Bryant R-22 service manual Temperature Thermistors, Thermistor Sensor Comparison

Temperature Thermistors

Thermistors are electronic devices which sense temperature. As the temperature increases, the resistance decreases. Thermistors are used to sense outdoor air (OAT) and coil temperature (OCT). Refer to Fig. 43 for resistance values versus temperature.

If the outdoor air or coil thermistor should fail, the control will flash the appropriate fault code. (See Table 21.)

IMPORTANT: The outdoor air thermistor and coil thermistor should be factory mounted in the final locations. Check to ensure thermistors are mounted properly per Fig. 46 and Fig. 47.

Thermistor Sensor Comparison

The control continuously monitors and compares the outdoor air temperature sensor and outdoor coil temperature sensor to ensure proper operating conditions. The comparison is:

SIn cooling if the outdoor air sensor indicates ≥ 10_F warmer than the coil sensor (or) the outdoor air sensor indicates ≥ 20_F cooler than the coil sensor, the sensors are out of range.

SIn heating if the outdoor air sensor indicates ≥ 35_F warmer than the coil sensor (or) the outdoor air sensor indicates ≥ 10_F cooler than the coil sensor, the sensors are out of range.

If the sensors are out of range, the control will flash the appropriate fault code as shown in Table 21.

OAT Thermistor must be locked in place with spherical nib end facing to- wards the front of the control box

The thermistor comparison is not performed during low ambient cooling or defrost operation.

Failed Thermistor Default Operation

Factory defaults have been provided in the event of failure of outdoor air thermistor (OAT) and/or outdoor coil thermistor (OCT).

If the OAT sensor should fail, low ambient cooling will not be allowed and the one-minute outdoor fan off delay will not occur. Defrost will be initiated based on coil temperature and time.

If the OCT sensor should fail, low ambient cooling will not be allowed. Defrost will occur at each time interval during heating operation, but will terminate after 5 minutes.

If there is a thermistor out of range error, defrost will occur at each time interval during heating operation, but will terminate after 5 minutes.

Count the number of short and long flashes to determine the appropriate flash code. Table 21 gives possible causes and actions related to each error.

OCT Thermistor must be secured tight on stub tube.

Fig. 46 – Outdoor Air Thermistor (OAT) Attachment

Fig. 47 – Outdoor Coil Thermistor (OCT) Attachment

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