Example
Natural Gas No. of Seconds Time Per Cubic BTU Per
BTU Content Per Hour Foot in Seconds Hour
1,000 3,600 48 75,000
1,000 x 3,600 ÷ 48 = 75,000 BTUH
3. Relight all appliances and ensure all pilots are operating.
Orifice Sizing
NOTE: Factory sized orifices for natural and LP gas are listed in
the furnace Technical Support manual.
Ensure furnace is equipped with the correct main burner orifices.
Refer to Table 3 for correct orifice size for a given heating value
and specific gravity for natural and propane gas. Note that this
chart is ONLY for installations below 2000' in altitude.
Table 3
Orifice Sizes (below 2000')
Heating Value
Gas Manifold Specific (BTU per
Type Pressure Gravity Cubic Ft.)
Natural 800
3.5" w.c. 900
0.9kPa 0.6
1000
W.C.
1100
10" W.C.
Propane 2.5kPa 1.53 2500
w.c.
Orifice Size
(Drill #)
40
41
42
43
54
OperationAbove 2000' Altitude
Fire, Explosion, Poison carbon monoxide gas haz-
ard.
These units may be used at full input rating when installed at alti-
tudes up to 2000'. When installed above 2000', the input must be
decreased 4% for each 1000' above sea level. This may be ac-
complished by a simple adjustment of manifold pressure or an ori-
fice change, or a combination of a pressure adjustment and an
orifice change. The changes required depend on the installation
altitude and the heating value of the fuel. TABLES 4 & 5 show the
proper furnace manifold pressure and gas orifice size to achieve
proper performance based on elevation above sea level for both
natural gas and propane.
To use the natural gas table, first consult your local gas utility for
the heating value of the gas supply. Select the heating value on
the vertical border and follow across the table until the appropriate
elevation for the installation is reached. The first value in the box
at the intersection of the heating value and elevation will be the
manifold pressure required. If a gas orifice change is also re-
quired, the box is shaded. The required orifice size is shown at the
bottom of the table.
Sea Level
High Altitude Input Rate = Nameplate x (Multiplier)
Input Rate
Elevation High Altitude
Multiplier
2000'-2999' 0.92
3000'-3999' 0.88
4000'-4999' 0.84
5000'-5999' 0.80
6000'-6999' .0.76
7000'-8000' .0.72
This conversion shall be done by a qualified service
agency in accordance with the Manufacturer's
instructions and all applicable codes and require-
ments, or in the absence of local codes, the applica-
ble national codes.
Failure to follow these instructions exactly can re-
sult in property damage, personal injury and/or
death.
For installations above 4000', the inlet air restrictor of the com-
bustion air blower MUST be changed, whether gas has been der-
ated by the utility or orifices have been changed.
A High Altitude Kit is available which includes restrictors, orifices
and installation instructions.
Natural Gas - 1160993
LP Gas - 1160992
MANIFOLD PRESSURE AND ORIFICE SIZE FOR HIGH ALTITUDE APPLICATIONS
Table 4
HEATING VALUE
BTU/CU. FT.
800
850
900
950
t 000
t 050
1100
Orifice Size
NATURAL GAS
2000 to
2999
3.5" wc
3.5" wc
3.5" wc
3.3" wc
3.0" wc
2.7" wc
2.5" wc
#42
MEAN ELEVATION FEET ABOVE SEA LEVEL
3000 to 4000 to
3999 4999
3.5" wc 3.5" wc
3.5" wc 3.5" wc
3.4" wc 3.1" wc
3.1" wc 2.8" wc
2.8" wc 2.5" wc
2.5" wc 2.3" wc
2.3" wc
#42
;HADED AREA REQUIRES ORIFICE CHANGE.
5000 to
5999
3.5" wc
3.2" wc
2.8" wc
2.5" wc
2.3" wc
3.5" wc
6000 to
6999
3.2" wc
2.9" wc
2.5" wc
2.3" wc
7000 to
8000
2.9" wc
2.6" wc
2.3" wc
3.5" wc
3.5" wc ' 3.1" wc
3.2" wc • 2.8" wc
3.5" wc " 3.2" wc 2.9" wc • 2.6" wc
#45 " #45 " #45 " #45
NO SHADING INDICATES MANIFOLD PRESSURE CHANGE ONLY.
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