41
TABLE 11
Natural Gas Orifice Drill Size (4% per 1000 ft. De-Rate)
IMPORTANT: For 90+ Furnaces only. Do not use this chart for any 80+ Furnace.
Burner Input (per burner) 15,000 BTU @ Sea Level
Sea level
2000 to
3000 to
4000 to
5000 to
6000 to
7000 to
8000 to
to 1999 ft 2999 ft
3999 ft
4999 ft
5999 ft
6999 ft
7999 ft
8999 ft
47
48
48
49
49
49
50
50
48
49
49
49
50
50
50
51
49
50
50
50
51
51
51
52
50
51
51
51
51
52
52
52
51
51
52
52
52
53
53
53
RGFG- HIGH ALTITUDE
CONVERSION
FOR ALL MODELS, A PRESSURE
SWITCH CHANGE IS NOT NECES-
SARY AT ANY ELEVATION UP TO
8,000 FT. HOWEVER, AT ELEVATIONS
ABOVE 5,000 FT., AN ORIFICE
CHANGE MAY BE REQUIRED. FUR-
NACES ARE NOT TESTED ABOVE
8,000 FT. AND CANNOT BE
INSTALLED ABOVE 8,000 FT.
Orifice Selection for High Altitude
Applications For U.S. Installations
Natural Gas
INSTALLATION OF THIS FURNACE AT
ALTITUDES ABOVE 2000 FT (610 m)
SHALL BE IN ACCORDANCE WITH
LOCAL CODES OR, IN THE ABSENCE
OF LOCAL CODES, THE NATIONAL
FUEL GAS CODE, ANSI Z223.1/NFPA
54 OR NATIONAL STANDARD OF
CANADA, NATURAL GAS AND
PROPANE INSTALLATION CODE, CAN
B149.1.
Elevations above 2000 ft. require the
furnace to be de-rated 4% per thousand
feet. NOTE: Factory installed orifices are
calculated and sized based on a sea
level Natural Gas heating value of 1075
BTU per cubic ft. Regional reduced
heating values may nullify the need to
change orifices except at extreme alti-
tudes. Table 11 shows some quick con-
versions based on elevation and gas
heating value. This table is combined
and simplified from Tables F1 and F4 of
the National Fuel Gas Code.
EXAMPLES
The following are examples of orifice
sizing using the National Fuel Gas
Code Appendix F. For a simplified esti-
mation of orifice size based on heating
value and elevation use Tables 11 and
12. However, calculations are the best
method.
Example: 900 BTU/ft
3
Regional
Natural Gas Heating Value
I / H = Q
15000 / 900 = 16.68 ft
3
/hr
I = Sea Level input (per burner): 15000
H = Sea Level Heating Value: 900
Q = 16.68 ft
3
Natural Gas per hour.
From Table F.1 of National Fuel Gas
Code Handbook, 2002 (3.5
ⴖ
w.c. col-
umn)
Orifice required at Sea Level: # 48
From Table F.4 of National fuel Gas
Code Handbook, 2002
Orifice required at 5000 ft. elevation
(4% de-rate per thousand ft): # 50
Orifice required at 8000 ft. elevation
(4% de-rate per thousand ft): # 51
Annual Avg. Heat
Value (btu per ft
3
)
850
900
1000
1075
1170
Example: 1050 BTU/ft
3
Regional
Natural Gas Heating Value
I / H = Q
15000 / 1050 = 14.63 ft
3
/hr
I = Sea Level input (per burner):
15000
H = Sea Level Heating Value: 1050
Q= 14.28 ft
3
Natural Gas.
From Table F.1 of National Fuel Gas
code Handbook, 2002 (3.5
ⴖ
w.c. col-
umn)
Orifice required at Sea Level: # 50
From Table F.4 of National Fuel Gas
code Handbook, 2002
Orifice required at 5000 ft. elevation
(4% de-rate per thousand ft): # 51
Orifice required at 8000 ft. elevation
(4% de-rate per thousand ft): # 52
CAUTION
!
!
WARNING
HIGH ALTITUDE INSTALLATIONS IN THE U.S.
