Rinnai HE Series How To Use Manual page 177

1. Check sensor wiring harness for loose, broken or damaged connections from sensor back to PC
board.
2. Turn off water supply to water heater and drain system down. With no water left in hot water
supply lines remove the sensor and check it for scale build up. Clean off any substance found on
this component. Proceed to item #3 below.
3. With water supply still isolated and sensor removed from unit, check the resistance readings of
this component using a volt/ohm meter capable of reading 20K ohms. Set meter to proper
setting for checking 20K ohms and insert meter leads into each end of the sensor plug. Then
apply heat to the sensor bulb. You will notice the sensor resistance reading start to drop when
heat is applied. A simple way to apply heat is to place the sensor bulb between your thumb and
another finger and apply pressure. The heat from your body will cause the resistance reading to
decrease. If the sensor reading starts to decrease with heat applied normally that indicates the
component is functioning properly. The resistance reading will increase if ice is placed against
the bulb. Typical resistance values are 11.4 – 14K ohms at 59 degrees ⁰F, 6.4 – 7.7K ohms at 86
⁰F, 3.6 – 4.5K ohms at 113⁰F, 2.2 – 2.7K at 140 ⁰F or 0.6 – 0.8K at 221 ⁰F.
If sensor readings are correct re‐install this component ensuring the small O‐ring is still intact in
sensor well before installation. Place a small amount of grease or lubricant on the O‐ring to
prevent damaging it during installation. Turn water supply back on and check for leaks around
this component. Next re‐fire the unit to see if you still get a code 51. If so, replace the sensor.
Code 51 Troubleshooting Procedure
Inlet Water Temperature Sensor
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