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SERVICE AND TECHNICAL SUPPORT MANUAL
in Table 3 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. (610 M) to 4500 ft. (1372 M) above sea
level. The natural gas manifold pressures in Table 3 adjust for
BOTH altitude and natural gas heating value.
Table 1
Altitude Derate Multiplier for U.S.A.
ALTITUDE
PERCENT
FT. (M)
OF DERATE
0–2000
0
(0−610)
2001–3000
4−6
(610−914)
3001–4000
6−8
(914−1219)
4001–5000
8−10
(1219−1524)
5001–6000
10−12
(1524−1829)
6001–7000
12−14
(1829−2134)
7001–8000
14−16
(2134−2438)
8001–9000
16−18
(2438−2743)
9001–10,000
18−20
(2743−3048)
* Derate multiplier factors are based on midpoint altitude for altitude range.
NOTE: For Canadian altitudes of 2000 to 4500 ft. (610 to
1372 M), use USA altitudes of 2001 to 3000 ft. (610 to 914
M) in Table 3.
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 Service and
Technical Manual 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 in Table 3.
4. Find closest natural gas heat value and specific gravity in
Table 3. Follow heat value and specific gravity lines to
point of intersection to find orifice size and low−and
high−heat
manifold
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 3.
Use only factory−supplied orifices. See EXAMPLE.
EXAMPLE 1:
0 - 2000 ft. (0 - 609.6M) altitude
Heating value = 1050 Btu/cu ft.
Specific gravity = 0.62
6
DERATE MULTIPLIER
FACTOR*
1.00
0.95
0.93
0.91
0.89
0.87
0.85
0.83
0.81
pressure
settings
for
proper
Specifications subject to change without notice.
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) for high heat, 1.4-in.
w.c. (349 Pa) for low 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. wc. = 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. (1121 Pa).
1. Make sure the gas supply is turned off to the furnace and
at the electric switch on the gas valve.
2. Loosen set screw on inlet tower pressure tap no more
than one full turn with a 3/32−in. hex wrench or 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. (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.
11. Turn gas supply manual shutoff valve to OFF position.
12. Turn off furnace power supply.
13. Remove water column manometer or similar device from
manifold pressure tap.
WARNING
!
FIRE HAZARD
Failure to follow this warning could result in personal injury,
death, and/or property damage.
Inlet pressure tap set screw must be tightened and 1/8−in.
NPT pipe plug must be installed to prevent gas leaks.
14. Tighten set screw on inlet tower pressure tap with
3/32−in. hex wrench, or if 1/8−in. NPT plug was
removed, apply pipe dope sparingly to end of plug and
re−install in the gas valve.
15. Re-install cap over adjustment screw on the top of the
gas valve.
Adjust Manifold Pressure
1. Adjust manifold pressure to obtain low fire input rate.
(See Figure 3)
a. Turn gas valve ON/OFF switch to OFF.
b. Loosen set screw on manifold tower pressure tap no
more than one full turn with a 3/32−in. hex wrench, or
remove the 1/8 inch NPT plug from the manifold
pressure tap on the gas valve. See Figure 3.
c. Connect a water column manometer or similar device
to manifold pressure tap.
d. Turn gas valve ON/OFF switch to ON.
Gas Furnace: (F/G)9MVE
440 04 4801 04
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