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The furnaces are shipped from the factory with orifices
and gas regulator settings for natural gas operation at sea
level altitudes. At 2,000 feet, the NFGC requires that this
appliance be derated 4% for each 1,000 feet of altitude.
For example, the input needs to be reduced 8% at 2,000
feet, 12% at 3,000 feet, etc. This deration is in reference
to the input rate and gas heating value at sea level.
To derate the furnace requires knowing the heating value of
the gas at the installation site. Heating values at particular
job sites vary for two reasons:
1. The chemical mixture of the gas varies across regions
and is expressed as the “sea level heating value”.
2. The heating value varies by altitude. For this reason,
especially in high altitude areas, the local gas utility
specifies the heating value at the residence’s gas meter
as the “local value”.
For added flexibility, two tables have been provided for
natural gas installations with high or low heating values
at sea level. Table 15 & Table 16 (page 44) contain the
manifold pressure and orifice sizes to use at various
altitudes. Table 15 (HIGH) is for natural gas installations
with a heating value of more than 1,000 Btu per cubic foot
and Table 16 (LOW) is for less than 1,000 Btu per cubic
foot. To determine which table to use:
1. Consult the local utility for the local heating value at
your installation site.
2. From Table 14 (page 43), find your local heating value
as supplied by the utility company. Follow down the
column and stop at your altitude level.
3. If your sea level heating value is HIGH, use
Table 15 or
if it’s LOW, use Table 16. See Example below.
EXAMPLE
.................................................. 5,000 feet
...........................................Natural Gas
.............................. 750
From
Table 14, find 750 and follow down the column,
stop at the 5,000 feet row. The heating value listed is
LOW. Table 16 will be used to determine orifice size
and manifold pressure.
After changing the regulator pressure or the orifices, it is
required that you measure the gas input rate. This may be
accomplished in the usual way, by clocking the gas meter
and using the local gas heating value. See Verifying and
Adjusting the Input Rate section on
page 28.
IMPORTANT NOTE:
WARNING:
Conversion to LP / Propane is detailed in the installation
instructions provided with the conversion kit. Generally,
this will require the replacement of the burner orifices and
the spring found under the cap screw on the pressure
regulator. Approved conversion kits are listed below:
• Installations in the United States: For Propane (LP)
conversion or for LP gas installations in altitudes
between 2,000 ft. and 10,000 ft. above sea level, use
the LP & High Altitude LP Gas Conversion Kit (P/N
904914) for Installations in the United States. Please
.
• Installations in Canada: For Propane (LP) conversion
or for LP Gas installations in altitudes between zero and
4,500 ft. above sea level, use the LP & High Altitude
LP Gas Conversion Kit (P/N 904915) for Installations
in Canada.
If the installation is in the U.S. and above 2,000 ft., refer
to Table 13 (page 43) to determine the correct orifice size
and regulator pressure. See Example below.
EXAMPLE
.................................................. 5,000 feet
.........................................Propane Gas
................................72,000
From
Table 13, find 5,000 and follow across the row,
stop at the 72,000 btu column. The manifold pressure
listed is 10.0 and the orifice size is 57.
When conversion is complete, verify the manifold pressure
and input rate are correct as listed in the table.
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