280241-YIM-D-0909
22 Johnson Controls Unitary Products
CHECKING GAS 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 14.
If the actual input is not within 5% of the furnace rating with
allowance being made for the permissible range of the regu-
lator 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.)
TABLE 14: 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 28 seconds in the first column of Table 14. 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. Multi-
ply 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.
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