5370598-BIM-C-0518
Johnson Controls Ducted Systems
51
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 46 seconds for the hand on a 1
cubic foot dial to make a revolution with a 80,000 Btuh furnace
running. To determine rotations per minute, divide 60 by 46 =
1.30. To calculate rotations per hour, multiply 1.30
60 = 78.
Multiply 78
1 (0.5 if using a 1/2 cubic foot dial) = 78. Multiply
78
(the Btu rating of the gas). For this example, assume the
gas has a Btu rating of 1050 Btu/ft.
3
. The result of 81,900 Btuh
is within 5% of the 80,000 Btuh rating of the furnace.
Manifold Gas Pressure Adjustment
This gas furnace has two heat stages. Therefore, the gas valve
has two adjustment screws located under a plastic protective
cover. The second stage (100% input) adjustment screw is
adjacent to the “HI” marking on the valve and the first stage
(60% input) adjustment screw is located adjacent to the “LO”
marking on the valve (See Figure 30).
Manifold pressure adjustment procedure.
Adjust second stage (100% input) pressure first, then adjust
first stage (70% input) pressure.
1.
Turn off all power to the unit.
2.
Using the outlet pressure port on the gas valve, connect a
manometer to monitor the manifold pressure.
3.
Remove plastic cap covering HI and LO pressure
adjustment screws.
4.
Turn on power to the unit.
5.
Set thermostat to call for second stage heat and start
furnace.
6.
If necessary, using a screwdriver, turn the
second stage
adjustment screw (adjacent to the “HI” marking on the
valve) clockwise to increase manifold pressure or
counterclockwise to decrease manifold pressure.
Be sure
not to over-fire the unit on second stage
.
7.
After the high manifold pressure has been checked, adjust
the thermostat to call for first stage heat.
8.
If necessary, using a screwdriver, turn the first stage
adjustment screw (adjacent to the “LO” marking on the
valve) clockwise to increase manifold pressure or
counterclockwise to decrease manifold pressure.
Be sure
not to under-fire the unit on first stage
.
9.
Once pressure has been checked, replace the plastic cap
covering the HI and LO pressure adjustment screws.
NOTE:
When using natural gas, the manifold pressure for
second stage (100% input) should be 3.5 IWG ± 0.3.
The manifold pressure for first stage (60% input) when
using natural gas should be 1.5 IWG ± 0.3.
Adjustment Of Temperature Rise
The temperature rise (the difference of temperature between the
return air and the heated air from the furnace) must lie within the
range shown on the CSA rating plate and the data in Table 10.
After the temperature rise has been determined, the CFM can
be calculated as follows:
After about 20 minutes of operation, determine the furnace
temperature rise. Take readings of both the return air and the
heated air in the ducts (about 6 feet from the furnace) where
they will not be affected by radiant heat. Increase the blower
CFM to decrease the temperature rise; decrease the blower
CFM to increase the rise (See SUPPLY AIR DRIVE
ADJUSTMENT).
NOTE:
Each gas heat exchanger size has a minimum
allowable CFM. Below this CFM, the limit will open.
Table 27: 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
Table 28: Gas Heat Stages
Unit
# of Burner
Tubes
1st Stage
Input (Mbh)
2nd Stage
Input
(Mbh)
Total
Input
(Mbh)
Size
Opt.
-A3
N06
4
45
15
60
N08
4
56
24
80
N12
6
84
36
120
-A4
N06
4
45
15
60
N08
4
56
24
80
N12
6
84
36
120
-A5
N08
4
56
24
80
N12
6
84
36
120
N16
8
112
48
160
