335224-BIM-B-1208
60
Johnson Controls Unitary Products
4.
Using the number of seconds it takes for one revolution
of the dial, calculate the cubic feet of gas consumed per
hour. (See example below).
5.
If necessary, adjust the high pressure regulator as dis-
cussed in the section “Manifold Gas Pressure Adjust-
ment”.
Be sure not to over-fire
the furnace on Second
stage. If in doubt, it is better to leave the Second stage of
the furnace slightly under-fired. Repeat Steps 1-5.
To determine the rate of gas flow (First Stage)
1.
Turn off all other gas appliances connected to the gas
meter.
2.
Turn on the furnace and make sure the thermostat is
calling for first stage (60% input) heat.
3.
Even when the thermostat is calling for first stage heat,
the unit will light on second stage and will run on Second
stage for 1 minute. Allow this one-minute time period to
expire and be certain the unit is running on first stage.
4.
Measure the time needed for one revolution of the hand
on the smallest dial on the meter. A typical gas meter
has a 1/2 or a 1 cubic foot test dial.
5.
Using the number of seconds it takes for one revolution
of the dial, calculate the cubic feet of gas consumed per
hour (See example).
6.
If necessary, adjust the low pressure regulator as dis-
cussed in the section “Manifold Gas Pressure Adjust-
ment”.
Be sure not to under-fire
the furnace on first
stage. If in doubt, it is better to leave the first stage of the
furnace slightly over-fired (greater than 60% input).
Repeat Steps 1-6.
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 as it varies widely from
area to area).
EXAMPLE:
By actual measurement, it takes 19 seconds for the hand on
a 1 cubic foot dial to make a revolution with a 192,000 Btuh
furnace running. To determine rotations per minute, divide 60
by 19 = 3.16. To calculate rotations per hour, multiply 3.16
•
60 = 189.6. Multiply 189.6
•
1 (0.5 if using a 1/2 cubic foot
dial) = 189.6. Multiply 189.6
•
(the Btu rating of the gas). For
this example, assume the gas has a Btu rating of 1050
Btu/ft.
3
. The result of 199,000 Btuh is within 5% of the
192,000 Btuh rating of the furnace.
TABLE 40: GAS RATE CUBIC FEET PER HOUR
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
