5750149-UIM-D-1219
10 Johnson Controls Ducted Systems
HIGH ALTITUDE GAS ORIFICE CONVERSION
This furnace is constructed at the factory for natural gas-fired operation
at 0 – 7,999 feet (0 – 2,438 m) above sea level.
The manifold pressure must be changed in order to maintain proper
and safe operation when the furnace is installed in a location where the
altitude is greater than 7,999 feet (2,438 m) above sea level. Refer to
Table 4 for proper manifold pressure settings.
HIGH ALTITUDE PRESSURE SWITCH CONVERSION
For installation where the altitude is less than 5,000 feet (1,524m), it is
not required that the pressure switch be changed unless you are in an
area subject to low pressure inversions.
FIGURE 12: Gas Piping
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Table 4: Nominal Manifold Pressure - High Fire
Manifold Pressures Manifold Pressures
Gas Heating Value
(BTU/cu ft.)
Altitude (feet)
Gas Heating Value
(MJ/cu m)
Altitude (m)
0-7999 8000-8999 9000-9999 0-2437 2438-2742 2743-3048
in. w.c. in. w.c. in. w.c. kPa kPa kPa
800 3.5 3.5 3.5 29.8 0.87 0.87 0.87
850 3.5 3.5 3.5 31.7 0.87 0.87 0.87
900 3.5 3.5 3.5 33.5 0.87 0.87 0.87
950 3.5 3.5 3.3 35.4 0.87 0.87 0.81
1000 3.5 3.2 2.9 37.3 0.87 0.80 0.73
1050 3.5 2.9 2.7 39.1 0.87 0.73 0.67
1100 3.2 2.7 2.4 41.0 0.80 0.66 0.61
2500 (LP) 9.8 8.2 7.5 93.2 (LP) 2.44 2.03 1.86
Table 5:
Nominal Manifold Pressure - Low Fire
Manifold Pressures Manifold Pressures
Gas Heating Value
(BTU/cu ft.)
Altitude (feet)
Gas Heating Value
(MJ/cu m)
Altitude (m)
0-7999 8000-8999 9000-9999 0-2437 2438-2742 2743-3048
in. w.c. in. w.c. in. w.c. kPa kPa kPa
800 1.7 1.7 1.7 29.8 0.42 0.42 0.42
850 1.7 1.7 1.7 31.7 0.42 0.42 0.42
900 1.7 1.7 1.7 33.5 0.42 0.42 0.42
950 1.7 1.7 1.5 35.4 0.42 0.42 0.38
1000 1.6 1.5 1.4 37.3 0.41 0.37 0.34
1050 1.5 1.4 1.3 39.1 0.37 0.34 0.31
1100 1.3 1.2 1.1 41.0 0.34 0.31 0.28
2500 (LP) 4.1 3.8 3.5 93.2 (LP) 1.03 0.95 0.87
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