Checking Temperature Sensors - Lochinvar KNIGHT 400 Service Manual

3
Troubleshooting

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.
Table 3-2A - Inlet Water/System Sensor Resistance vs.
Temperature
Temperature Resistance Temperature Resistance
50 18,780
68 12,263
86 8,194
104 5,592
122 3,893
140 2,760
Table 3-2C - Outlet Water Sensor Resistance vs. Temperature
(Wire Color - R/BK and Y)
Temperature Resistance Temperature Resistance Temperature Resistance Temperature Resistance
50 19,553
68 12,690
86 8,406
104 5,715
122 3,958
140 2,786
Table 3-2D - Flue Sensor Resistance vs. Temperature
(Wire Color - W/B and Y)
Temperature Resistance Temperature Resistance Temperature Resistance Temperature Resistance
50 40,030
68 25,030
86 16,090
104 10,610
122 7,166
140 4,943
(continued)
158 1,990
176 1,458
194 1,084
212 817
S1a
158 2,004
176 1,464
194 1,084
212 816
S3a
158 3,478
176 2,492
194 1,816
212 1,344
Service Manual
Table 3-2B - Outdoor Air Sensor Resistance vs.
Temperature
Temperature Resistance Temperature Resistance
-50 490,813
-40 336,606
-30 234,196
-20 165,180
10 118,018
0 85,362
10 62,465
(Wire Color - G and Y)
50 40,030
68 25,030
86 16,090
104 10,610
122 7,166
140 4,943
(Wire Color - PR and Y)
50 258,500
68 125,500
86 80,220
104 52,590
122 35,270
140 24,160
20 46,218
30 34,558
40 26,099
50 19,900
60 15,311
70 11,883
80 9,299
S1b
158 3,478
176 2,492
194 1,816
212 1,344
S3b
158 16,870
176 12,000
194 8,674
212 6,369
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