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Bosch 96% AFUE Gas Furnace Installation, Operation & Maintenance
04.2019 | Bosch Thermotechnology Corp.
Data subject to change
9.2 High Altitude Derate
Alternately, in high altitude applications, a standard derate for altitude from
National Fuel Gas Code ANSI Z223.1 of 4% per 1000 feet above sea level may
be taken. Refer to the most recent version of ANSI Z223.1 for correct gas orifi ce
based on your specifi c application. The orifi ces must be selected using the
specifi cations listed in the table below. The furnace derate is 4% for each 1,000
feet above sea level.
Table 14 is based upon a heating value of approximately 1,000 Btu/ft³.
In Canada, the input rating must be derated by 10 percent for altitudes of 2,000 ft.
to 4,500 ft. above sea level by an authorized Gas Supplier or Dealer.
When an appliance is installed at elevations above 4500 ft, the certifi ed high-
altitude input rating shall be reduced at the rate of 4% for each additional 1000 ft.
In some areas the gas supplier may artifi cially derate the gas in an eff ort to
compensate for the eff ects of altitude. If the gas is artifi cially derated, the
appropriate orifi ce size must be determined based upon the BTU/ft³ content of the
derated gas and the altitude. Refer to the latest version of NFPA54/ANSI Z223.1
and information provided by the gas supplier to determine the proper orifi ce size.
US installation
Input Rate
KBTU/H
Number of burners
Elevation (Ft) Elevation (Ft) Elevation (Ft) Elevation (Ft) Elevation (Ft)
0-2000 2000-4000 4000-6000 6000-8000 8000-10000
NG** LP* NG** LP* NG** LP* NG** LP* NG** LP*
60 3 45554756485749585059
80 4 45554756485749585059
100 5 45554756485749585059
120 6 45554756485749585059
Table 14 High Altitude Derate Orifi ce Size Chart (Natural and LP Gas*)
* LP orifi ce based on 10 in. WC manifold pressure
- The input to the furnace must be checked AFTER reorifi cing.
** NG denotes natural gas
Canada installation
Input Rate
KBTU/H
Number of burners
Elevation (Ft) Elevation (Ft) Elevation (Ft) Elevation (Ft) Elevation (Ft)
0-2000 2000-4000 4000-6000 6000-8000 8000-10000
NG** LP* NG** LP* NG** LP* NG** LP* NG** LP*
60 3 45554756485749585059
80 4 45554756485749585059
100 5 45554756485749585059
120 6 45554756485749585059
Table 15 High Altitude Derate Orifi ce Size Chart (Natural and LP Gas*)
* LP orifi ce specifi cation based on 10 in. WC manifold pressure (as specifi ed on the
unit's rating plate for LP).
- The input to the furnace must be checked AFTER reorifi cing.
** NG denotes natural gas
For Canada applications, regulation requires 10% derating
between 2000-4500 ft. When an appliance is installed at elevations
above 4500 ft, the certifi ed high altitude input rating shall be reduced at
the rate of 4% for each additional 1000 ft.
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