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In Canada, the input rating must be reduced by 5 percent for
altitudes of 2000 ft. (609.6M) to 4500 ft. (1371.6M) above sea
level. The natural gas manifold pressures in Tables 21 and 22
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 2000 ft. (611 to 914 M) in Table
21.
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.
THERE ARE TWO MANIFOLD PRESSURE TABLES
Use Table 21 for all models
EXCEPT
987MA60060V21.
Use Table 22 for Model 987MA60060V21
ONLY.
NOTICE
To do this:
1. Obtain average yearly gas heat value (at installed altitude)
from local gas supplier.
2. Obtain average yearly specific gravity from local gas
supplier.
3. Find installation altitude range for your installation in the
manifold pressure tables. See Table 21 for 20,000 Btuh
Max--Heat/8,000 Btuh min--heat per burner or Table 22 for
20,200 Btuh Max--Heat/8,000 Btuh Min--Heat per burner.
4. Find closest natural gas heat value and specific gravity in
Table 21 or Table 22 depending on furnace gas input rate.
5. Follow heat value and specific gravity lines to point of
intersection to find orifice size and maximum and minimum
manifold pressure settings for proper operation.
6. Check and verify burner orifice size in furnace. NEVER
ASSUME ORIFICE SIZE.
ALWAYS CHECK AND
VERIFY.
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.
NOTICE
7. Replace original orifice with correct size, if required by
Table 21 or Table 22 depending on furnace gas input rate.
Use only factory--supplied orifices. See EXAMPLE 1.
EXAMPLE 1 -- 80,000 BTUH INPUT:
EXAMPLE: 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. for maximum heat, 0.55--in. w.c.
for minimum heat.
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.
for natural gas. The maximum inlet gas pressure is 13.6 in. of water
column. If the inlet pressure is too low, you will not be able to
adjust the manifold pressure to obtain the proper input rate. To
check the inlet gas pressure:
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 the R to W/W1 and W2 thermostat connections at
the furnace control board.
8. When main burners ignite, confirm inlet gas pressure is
Between 4.5 in. w.c. and 13.6 in. w.c.
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.
11. Turn gas supply manual shutoff valve to OFF position.
12. Turn off furnace power supply.
13. Remove manometer from the inlet pressure tap of the gas
valve.
FIRE HAZARD
Failure to follow this warning could result in personal
injury, death, and/or property damage.
Reinstall manifold pressure tap plug in gas valve to prevent
gas leak.
!
WARNING
14. Apply pipe dope sparingly to end of inlet gas pipe plug and
re--install in the gas valve.
Adjust Manifold Pressure -- Maximum Heat
For proper operation and long term reliability, the manifold
pressure must be adjusted /--2 percent of input rate on
furnace rating plate.
The modulating furnace manifold pressure is set at two points. The
first point is Maximum Heat. The second point is Minimum Heat.
Do not adjust Intermediate Heat manifold pressure. Intermediate
Heat manifold pressure is checked as part of the temperature rise,
but is not adjustable. Always adjust Maximum Heat first, then
Minimum Heat.
DO NOT set Maximum Heat manifold pressure less than
3.2--in. w.c. or more than 3.8 in. w.c. for natural gas. If
required manifold pressure is outside this range, change main
burner orifices to obtain manifold pressure in this range.
NOTICE
To adjust manifold pressure to obtain input rate for Maximum
Heat:
987MA
