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Checking Gas Heat Input
This unit has two stages of gas heat. The first stage is 60% of
the full fire input and is considered the minimum input for the
furnace. The intended input for each furnace is shown in
Table 29. The table applies to units operating on 60 Hz power
only.
To determine the rate of gas flow (Second 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 Second stage (100% input) heat.
3.
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.
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 "Manifold 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)
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 below).
6.
If necessary, adjust the low pressure regulator as
discussed in the section "Manifold Gas Pressure
Adjustment". 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.
Table 28: Gas Rate Cubic Feet Per Hour
Seconds for
One Rev.
10
12
14
16
18
20
22
Johnson Controls Ducted Systems
Size of Test Dial
1/2 cu. ft.
1 cu. ft.
180
360
150
300
129
257
113
225
100
200
90
180
82
164
Table 28: Gas Rate Cubic Feet Per Hour
Seconds for
One Rev.
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 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.
199,000 Btuh is within 5% of the 192,000 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 33).
Manifold pressure adjustment procedure.
Adjust second stage (100% input) pressure first, then adjust
first stage (60% 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.
5565606-TIM-B-1018
Size of Test Dial
1/2 cu. ft.
1 cu. ft.
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
59
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