Standard Orifice Sizing And High Altitude Derate - Bard W24G4 Installation Instructions Manual

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.
W**G Rated
Sea Level
Input
41,000 40,500 39,204
45,000 45,000 43,560
61,000 60,750 58,806
68,000 67,500 65,340
75,000 75,000 72,600
81,000 81,000 78,408
90,000 90,000 87,120
100,000 100,000 96,800
113,000 112,500 108,900
125,000 125,000 121,000
Natural Gas Derate Capacities
for All Models
1000 2000 3000
37,908 36,612
42,120 40,680
56,862 54,918
63,180 61,020
70,200 67,800
75,816 73,224
84,240 81,360
93,600 90,400
105,300 101,700
117,000 113,000
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 10 below and Orifice Tables 11A and
11B on following pages.
TABLE 10
4000 5000 6000
35,640 34,992 34,182
39,600 38,880 37,980
53,460 52,488 51,273
59,400 58,320 56,970
66,000 64,800 63,300
71,280 69,984 68,364
79,200 77,760 75,960
88,000 86,400 84,400
99,000 97,200 94,950
110,000 108,000 105,500
7000 8000 9000
33,696 33,048 32,643
37,440 36,720 36,270
50,544 49,572 48,965
56,160 55,080 54,405
62,400 61,200 60,450
67,392 66,096 65,286
74,880 73,440 72,540
83,200 81,600 80,600
93,600 91,800 90,675
104,000 102,000 100,750
Manual 2100-721B
Page
10,000
32,076
35,640
48,114
53,460
59,400
64,152
71,280
79,200
89,100
99,000
31 of 54