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MANIFOLD PRESSURE AND ORIFICE SIZE FOR HIGH ALTITUDE APPLICATIONS
Table 7
Heat Value
Heat Value
0- -1999
Btu/Cu.Ft.
(² ² ² ² × × × × wc)
800
3.5
850
3.5
900
3.5
950
3.5
1000
3.5
1050
3.2
1100
2.9
*
* #42 Orifice size for 50,000, 75,000 100,00 & 125,000 BTUH models
Orifice Size
* #44 Orifice size for 80,000 BTUH model
General Derating Rules
1.
For operation with natural gas at altitudes above 2,000¢, ori-
fice change and/or manifold pressure adjustment may be re-
quired to suit gas supplied. Check with gas supplier. If orifice
sizing is needed, it should be based on reducing the input
rating by 2% (natural) or 4% (LP) for each 1,000¢ above sea
level. See Table 7 and Figure 28 for required pressure
change and/or orifice change for high altitudes.
2.
For operation with LP gas at altitudes above 2,000¢, gas ori-
fices MUST be changed and manifold pressure MUST be
maintained as per Table 6. For the 50,000, 75,000, 100,000
and 125,000 BTUH Orifice sizes are #54 for 0--2000¢ above
sea level, #55 for 2000¢ --7000¢ above sea level, and #56 for
7000¢ --8000¢ above sea level. For the 80,000 BTUH models,
the orifice size is #55 for 0 -- 4,000¢ above seal level and #56
for 5,000¢ -- 8,000¢ above sea level. Orifices can be ordered
through your distributor. (See Figure 28)
Changing Orifices
Figure 28
1
3
1
/
² to 1
/
²
8
16
NOTE: The derating of these units at 2% (Natural) and 4% (L.P.)
has been tested and approved by AGA.
High Altitude Air Pressure Switch
Altitudes over 4,000¢ require a different air pressure switch than the
one installed at the factory. Check parts list for pressure switch and
consult your distributor for part number and availability. In Canada,
provincial codes may govern installation of switch. Check with gov-
erning authorities.
440 01 1020 02
2000- -2999
3000- -3999
(² ² ² ² × × × × wc)
(² ² ² ² × × × × wc)
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.4
3.3
3.1
3.0
2.8
2.7
*
*
Measure from face of orifice
to the back side of the
manifold.
NATURAL GAS
Elevation Above Sea Level
4000- -4999
5000- -5999
(² ² ² ² × × × × wc)
(² ² ² ² × × × × wc)
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.3
3.2
3.0
2.9
2.7
2.6
2.5
*
*
Changing Orifices for High Altitude
!
ELECTRICAL SHOCK, FIRE OR EXPLOSION HAZARD
Failure to properly install orifices can result in death,
personal injury and/or property damage.
Turn OFF electric power (at disconnect) and gas supply
(at manual valve in gas line) when installing orifices.
Installation of orifices requires a qualified service
technician.
NOTE: Main burner orifices can be changed for high altitudes.
1.
Disconnect gas line from gas valve.
2.
Remove manifold from furnace.
3.
Remove the orifices from the manifold and replace them
with properly sized orifices.
4.
Tighten orifices so it is seated and gas tight. (See Figure 28)
5.
Reinstall manifold. Ensure burners do NOT bind on new ori-
fices.
Natural Gas Input Rating Check
NOTE: The gas meter can be used to measure input to furnace.
Rating is based on a natural gas BTU content of 1,000 BTU's per
cubic meter. Check with gas supplier for actual BTU content.
1.
Make sure burner compartment door is in place before per-
forming the following steps.
2.
Turn OFF gas supply to all appliances and start furnace.
Natural Gas
No. of Seconds
BTU Content
1,000
1,000 x 3,600 ¸ 48 = 75,000 BTUH
3.
Time how many seconds it takes the smallest dial on the gas
meter to make one complete revolution. Refer to Example.
4.
Relight all appliances and ensure all pilots are operating.
NOTE: If meter uses a 2 cubic foot dial, divide results (seconds) by
two.
6000- -6999
(² ² ² ² × × × × wc)
3.5
3.5
3.5
3.2
2.9
2.6
2.4
*
Example
Time Per Cubic
Per Hour
Foot in Seconds
3,600
48
7000- -7999
(² ² ² ² × × × × wc)
3.5
3.5
3.4
3.1
2.8
2.5
2.3
*
BTU Per
Hour
75,000
27
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