Bradford White 20A Service Manual download pdf (Page 26)

°F012345678 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

In Degrees F

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.

Page 26

SERVICE PROCEDURE D24-I

Thermostat Circuit Testing

Upper thermister location

(applicable models)

Lower thermister

access located inside

control box.

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:

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.

Are readings

correct?

Replace

thermister

Check harness

continuity.

Continuity okay?

Replace

control board

Replace

harness

CAUTION

Be Careful When Making Resistance

Measurements Not to Damage or

Deform Connectors or Connector Pins.

Conditions: Upper or Lower Sensor Reading

aulty, High Water Temperature, or suspect

thermostat is not accurate.

Checking resistance of

upper sensor

Checking resistance of

lower sensor (center and

outside pins)

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Bradford White 20A Service Manual Page 26

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