5750156-UIM-B-0919
Johnson Controls Ducted Systems
21
CHECKING GAS HEAT INPUT
Natural Gas
1.
Turn off all other gas appliances connected to the gas meter.
2.
With the unit 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 cubic foot 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 unit heating rating with allow-
ance being made for the permissible range of the regulator setting,
replace the orifice spuds with spuds of the proper size.
NOTICE
To find the BTU input, multiply the number of cubic feet of gas con-
sumed 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.)
Table 19:
Gas Rate Cubic Feet Per Hour
1
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 1 Cubic Foot column, where you see that 95
cubic feet of gas per hour are consumed by the furnace at that rate. Multiply
95 x 1050 (or the BTU rating of the gas obtained from the local gas com-
pany). The result is 99,750 BTUH, which is close to the 100,000 BTUH rat-
ing of the unit.
Seconds for
One Revolution
Size of Test Dial
1/2 Cubic Foot
1 Cubic Foot
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
