5725848-XIM-A-0319
44 Johnson Controls Ducted Systems
meter. A typical gas meter usually has a 1/2 or a 1 cubic
foot test dial.
3. Using the number of seconds for each revolution and the
size of the test dial increment, find the cubic feet of gas
consumed per hour from the Gas Rate - Cubic Feet Per
Hour (Table 25).
If the actual input is not within 5% of the furnace rating (with
allowance being made for the permissible range of the regulator
setting), replace the orifice spuds with spuds of the proper size.
NOTE: To find the Btu input, multiply the number of cubic feet
of gas consumed per hour by the Btu content of the gas
in your particular locality (contact your gas company for
this information - it varies widely from city to city.)
Adjustment of temperature rise
The temperature rise (or temperature difference between the
return air and the heated air from the furnace) must lie within
the range shown on the rating plate and the data in the Gas
Heat Application Table 3.
After the temperature rise has been determined, the Volumetric
Flow Rate can be calculated as follows:
After about 20 minutes of operation, determine the furnace
temperature rise. Take readings of both the return air and the
heated air in the ducts (about six feet from the furnace) where
they will not be affected by radiant heat. Increase
the blower
Volumetric Flow Rate to decrease the temperature rise;
decrease the blower Volumetric Flow Rate to increase the rise.
Refer to the Blower Motor and Drive Data Table 19.
Table 25: Gas rate - cubic feet per hour
1
Seconds for one
rev.
Size of Test Dial
1/2 cu. ft. 1 cu. ft.
10 180 360
12 150 300
14 129 257
16 113 225
18 100 200
20 90 180
22 82 164
24 75 150
26 69 138
28 64 129
30 60 120
32 56 113
34 53 106
36 50 100
38 47 95
40 45 90
42 43 86
44 41 82
46 39 78
48 37 75
50 36 72
52 35 69
54 34 67
56 32 64
58 31 62
60 30 60
1. By actual measurement, it takes 38 seconds for the hand on
the 1-cubic foot dial to make a revolution with a 100,000 Btuh
furnace running. Using this information, located 38 seconds
in the first column in the table above. Read across to the col-
umn headed “1 Cubic Foot”, where you will see that 95 cubic
feet of gas per hour are consumed by the furnace at that rate.
Multiply 95 X 1050 (the Btu rating of the gas obtained from
the local gas company). The result is 99,750 Btuh, which is
close to the 100,000 Btuh rating of the furnace.
Table 25: Gas rate - cubic feet per hour
1
Seconds for one
rev.
Size of Test Dial
1/2 cu. ft. 1 cu. ft.
CFM
Btuh Input x 0.8
=
108..xFTempRise
o
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