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Table 16 – Altitude Derate Multiplier for U.S.A.*
ALTITUDE
PERCENT
OF
DERATE
DERATE
MULTIPLIER
FACTOR*
FT.
M
0–2000
0---610
0
1.00
2001–3000
610---914
4---6
0.95
3001–4000
914---1219
6---8
0.93
4001–5000
1219---1524
8---10
0.91
5001–6000
1524---1829
10---12
0.89
6001–7000
1829---2134
12---14
0.87
7001–8000
2134---2438
14---16
0.85
8001–9000
2438---2743
16---18
0.83
9001–10,000
2743---3048
18---20
0.81
*Derate multiplier factors are based on midpoint altitude for altitude range.
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. 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. 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.
Inlet pressure tap set screw must be tightened and
⅛
--in.
NPT pipe plug must be installed to prevent gas leaks.
!
WARNING
14. Tighten set screw on inlet tower pressure tap with 3/32--in.
hex wrench, or if
⅛
--in. NPT plug was removed, apply pipe
dope sparingly to end of 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:
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 manifold 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 manifold pressure tap on the
gas valve. See
Fig. 61
.
3. Connect a manometer to the outlet 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. After the main burners ignite and the blower starts, confirm
Maximum Heat manifold pressure is correct, based on the
manifold pressure tables in the installation instructions.
9. To adjust the Maximum Heat manifold pressure, slowly
turn adjusting screw counterclockwise to decrease manifold
pressure or clockwise to increase manifold pressure. Turn
adjustment no more than one click per second until you
obtain the required manifold pressure. See Fig. 60.
10. Main burner flame should be clear blue, almost transparent.
11. After adjusting the Maximum Heat manifold pressure,
remove jumpers across thermostat connections to terminate
the call for heat.
12. Wait for blower off-delay to finish then reset 115--v power
to furnace.
