale and wiring

Scale on the sensor insulates it, so the reading lags the true temperature. Wiring that has been pinched during service, or chewed by rodents, gives open or erratic readings. Looking at the visible wiring, without opening the unit, is the safe first check.

What the technician does

The technician measures the sensor's resistance at a known temperature, cleans scale from it or replaces it. Replacing a thermistor is a small part, and the repair is usually quick once the cause is found.

Why a wrong reading is dangerous

If the thermistor reads warmer than the true temperature, the control adds too little heat and the water is cold. If it reads cooler, the control adds too much and the water scalds. Because either error is unwelcome, the control refuses to run on a reading it cannot trust.

Cost and parts

The thermistor is a small, inexpensive part, and most of a repair bill is the visit. Because the part is cheap, many technicians replace it rather than clean it. If the heater is old, ask whether the outlet sensor and the flame sensing parts are due too, so that a single visit covers the likely failures.