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 & Table 4 for correct orifice size for a given heat-
ing value and specific gravity for natural and propane gas.
OperationAbove 2000' Altitude
Fire, Explosion, Poison carbon monoxide gas
hazard.
This conversion shall be done by a qualified
service agency in accordance with the
Manufacturer's instructions and all applicable
codes and requirements, or in the absence of
local codes, the applicable national codes.
Failure to follow these instructions exactly can
result in property damage, personal injury and/
or death.
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. TABLE 3 & TABLE 4
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
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.
MANIFOLD PRESSURE AND ORIFICE SIZE FOR HIGH ALTITUDE APPLICATIONS
NATURAL GAS
MEAN ELEVATION FEET ABOVE SEA LEVEL
HEATINGVALUE 0 to 2000 to 3000 to 4000 to 5000 to 6000 to 7000 to
BTU/CU. FT. 1999 2999 3999 4999 5999 6999 8000
800 3.5" wc 3.5" wc 3.5" wc 3.5" wc 3.5" wc 3.2" wc 2.9" wc
850 3.5" wc 3.5" wc 3.5" wc 3.5" wc 3.2" wc 2.9" wc 2.6" wc
900 3.5" wc 3.5" wc 3.4" wc 3.1" wc 2.8" wc 2.5" wc 2.3" wc
950 3.5" wc 3.3" wc 3.1" wc 2.8" wc 2.5" wc 2.3" wc
t000 3.5" wc 3.0" wc 2.8" wc 2.5" wc 2.3" wc
t050 3.2" wc 2.7" wc 2.5" wc 2.3" wc
1100 2.9" wc 2.5" wc 2.3" wc
Orifice Size #42 #42 #42
SHADED AREA REQUIRES ORIFICE CHANGE. NO SHADING INDICATES MANIFOLD PRESSURE CHANGE ONLY.
PROPANE
MEAN ELEVATION FEET ABOVE SEA LEVEL
0 to 2000 to 3000 to 4000 to 5000 to 6000 to 7000 to
1999 2999 3999 4999 5999 6999 8000
10.0" wc 10.0" wc 9.4" wc 10.0" wc 9.8" wc 8.8" wc 7.9" wc
#54 #54 #54 #55 #55 #55 #55
HEATING VALUE
BTU/CU. FT.
2500
Orifice Size
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