Manual 2100-365
Page 24
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.
TABLE 7
NATURAL GAS DERATE CAPACITIES
FOR ALL MODELS
detaRGW
tupnI
aeS
leveL000,1000,2000,3000,4000,5000,6000,7000,8000,9000,01
005,04
000,54
005,04
000,54
402,93
065,34
809,73
021,24
216,63
086,04
046,53
006,93
299,43
088,83
281,43
089,73
696,33
044,73
840,33
027,63
346,23
072,63
670,23
046,53
057,06
005,76
057,06
005,76
608,85
043,56
268,65
081,36
819,45
020,16
064,35
004,95
884,25
023,85
372,15
079,65
445,05
061,65
275,94
080,55
569,84
504,45
411,84
064,35
000,57
000,18
000,57
000,18
006,27
804,87
002,07
618,57
008,76
422,37
000,66
082,17
008,46
489,96
003,36
463,86
004,26
293,76
002,16
690,66
054,06
682,56
004,95
251,46
000,09
000,001
000,09
000,001
021,78
008,69
042,48
006,39
063,18
004,09
002,97
000,88
067,77
004,68
069,57
004,48
088,47
002,38
044,37
006,18
045,27
006,08
082,17
002,97
005,211
000,521
005,211
000,521
009,801
000,121
003,501
000,711
007,101
000,311
000,99
000,011
002,79
000,801
059,49
005,501
006,39
000,401
008,19
000,201
576,09
057,001
001,98
000,99
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.
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.
To Next Page
