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At higher altitudes, the density of the air is reduced.
Therefore, for proper combustion, the quantity of gas
burned in the furnace must also be reduced. This is
called derating. Furnaces must be derated when
installed at altitudes greater than 2,000 feet above sea
level.
At elevations above 2,000 feet the furnace must be
derated. It is the installer's responsibility to see that
the furnace input rate is adjusted properly. Derating
must be achieved by reducing the size of the main
burner orifices. Derating the furnace by adjusting the
manifold pressure lower than the range specified in
the Section 16, "Manifold Pressure Adjustment" is
considered to be an improper procedure.
N
A
T
U
R
A
L
G
A
G
a
s
H
e
t a
*
U
p
o t
V
a
u l
e
2
0
0
0
B
T
U
C /
. u
F
. t
F
e
t e
8
0
0
-
8
4
9
2
7 .
0
8
8
0
-
8
9
9
2
6 .
0
9
0
0
-
9
4
9
2
5 .
0
9
5
0
-
9
9
9
2
4 .
5
1
0
0
0
-
1
0
4
9
2
4 .
0
2
3 .
0
1
0
5
0
1 -
1
0
0
* At standard conditions: 30.00 inches Mercury, 60F, saturated.
NOTE: 2.40
Standard factory installed orifice size. 2.30 is shipped with the unit for field installed optional 10% derate.
All orifice sizes not shown with black background are available in U.S. Natural Gas High Altitude Kit, Bard
Part No. GCK-US-N1. Orifice sizes shown with black background available separately.
N
A
T
U
R
A
L
G
A
S
G
a
s
H
e
t a
*
U
p
o t
V
a
u l
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2
0
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0
B
T
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F
. t
F
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8
0
0
-
8
4
9
2
5 .
0
8
8
0
-
8
9
9
2
5 .
0
2
4 .
0
9
0
0
-
9
4
9
9
5
0
-
9
9
9
2
3 .
5
1
0
0
0
-
1
0
4
9
2
3 .
0
1
0
5
0
1 -
1
0
0
2
2 .
5
* At standard conditions: 30.00 inches Mercury, 60F, saturated.
NOTE: 2.30
Orifices are shipped with the unit for field installed optional 10% input derate.
2.40 is the factory installed orifice size for full rated input.
All other orifice sizes not shown with black background are available in U.S. Natural Gas High Altitude Kit,
Bard Part No. GCK-US-N1. Orifice sizes shown with black background available separately.
Manual 2100-365
Page 22
HIGH ALTITUDE DERATE CHARTS
U.S. INSTALLATIONS ONLY
S
- -
O
R
F I
C I
E
S
Z I
E
C
H
A
R
2
0
0
1
3
0
0
1
4
o t
o t
3
0
0
0
4
0
0
0
5
F
e
t e
F
e
t e
2
6 .
0
2
5 .
0
2
5 .
0
2
4 .
5
2
4 .
0
2
3 .
5
2
3 .
5
2
3 .
0
2
3 .
0
2
3 .
0
2
2 .
5
2
2 .
0
- -
O
R
F I
C I
E
S
Z I
E
C
H
A
R
T
F
2
0
0
1
3
0
0
1
4
o t
o t
3
0
0
0
4
0
0
0
5
F
e
t e
F
e
t e
F
2
4 .
5
2
4 .
5
2
2
4 .
0
2
3 .
5
2
2
3 .
0
2
3 .
0
2
2
2 .
5
2
2 .
5
2
2
2 .
0
2
2 .
0
2
2
1 .
5
2
1 .
5
2
U. S. INSTALLATIONS ONLY
Ratings of gas utilization equipment are based on sea
level operation and need not be changed for operation
at elevations up to 2,000 feet. For operation at
elevations above 2,000 feet, and in the absence of
specific recommendations from the local authority
having jurisdiction, equipment ratings shall be
reduced at the rate of 4 percent per each 1,000 feet
above sea level before selecting appropriately sized
equipment. (Ref. ANSI Z223.1 - latest edition). See
Table 7 for high altitude derate information, and for
fuel gas BTU ratings from 800-1100 BTU per cubic
foot.
TABLE 7
T
F
O
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F
A
C
T
O
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S
T
A
N
D
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0
0
1
5
0
0
1
6
0
0
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0
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6
0
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7
0
0
F
e
t e
F
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t e
F
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t e
2
4 .
2
5 .
0
2
4 .
5
2
4 .
0
2
4 .
0
2
3 .
2
3 .
0
2
3 .
5
2
2 .
2
3 .
0
2
2 .
5
2
2 .
2
2 .
0
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2 .
0
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1 .
2
1 .
2
1 .
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2
1 .
5
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3 .
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3 .
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3 .
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3 .
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2 .
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2 .
5
2
2 .
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2 .
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2 .
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1 .
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1 .
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1 .
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1 .
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1 .
0
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1 .
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5
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2 .
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2
2 .
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2 .
0
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1 .
5
2
1 .
0
2
1 .
5
2
1 .
0
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2
0 .
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2
0 .
0
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0
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9
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0
2
2 .
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1 .
5
2
1 .
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2
0 .
5
2
0 .
0
1
9 .
5
1
9 .
0
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