Bradford-White Corp D-38T-155 service manual Upper Sensor, Lower Sensor

Conditions: Upper or Lower Sensor Reading Faulty, High Water Temperature, or suspect thermostat is not accurate.

SERVICE PROCEDURE D24-I

Thermostat Circuit Testing

Sensor Resistance Testing

Upper Sensor

1.Determine resistance value of upper sensor. Test across grey wires.

2.Draw quart of water off T&P valve. Using a thermometer, determine water temperature.

3.Use table below to verify correct resistance per water temperature measured.

Lower Sensor:

Upper thermister location

(applicable models)

10

1. Determine resistance value of lower sensor. Test across center wire (common) to each outside wire. Resistance of both thermisters in the lower sensor should be close to each other. If the resistance values for both thermisters are not close to each other, replace the lower sensor. The dual thermisters are used to provide high limit protection in case the thermostat circuit fails to shut off the water heater.

2. Draw quart of water off Drain Valve.

Using a thermometer, determine water temperature.

3.Use table below to verify correct resistance per water temperature measured.

Checking resistance of

upper sensor

Lower thermister

access located inside

control box.

11

Are readings N		Replace
correct?		thermister	Checking resistance of
Y
			lower sensor (center and
Check harness		Replace	outside pins)
	N
continuity.
		harness	CAUTION
Continuity okay?
Y			Be Careful When Making Resistance
			Measurements Not to Damage or
Replace			Deform Connectors or Connector Pins.
control board		Sensor Resistance at Various Temperatures

Example: If water temperature is 84°F, then the resistance through the sensor would be 8449 (see shaded area).

NOTE: Sensor resistance increases as the temperature falls.

In Degrees F

°F	0	1	2	3	4	5	6	7	8	9
40	26109	25400	24712	24045	23399	22771	22163	21573	21000	20445
50	19906	19383	18876	18383	17905	17440	16990	16553	16128	15715
60	15314	14925	14548	14180	13823	13477	13140	12812	12494	12185
70	11884	11592	11308	11032	10763	10502	10248	1000	9760	9526
80	9299	9078	8862	8653	8449	8250	8057	7869	7685	7507
90	7333	7165	7000	6839	6683	6531	6383	6238	6098	5961
100	5827	5697	5570	5446	5326	5208	5094	4982	4873	4767
110	4663	4562	4464	4368	4274	4183	4094	4006	3922	3839
120	3758	3679	3602	3527	3453	3382	3312	3244	3177	3112
130	3048	2986	2925	2866	2808	2752	2697	2643	2590	2538
140	2488	2439	2391	2344	2298	2253	2209	2166	2124	2083
150	2043	2004	1966	1928	1891	1856	1820	1786	1753	1720
160	1688	1656	1625	1595	1566	1537	1509	1481	1454	1427
170	1402	1376	1351	1327	1303	1280	1257	1235	1213	1191
180	1170	1150	1129	1110	1090	1071	1053	1035	1017	999
190	982	965	949	933	917	901	886	871	857	842
200	828	814	801	788	775	762	749	737	725	713

Page 26

26