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3 Troubleshooting (continued)
35
Service Manual
Table 3-2A - Inlet Water/System Sensor Resistance vs.
Temperature
Temperature Resistance Temperature Resistance
50 18,780 158 1,990
68 12,263 176 1,458
86 8,194 194 1,084
104 5,592 212 817
122 3,893
140 2,760
Table 3-2B - Outdoor Air Sensor Resistance vs.
Temperature
Temperature Resistance Temperature Resistance
-50 490,813 20 46,218
-40 336,606 30 34,558
-30 234,196 40 26,099
-20 165,180 50 19,900
10 118,018 60 15,311
0 85,362 70 11,883
10 62,465 80 9,299
Checking temperature sensors
The boiler temperature sensors (inlet water, outlet water, system water, flue, and outdoor air) are all resistance type devices. The
following tables show the correct values for the sensors at various temperatures. Use an ohmmeter to read the resistance of the
sensor at a known temperature. If the resistance of the sensor does not closely match its corresponding table, replace the sensor
It is important to note that the flue and outlet water sensors have two temperature sensing devices in one housing. These devices
are designated as S1a/S1b, outlet sensor and S3a/S3b, flue sensor. Please reference the wiring diagram in the Knight XL
Installation and Operation Manual for correct terminal location.
S1a
(Wire Color - R/BK and Y)
S1b
(Wire Color - G and Y)
Temperature Resistance Temperature Resistance Temperature Resistance Temperature Resistance
50 19,553 158 2,004 50 40,030 158 3,478
68 12,690 176 1,464 68 25,030 176 2,492
86 8,406 194 1,084 86 16,090 194 1,816
104 5,715 212 816 104 10,610 212 1,344
122 3,958 122 7,166
140 2,786 140 4,943
Table 3-2C - Outlet Water Sensor Resistance vs. Temperature
S3a
(Wire Color - W/B and Y)
S3b
(Wire Color - PR and Y)
Temperature Resistance Temperature Resistance Temperature Resistance Temperature Resistance
50 40,030 158 3,478 50 258,500 158 16,870
68 25,030 176 2,492 68 125,500 176 12,000
86 16,090 194 1,816 86 80,220 194 8,674
104 10,610 212 1,344 104 52,590 212 6,369
122 7,166 122 35,270
140 4,943 140 24,160
Table 3-2D - Flue Sensor Resistance vs. Temperature