Atwood AFS Series Training Manual page 41

#2- Voltages
Voltage to the furnace should be between 10.5 and 13.5 VDC during operation with the interior lights ON and OFF. This check should be
made with the battery, converter or generator when applicable. Use a digital or analog multi meter when taking voltage readings. Do not use a
test light. It does not provide enough useful information for proper diagnosis.
#3- Minimum Ducting Requirements
The various BTU sizes of 85, 89 and 2-stage series furnaces require a minimum number of square inches of heat ducting. The most common
size of soft ducting is 4" diameter. Therefore, since a 4" duct is equal to 12 square inches, we can call out the minimum number of ducts
needed for the models of furnaces as noted below.
Furnace Model
8516, 8520
8525, 8531, 1522
8535, 8935, 8940, 2334, 2540
% Full +20°F 0°F
60%
36,000 18,000 12,750
50% 32,400 16,200 12,150
40% 28,800 14,400 11,400
30% 25,200 12,600 10,450
20% 21,600 10,800 8,100
10%
16,200 8,100
65 lb. Under Mount LP Gas Tank BTU available at
% Full +20°F 0°F
60% 95,600 47,800 36,000
50%
86,000 43,000 32,250
40% 77,000 38,500 29,250
30% 68,000 34,000 25,500
20% 58,000 29,000 21,750
10%
43,200 21,600 16,200
Minimum Ducting Minimum Ducts
24 sq. in.
36 sq. in.
48 sq. in.
-5°F -10°F
8,500
8,100
7,600
7,300
5,400
6,075 4,050
-5°F -10°F
23,900
31,500
19,250
17,000
14,500
10,800
2
3
4
41
-15°F
4,250
4,050
3,800
3,150
2,700
2,025
-15°F
12,100
11,750
9,625
8,500
7,250
5,400