4
440 04 6001 00
Specifications subject to change without notice.
FURNACE DAMAGE HAZARD
Failure to follow this caution may result in reduced
furnace life.
DO NOT redrill orifices. Improper drilling (burrs,
out
−
of
−
round holes, etc.) can cause excessive burner
noise and misdirection of burner flames. This can
result in flame impingement of heat exchangers,
causing failures. (See Figure 1.)
CAUTION
!
BURNER
ORIFICE
A93059
Figure 1
−
Orifice Hole
For proper operation and long term reliability the furnace
input rate must be /
−
2 percent of input rate on
furnace rating plate, or as adjusted for altitude.
The gas input rate on rating plate is for installations at
altitudes up to 2000 ft. (609.6M).
NOTICE
The NATURAL GAS manifold pressure adjustments in Table
4 compensate for BOTH altitude AND gas heating value. DO
NOT apply an additional de
−
rate factor to the pressures
shown in Table 4. The values in this Table and NOT
referenced to sea level; they are AS
−
MEASURED AT
ALTITUDE.
The heating content of natural gas at altitude may already
provide for a reduction in capacity or altitude. Refer to Table
4. No adjustments to the furnace may be necessary at
altitude for certain gas heating values.
Refer to the instructions provided in the factory-specified
Propane conversion kit for instructions for setting gas
manifold pressures for Propane applications.
In the USA, the input rating for altitudes above 2000 ft. (610
M) must be reduced by 2 percent for each 1000 ft. (305 M)
above sea level refer to Table 2. The natural gas manifold
pressures in Table 4 adjust for BOTH altitude and natural
gas heating value.
In Canada, the input rating must be reduced by 5 percent for
altitudes of 2000 ft. to 4500 ft. (610 to 1372 M) above sea
level. The natural gas manifold pressures in Table 4 adjust
for BOTH altitude and natural gas heating value.
NOTE
: For Canadian altitudes of 2000 to 4500 ft. (610 to
1372 M), use USA altitudes of 2001 to 3000 ft. (611 to 914
M) in Table 4
To adjust manifold pressure to obtain the proper input rate,
first, determine if the furnace has the correct orifice installed.
At higher altitudes or different gas heat contents, it may be
necessary to change the factory orifice to a different orifice.
Tables have been provided in the furnace installation
instructions to match the required orifice to the manifold
pressure to the heat content and specific gravity of the gas.
To do this:
1. Obtain average yearly gas heat value (at installed
altitude) from local gas supplier.
2. Obtain average yearly gas specific gravity from local
gas supplier.
3. Find installation altitude range for your installation in
the manifold pressure tables in Table 4.
4. Find closest natural gas heat value and specific grav-
ity in Table 4. Follow heat value and specific gravity
lines to point of intersection to find orifice size and
low-and high-heat manifold pressure settings for
proper operation.
5. Check and verify burner orifice size in furnace.
NEVER ASSUME ORIFICE SIZE. ALWAYS CHECK
AND VERIFY.
NOTICE
If orifice hole appears damaged or it is suspected to have
been redrilled, check orifice hole with a numbered drill bit of
correct size. Never redrill an orifice. A burr
−
free and squarely
aligned orifice hole is essential for proper flame
characteristics.
6. Replace orifice with correct size, if required by Table
4. Use only factory
−
supplied orifices. See EXAMPLE
1.
EXAMPLE 1:
0 - 2000 ft. (0 - 609.6M) altitude
Heating value = 1050 Btu/cu ft.
Specific gravity = 0.62
Therefore: Orifice No. 44
(Furnace is shipped with No. 44 orifices. In this example, all
main burner orifices are the correct size and do not need to
be changed to obtain proper input rate.)
Manifold pressure: 3.4-in. w.c. (847 Pa).
NOTE
: To convert gas manifold Table pressures to Pascals,
multiply the in.w.c. value by 249.1 Pa/in. w.c. (1 in. w.c. =
249.1 Pa).
Check Inlet Gas Pressure
The inlet gas pressure must be checked with the furnace
operating in maximum heat. This is necessary to make sure
the inlet gas pressure does not fall below the minimum
pressure of 4.5 in. w.c.
1. Make sure the gas supply is turned off to the furnace
and at the electric switch on the gas valve.
2. Remove the 1/8 in. NPT plug from the inlet pressure
tap on the gas valve.
3. Connect a manometer to the inlet pressure tap on gas
valve.
4. Turn on furnace power supply.
5. Turn gas supply manual shutoff valve to ON position.
6. Turn furnace gas valve switch to ON position.
7. Jumper R and W thermostat connections at the
furnace control board.
8. When main burners ignite, confirm inlet gas pressure
is between 4.5 in. w.c. (1125 Pa) and 13.6 in. w.c.
(3388 Pa).
9. Remove jumper across thermostat connections to
terminate call for heat. Wait until the blower off delay
is completed.
10. Turn furnace gas valve electric switch to OFF
position.
