Checking Temperature Sensors - Lochinvar 402 Service Manual

3
Troubleshooting

Checking temperature sensors

The appliance temperature sensors (inlet water, outlet water, system water, tank 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.
Table 3B Inlet/Outlet/System/Tank Sensor Resistance vs. Temperature
S1a Inlet/Outlet/System/Tank
(Wire Color - R and W)
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 3C Outdoor Air Sensor Resistance vs. Temperature
Temperature
-50
-40
-30
-20
-10
0
10
Copper-fin II/IIE Boiler and Water Heater
(continued)
158 2,004
176 1,464
194 1,084
212 816
Resistance
490,813
336,606
234,196
165,180
118,018
85,362
62,465
S1b Outlet Only
(Wire Color - Bk and G)
50 40,030
68 25,030
86 16,090
104 10,610
122 7,166
140 4,943
Temperature
20
30
40
50
60
70
80
Service Manual
158 3,478
176 2,492
194 1,816
212 1,344
Resistance
46,218
34,558
26,099
19,900
15,311
11,883
9,299
35