Bard W24G2DA, Installation Instructions Manual

Page: 30 / 69

Manual 2100-592C
Page
30 of 69
21. STANDARD ORIFICE SIZING AND
HIGH ALTITUDE DERATE
This furnace is shipped with fixed gas orifices for use
with natural gas and sized for 1000 BTU/cubic foot
gas. Make sure actual gas input does not exceed rating
plate input. The orifices may need to changed to get
the correct gas input. Whether they need changed
or not depends on input and the gas heat value at
standard conditions and elevation. Consult the local
gas supplier for gas heat value and any special derating
requirements. See Section 20 for more information.
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. This unit must be derated when
installed at altitudes greater than 6,000' above sea
level. A high altitude pressure switch must also be
installed for operation above 6,000'. High altitude
pressure switch kit number 8620-189 is designed for
this application.
Table 7
Natural Gas Derate Capacities for All Models
W**G Rated
Input
Sea Level 1000 2000 3000 4000 5000 6000 7000 8000 9000 10,000
41,000
45,000
40,500
45,000 39,204
43,560 37,908
42,120 36,612
40,680 35,640
39,600 34,992
38,880 34,182
37,980 33,696
37,440 33,048
36,720 32,643
36,270 32,076
35,640
61,000
68,000
60,750
67,500 58,806
65,340 56,862
63,180 54,918
61,020 53,460
59,400 52,488
58,320 51,273
56,970 50,544
56,160 49,572
55,080 48,965
54,405 48,114
53,460
75,000
81,000
75,000
81,000 72,600
78,408 70,200
75,816 67,800
73,224 66,000
71,280 64,800
69,984 63,300
68,364 62,400
67,392 61,200
66,096 60,450
65,286 59,400
64,152
90,000
100,000
90,000
100,000
87,120
96,800 84,240
93,600 81,360
90,400 79,200
88,000 77,760
86,400 75,960
84,400 74,880
83,200 73,440
81,600 72,540
80,600 71,280
79,200
113,000
125,000
112,500
125,000 108,900
121,000 105,300
117,000 101,700
113,000 99,000
110,000
97,200
108,000
94,950
105,500
93,600
104,000
91,800
102,000
90,675
100,750
89,100
99,000
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 19, “Manifold Pressure Adjustment” is
considered to be an improper procedure.
Above 6,000' elevation orifice changes are required,
and capacity reductions are a function of altitude
impact and orifice change. Pressure switch change is
required above 6,000' elevation. For natural gas see
Altitude Table 7 below and Orifice Tables 8 and 8A on
following pages.