Bradford White 25X78B N Series, Service Manual

Page: 17 / 28

W A RN I NG
Stored w ater may be HOT wh en
per f ormin g t h e f ollo w in g steps in t h is
procedure. T ake necessary precaution to
prevent personal injury
SE R VI C E PROC E D U R E A G -III
H on e yw e ll T h e r m ow e ll Te stin g a nd
R e p l a c eme nt
D E T E R MI N E W A T E R H EA T E R T A N K T EM P E R A T U R E
NOT E:
I t is i m port a nt to u nd e rst a nd onc e th e r e sist a nc e
f or th e th e r m ow e ll is d e t e r m in e d, w a t e r f l ow
thro u g h th e w a t e r h ea t e r sho u l d not occ u r . P rior
to p e r f or m in g th e st e ps b e l ow t u rn o ff co l d w a t e r
s u pp l y to th e w a t e r h ea t e r . T his wi ll pr e v e nt co l d
w a t e r f l ow into th e t a n k affe ctin g th e r e sist a nc e
v a l ue o f th e th e r m ow e ll.
Step 1. Turn the gas control to the “OFF” position and
disconnect the thermowell harness from the gas control.
Step 2. Using a multimeter set to measure Ohms determine the resistance of the thermowell sensor (See
caution on this page).
Step 3. Hold one probe in the center pin and measure the outside positions pins (see figure 29 & 30, page 16).
Step 4. Draw approximately 1 quart of water from the drain valve and immediately measure the water
temperature using an accurate thermometer. It may be necessary to open a hot water faucet to allow
the heater to drain.
Step 5. Refer to Table 1 below to determine the correct resistance value for the water heater tank temperature.
Example: If the temperature of the water is 84°F, then the resistance through the sensor would be 8449 (see
shaded area). NOTE: Sensor resistance increases as the temperature falls.
°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 1953 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
I n De g rees F
P a g e 17
Sensor R esistance at Various T emperatures
(T able 1 )
C AU T I ON
DO NOT use standard multimeter
probes to test. D oin g so w ill dama g e
connector. Use special pin type
electronic probes or small diameter
w ire pins inserted into connector
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