830841-BIM-D-0512
Johnson Controls Unitary Products
31
Direct Drive Blower
All units have direct drive, constant CFM blower motors.
Checking Gas Heat Input
Natural Gas
1.
Turn off all other gas appliances connected to the gas meter.
2.
With the furnace turned on, measure the time needed for
one revolution of the hand on the smallest dial on the
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 Table 19.
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 since it varies widely from city to city.)
BTUH Output
1.08 x CFM
OR
CFM =
BTUH Output
Degrees F Temp Rise =
1.08 x Degrees F Temp Rise
Table 19: 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.
EXAMPLE
: By actual measurement, it takes 38 seconds for the
hand on the 1-cubic foot dial to make a revolution with just a
100,000 BTUH furnace running. Using this information, locate
38 seconds in the first column of Table 19. Read across to the
column 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.
