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No high altitude pressure switch changes are required;
only a 4% derate at altitudes above 2,000 feet.
NATURAL GAS
NSTALLATION OF THIS FURNACE AT ALTITUDES
I
ABOVE 2000 FT. (610 M) SHALL BE IN ACCOR-
DANCE WITH LOCAL CODES, OR IN THE ABSENCE
OF LOCAL CODES, THE NATIONAL FUEL GAS
CODE, ANSI Z223.1/NFPA 54 OR IN CANADA, NATU-
RAL GAS AND PROPANE INSTALLATION CODE,
CSA B149.1.
ELEVATIONS ABOVE 2000 FT. REQUIRE THAT THE
FURNACE INPUT RATING BE ADJUSTED AND THAT
THE SIZE OF THE BURNER ORIFICES BE RE-CAL-
CULATED BASED ON ELEVATION AND GAS HEAT-
ING VALUE. THE BURNER ORIFICES MAY (OR MAY
NOT) NEED TO BE CHANGED. THE FOLLOWING EX-
AMPLES SHOW HOW TO DETERMINE IF AN ORI-
FICE CHANGE WILL BE NECESSARY AND HOW TO
DETERMINE THE NEW ORIFICE SIZE.
IN CANADA, AS AN ALTERNATE TO ADJUSTING
THE BURNER ORIFICE SIZE, THE MANIFOLD GAS
PRESSURE MAY BE ADJUSTED. THIS METHOD IS
COVERED LATER IN THIS SECTION. THIS METHOD
OF ADJUSTING MANIFOLD PRESSURE MAY ONLY
BE USED IN CANADIAN INSTALLATIONS.
NOTE: Factory installed orifices are calculated and sized
based on a sea level Natural Gas heating value of 1100
BTU per cubic ft. Regional reduced heating values may
nullify the need to change orifices except at extreme alti-
tudes.
The following are examples of orifice sizing using the Na-
tional Fuel Gas Code Annex F, Tables F.1 and F.4. For a
simplified estimation of orifice size based on heating value
and elevation, use Table 14. However, calculations are the
best method.
Example: 900 BTU/ft
3
Regional Natural Gas Heating
Value
I / H = Q
14000 / 900 = 15.56 ft.
3
I = Sea Level input (per burner): 14000
H = Sea Level Heating Value: 900
Q = 15.56 ft
3
Natural Gas per hour.
From Table E.1.1(a) of National Fuel Gas Code Handbook,
current edition (3.5 w.c. column)
Orifice required at Sea Level: #49
From Table E.1.1(d) of National Fuel Gas Code Handbook,
current edition
Orifice required at 5000 ft. elevation (4% de-rate per thou-
sand ft.): #51
Orifice required at 8000 ft. elevation (4% de-rate per thou-
sand ft.): #52
83
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