5566228-BIM-A-0818
Johnson Controls Ducted Systems
53
5.
If necessary, adjust the regulator as discussed in the
section "Manifold Gas Pressure Adjustment". Repeat
Steps 1-5.
Two Stage Gas Heat
This unit has two stages of gas heat. The first stage is 70% or
75% of the full fire input and is considered the minimum input
for the furnace. The intended input for each furnace is shown in
Table 28. 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 (70% 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 70% or 75% 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 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
Single Stage
This gas furnace has one stage of gas heat. Therefore, the gas
valve has one adjustment screw located under a plastic
protective cover on the valve (See Figure 33).
Manifold pressure adjustment procedure.
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 the pressure adjustment
screw.
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