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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. You may need to change orifices to get
correct gas input. Whether you do or not depends on
input, and your gas heat value at standard conditions
and elevation. Consult your 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 feet above sea
level. A high altitude pressure switch must also be
installed for operation above 6,000 feet. High altitude
pressure switch kit number 8620-189 is designed for
this application.
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I
Manual 2100-376
Page
28 of 80
NATURAL GAS DERATE CAPACITIES
FOR ALL MODELS
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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 feet 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 feet elevation. For Natural Gas
see the Altitude Table 7 below and the Orifice Tables 8
and 8A on following pages.
TABLE 7
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