International comfort products F8MVL Installation Instructions Manual page 38

Furnace gas input rate on rating plate is for installations at
altitudes up to 2000 Ft. (610 M). Furnace input rate must be
within +/−2 percent of furnace rating plate input. For altitudes
above 5500 Ft. (1676 M), a field−supplied high altitude
pressure switch is required.
1. Determine the correct gas input rate.
The input rating for altitudes above 2,000 ft. (610 M) must
be reduced by 4 percent for each 1,000 ft. (305 M) above
sea level.
For installations below 2000 Ft. (610 M), refer to the unit
rating plate. For installations above 2000 Ft. (610 M), mul-
tiply the input on the rating plate by the de−rate multiplier in
Table 11 for the correct input rate.
Table 11 – Altitude Derate Multiplier for U.S.A.
ALTITUDE PERCENT
FT. (M) OF DERATE
0–2000
0
(0−610)
2001–3000
8–12
(610−914)
3001–4000
12–16
(914−1219)
4001–5000
16–20
(1219−1524)
5001–6000
20–24
1524−1829)
6001–7000
24–28
(1829−2134)
7001–8000
28–32
(2134−2438)
8001–9000
32–36
(2438−2743)
9001–10,000
36–40
(2743−3048)
2. Determine the correct orifice and manifold pressure adjust-
ment. All models in all positions except Low NOx models in
downflow and horizontal positions use Table 13 (22,000
Btuh per burner.) Low NOx models in downflow or hori-
zontal positions must use Table 15 (21,000 Btuh per burn-
er.) See input listed on rating
a. Obtain average yearly gas heat value (at installed
altitude) from local gas supplier.
b. Obtain average yearly gas specific gravity from local
gas supplier.
c. Find installation altitude in Table 13 or 15.
d. Find closest natural gas heat value and specific
gravity in Table 13 or 15.
e. 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.
f. Check and verify burner orifice size in furnace. NEV-
ER ASSUME ORIFICE SIZE. ALWAYS CHECK AND
VERIFY.
NOTE: 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.
g. Replace orifice with correct size, if required by Table
13 or 15. Use only factory−supplied orifices. See
EXAMPLE 1.
38
DERATE MULTIPLIER
FACTOR*
1.00
0.90
0.86
0.82
0.78
0.74
0.70
0.66
0.62
plate.
Specifications are subject to change without notice.
EXAMPLE 1: 0–2000 ft. (0−610 M) altitude
For 22,000 Btuh per burner application use Table 13.
Heating value = 1000 Btuh/cu ft.
Specific gravity = 0.62
Therefore: Orifice No. 43*
Manifold pressure: 3.7−In. W.C. for high−heat
1.6−In. W.C. for low−heat
* Furnace is shipped with No. 43 orifices. In this example all
main burner orifices are the correct size and do
not need to be changed to obtain proper input rate.
3. Adjust manifold pressure to obtain low fire input rate. (See
Figure 48)
a. Turn gas valve ON/OFF switch to OFF.
b. Remove manifold pressure tap plug from gas valve.
c. Connect a water column manometer or similar
device to manifold pressure tap.
d. Turn gas valve ON/OFF switch to ON.
e. Move setup SW1—2 on furnace control to ON posi-
tion to lock furnace in low−heat operation. (See Ta-
ble 9)
f. Manually close blower door switch.
g. Jumper R and W/W1 thermostat connections on
control to start furnace. (See Figure 26)
h. Remove regulator adjustment cap from low heat gas
valve pressure regulator (See Figure 48) and turn
low−heat adjusting screw (3/16 or smaller flat−tipped
screwdriver) counterclockwise (out) to decrease in-
put rate or clockwise (in) to increase input rate.
NOTE: DO NOT set low−heat manifold pressure less than
1.4−In. W.C. or more than 1.7−In. W.C. for natural gas. If
manifold pressure is outside this range, change main burner
orifices.
i. Install low−heat regulator adjustment cap.
j. Move setup switch SW1−2 to off position after com-
pleting low−heat adjustment.
k. Leave manometer or similar device connected and
proceed to Step 4.
4. Adjust manifold pressure to obtain high fire input rate.
(See Figure 48)
a. Jumper R to W/W1 and W2 thermostat connections
on furnace control. This keeps furnace locked in
high−heat operation.
b. Remove regulator adjustment cap from high−heat
gas valve pressure regulator (See Figure 48) and
turn high heat adjusting screw (3/16−in. or smaller
flat−tipped screwdriver) counterclockwise (out) to
decrease input rate or clockwise (in) to increase in-
put rate.
NOTE: DO NOT set high−heat manifold pressure less than
3.2−In. W.C. or more than 3.8 In. W.C. for natural gas. If
manifold pressure is outside this range, change main burner
orifices to obtain manifold pressure in this range.
c. When correct input is obtained, replace caps that
conceal gas valve regulator adjustment screws.
Main burner flame should be clear blue, almost
transparent.
d. Remove jumpers R to W/W1 and R to W2.
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