Lochinvar 400-801 Checking temperature sensors, Temperature Resistance Temperature Resistance

Service Manual

3 Troubleshooting (continued)

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	158	1,990
68	12,263	176	1,458
86	8,194	194	1,084
104	5,592	212	817

1223,893

1402,760

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

Table 3-2C - Outlet Water Sensor Resistance vs. Temperature

		S1a							S1b
	(Wire Color - R/BK and Y)								(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 3-2D - Flue Sensor Resistance vs. Temperature
		S3a							S3b
		(Wire Color - W/B and Y)							(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

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