Johnson Controls ZL Series Installation Manual page 71

2. Turn on the furnace and make sure the
thermostat is calling for Second stage (100%
input) heat.
3. Measure the time needed for one revolution
of the hand on the lowest increment dial on
the meter. A typical gas meter has a 1/2 or a 1
cubic foot test dial.
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 discussed in the section
gas pressure adjustment. 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)
6. Turn off all other gas appliances connected to
the gas meter.
7. Turn on the furnace and make sure the
thermostat is calling for first stage heat.
8. 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.
9. Measure the time needed for one revolution
of the hand on the lowest increment dial on
the meter. A typical gas meter has a 1/2 or a 1
cubic foot test dial.
10. 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).
11. If necessary, adjust the low pressure regulator
as discussed in the section
adjustment. Be sure not to under-
pressure
fire the furnace on first stage. If in doubt, it
is better to leave the first stage of the furnace
slightly over-fired (Refer to Table 40 for input
value). Repeat Steps 1-6.
Table 40: Gas rate cubic feet per hour
Seconds for one rev.
10
12
14
Manifold
Manifold gas
Size of test dial
1/2 cu. ft. 1 cu. ft.
180 360
150 300
129 257
Johnson Controls Ducted Systems
Table 40: Gas rate cubic feet per hour
Seconds for one rev.
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
46
48
50
52
54
56
58
60
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 200,000 Btuh furnace run-
ning. 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
199,000 Btuh is within 2% of the 200,000 Btuh
rating of the furnace.
Manifold gas pressure adjustment
Two-stage
About this task:
This gas furnace has two heat stages. Therefore, the
gas valve has two adjustment screws located under
two cover screws. The second stage adjustment
screw is adjacent to the HI marking on the valve and
the first stage adjustment screw is located adjacent
to the LO marking on the valve (See Figure 29).
Manifold pressure adjustment procedure.
Installation Manual ZL Series 7.5- 12.5 Ton 60 Hertz R-410A
Size of test dial
1/2 cu. ft. 1 cu. ft.
113 225
100 200
90 180
82 164
75 150
69 138
64 129
60 120
56 113
53 106
50 100
47 95
45 90
43 86
41 82
39 78
37 75
36 72
35 69
34 67
32 64
31 62
30 60
3 . The result of
71