Standard Orifice Sizing And High Altitude Derate; Natural Gas Derate Capacities; Manual 2100-587J - Bard WG Series 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
WG Rated Input
45,000
50,000
67,500
75,000
90,000
100,000
112,500
125,000
Table 7
Natural Gas Derate Capacities for All Models
Sea Level 1000
45,000 43,560 42,120
50,000 48,400 46,800
67,500 65,340 63,180
75,000 72,600 70,200
90,000 87,120 84,240
100,000 96,800 93,600
112,500 108,900 105,300 101,700 99,000
125,000 121,000 117,000 113,000 110,000 108,000 105,500
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.
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.
2000 3000 4000
40,680 39,600
45,200 44,000
61,020 59,400
67,800 66,000
81,360 79,200
90,400 88,000
5000 6000
38,880 37,980
43,200 42,200
58,320 56,970
64,800 63,300
77,760 75,960
86,400 84,400
97,200 94,950
Manual 2100-587J
Page 29 of 59