51
ADJUSTING OR CHECKING
FURNACE INPUT
The maximum gas supply pressure to
the furnace should be 7" w.c.
for natural gas. The minimum gas
supply pressure for purposes of input
adjustment to the furnace should be 5"
w.c.
A properly calibrated magnehelic gauge
or manometer is required for accurate
gas pressure readings.
The manifold pressure should be set at
3.5" w.c. for natural gas. Only small
variations in the gas flow should be
made by means of the pressure
regulator adjustment. In no case should
the final manifold pressure vary more
than plus or minus 0.3" w.c. from the
above- specified pressures. To adjust
the pressure regulator, remove the
regulator cap and turn the adjustment
screw clockwise to increase pressure or
counterclockwise to decrease pressure.
Then replace the regulator cap securely.
Any necessary major changes in the
gas flow rate should be made by
changing the size of the burner orifices.
To change orifice spuds, shut off the
manual gas valve and remove the gas
manifold. On LP gas furnaces, the LP
gas supply pressure must be set
between 11" and 14" w.c. by means of
the tank or branch supply regulators.
The furnace manifold pressure should
be set at 10" w.c. at the gas control
valve. For elevations up to 2,000 feet,
rating plate input ratings apply. For high
altitudes (elevations over 2,000 ft.), see
conversion kit index for derating and
orifice spud sizes.
TABLE 9
METER TIME
METER TIME IN MINUTES AND SECONDS FOR NORMAL INPUT RATING OF FURNACES
EQUIPPED FOR NATURAL OR LP GAS
HEATING VALUE OF GAS BTU PER CU. FT.
METER
900
1000
1040
1100
2500 (LP)
INPUT
SIZE
BTU/HR
CU. FT.
MIN.
SEC.
MIN.
SEC.
MIN.
SEC.
MIN.
SEC.
MIN.
SEC.
ONE
1
12
1
20
1
23
1
28
3
20
45,000
TEN
12
0
13
20
13
50
14
40
33
20
ONE
0
54
1
0
1
3
1
6
2
30
60,000
TEN
9
0
10
0
10
24
11
0
25
0
ONE
0
44
0
48
0
50
0
53
2
0
75,000
TEN
7
12
8
0
8
19
8
48
20
0
ONE
0
36
0
40
0
42
0
44
1
40
90,000
TEN
6
0
6
40
7
0
7
20
16
40
ONE
0
31
0
34
0
36
0
38
1
26
105,000
TEN
5
10
5
40
6
0
6
20
14
20
ONE
0
27
0
30
0
31
0
33
1
15
120,000
TEN
4
30
5
0
5
10
5
30
12
30
Heating Value of Gas (BTU/FT
3
) x 3600 x correction factor
Formula: Input BTU/HR =
Time in Seconds (for 1 cu. ft.) of Gas
Checking furnace input is important to
prevent over firing beyond its design-
rated input. NEVER SET INPUT
ABOVE THAT SHOWN ON THE
RATING PLATE. Use the following
table or formula to determine input rate.
Start the furnace and measure the time
required to burn one cubic foot of gas.
Prior to checking the furnace input,
make certain that all other gas
appliances are shut off, with the
exception of pilot burners. Time the
meter with only the furnace in
operation.
SETTING INPUT RATE
The furnace is shipped from the factory
with #50 orifices. They are sized for
natural gas having a heating value of
1075 BTU/cu. ft. and a specific gravity
of .60.
Since heating values vary geo-
graphically, the manifold pressure
and/or gas orifice size may need to be
changed to adjust the furnace to its
nameplate input. Consult the local gas
utility to obtain the yearly average
heating value and orifice size required
to fire each individual burner at 15,000
BTU/HR.
NOTE:
Refer to the National Fuel Gas
Code for high altitude rate adjustment
above 2,000 ft.
