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Example:
3.50"w.c. Gas valve outlet pressure tap reading
-.15"w.c. Burner box inlet pressure (hose not connected)
3.65"w.c. Actual manifold pressure instead of the desired 3.5"w.c.
Figure 29
CHECKING DIRECT VENT MANIFOLD
PRESSURE WITH BURNER BOX REFERENCE
Manifold Pressure Adjustment:
The burner box pressure hose does not have to be
connected in order to set the manifold pressure if the
burner access panel is removed or no inlet air piping is
attached to furnace.
Turn OFF the gas and electrical before
preceeding!
Remove the manifold pressure tap pipe
plug from the gas valve and install a pressure tap and
connect it to a manometer as described in the previous
section. Turn on the gas and electrical supplies.
The gas control (Figure 27) has two separate pressure
regulator adjustment screws, one for 1
st
stage (marked
"LO") and the second one for 2
nd
stage (marked "HI").
The adjusting screws are positioned on either side of
the barbed fitting. The pressure regulator adjustment
is sensitive: one turn of the adjusting screw will
result in a relatively large change in manifold
pressure. Turn regulator-adjusting screws IN
( c l o c k w i s e ) t o i n c r e a s e p r e s s u r e , O U T
(counterclockwise) to decrease presure.
Set the unit on high fire and adjust the 2
nd
stage high fire
pressure regulator to the required setting. Disconnect the
violet wire from the "HI" terminal on the gas control and
the burners will drop down to the 1
st
stage low fire, then
adjust the 1
st
stage pressure regulator setting as required.
Gas input must never exceed the value shown on the
furnace rating label. These units are equipped for rated
input at manifold pressures of 2.3" w.c. (1
st
stage) and
3.5" w.c. (2
nd
stage) for natural gas. When these
furnaces have been converted for use with LP (propane)
gas, the manifold pressures are 6.4" w.c. (1
st
stage) and
10.0" w.c. (2
nd
stage). After proper adjustments,
reconnect violet wire and furnace should go up to 2
nd
stage high fire. Turn OFF gas, replace manifold pressure
tap pipe plug, regulator adjustment vent cap, and turn ON
gas.
At higher altitudes and varying heating values,
manifold pressure or orifice changes maybe
required. Consult Tables 7 and 8 for appropriate
values. Failure to follow this warning could lead to
a hazardous furnace operating condition and result
in serious bodily injury or loss of life.
Determining Furnace Input - Natural Gas ONLY:
NOTE:
All access doors must be in place when checking
gas input.
1.
Turn OFF all other gas appliances (except for pilot
burners) served by the same gas meter.
2.
With furnace operating in full heat cycle, note how
many seconds it takes for one full revolution of the
smallest dial on the meter. Typically, this will be a
1/2 - or - 1 - cubic foot test dial.
3.
Using the number of seconds for one revolution and
the size of the meter dial, determine the cubic foot
per hour of gas flow by using the formula provided
below or Table 5.
Cubic Ft/Hr =
Number of Dial Revolutions x Cubic Foot/Revolution x 3600
Time (in seconds) Required for Number of Timed Revolutions
TABLE 5
Gas Rate (Cubic Feet per Hour)
Seconds for
One
Revolution
TEST DIAL
1/2
Cubic
Feet
1
Cubic
Foot
2
Cubic
Feet
10
160
360
720
12
150
300
600
14
129
257
514
16
113
225
450
18
100
200
400
20
90
180
360
22
82
164
325
24
75
150
300
26
69
138
276
28
64
129
258
30
60
120
240
32
56
113
226
34
53
106
212
Seconds for
One
Revolution
TEST DIAL
1/2
Cubic
Feet
1
Cubic
Foot
2
Cubic
Feet
36
50
100
200
38
47
95
190
40
45
90
180
42
43
86
172
44
41
82
164
46
39
78
156
48
37
75
150
50
36
72
144
52
35
69
138
54
34
67
134
56
32
64
128
58
31
62
124
60
30
60
120
20558801
Issue 0442
Page 24 of 32
Summary of Contents for Gas-Fired Furnace
Page 13: ...FURNACE WIRING SPECIFICATIONS 20558801 Issue 0442 Page 5 of 32 ...
Page 21: ...Figure 8 20558801 Issue 0442 Page 13 of 32 VENT TERMINATION CLEARANCES ...
Page 36: ...SEQUENCE OF OPERATION DIRECT IGNITION SYSTEM CONTROL 20558801 Issue 0442 Page 28 of 32 ...
Page 40: ...WIRING DIAGRAM 20558801 Issue 0442 Page 32 of 32 ...