32
441 01 1411 04
Specifications are subject to change without notice.
Step 3
—
Adjustments
FIRE HAZARD
Failure to follow this warning could result in injury,
death and/or property damage.
DO NOT bottom out gas valve regulator adjusting
screw. This can result in unregulated manifold
pressure and result in excess overfire and heat
exchanger failures.
!
WARNING
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 49)
CAUTION
!
Furnace gas input rate on rating plate is for installations at
altitudes up to 2000 ft. (610 M). Furnace input rate must be
/
−
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), multiply the input on the rating
plate by the de
−
rate multiplier in Table 8 for the correct
input rate.
2. Determine the correct orifice and manifold pressure ad-
justment. There are two different orifice and manifold
adjustment tables. All models in all positions, except
Low NOx models in downflow or horizontal positions,
use Table 13 (22,000 Btuh/Burner).
Low NOx models in the downflow or horizontal posi-
tions must use Table 14 (21,000 Btuh/Burner). See in-
put listed on rating plate.
a. Obtain average yearly gas heat value (at installed alti-
tude) from local gas supplier.
b. Obtain average yearly gas specific gravity from local
gas supplier.
c. Find installation altitude in Table 13 or 14.
d. Find closest natural gas heat value and specific grav-
ity in Table 13 or 14.
e. Follow heat value and specific gravity lines to point of
intersection to find orifice size and manifold pressure
settings for proper operation.
f. Check and verify burner orifice size in furnace.
NEVER ASSUME ORIFICE SIZE. ALWAYS CHECK
AND VERIFY.
g. Replace orifice with correct size if required by Table
10, 11, 13 and 14. Use only factory
−
supplied orifices.
See EXAMPLE 1.
Table 10 – Altitude Derate Multiplier for U.S.A.
ALTITUDE
(FT. / M)
PERCENT
OF
DERATE
DERATE
MULTIPLIER
FACTOR*
0–2000 (0
−
610)
0
1.00
2001–3000 (610
−
914)
8–12
0.90
3001–4000 (914
−
1219)
12–16
0.86
4001–5000 (1219
−
1524)
16–20
0.82
5001–6000 (1524
−
1829)
20–24
0.78
6001–7000 (1829
−
2134)
24–28
0.74
7001–8000 (2134
−
2438)
28–32
0.70
8001–9000 (2438
−
2743)
32–36
0.66
9001–10,000 (2743
−
3048)
36–40
0.62
* Derate multiplier factors are based on midpoint altitude for altitude range.
EXAMPLE 1: (0 to 2000 ft. (0 to 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.
*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 correct input rate.
a. Turn gas valve ON/OFF switch to OFF.
b. Remove manifold pressure tap plug from gas valve.
(See Figure 48)
c. Connect a water column manometer or similar device
to manifold pressure tap.
d. Turn gas valve ON/OFF switch to ON.
e. Manually close blower door switch.
f. Set thermostat to call for heat.
g. Remove regulator seal cap and turn regulator adjust-
ing screw counterclockwise (out) to decrease input
rate of clockwise (in) to increase input rate.
h. Install regulator seal cap.
i. Leave manometer or similar device connected and
proceed to Step 4.
NOTE
: DO NOT set manifold pressure less than 3.2
−
in. w.c.
or more than 3.8
−
in. w.c. for natural gas at sea level. If
manifold pressure is outside this range, change main burner
orifices. Refer to Table 10, 11, 13 and 14.
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.
4. Verify natural gas input rate by clocking meter.
NOTE
: Gas valve regulator adjustment cap must be in place
for proper input to be clocked.
a. Turn off all other gas appliances and pilots served by
the meter.
b. Run furnace for 3 minutes in heating operation.
c. Measure time (in sec) for gas meter to complete 1 re-
volution and note reading. The 2 or 5 cubic feet dial
provides a more accurate measurement of gas flow.
d. Refer to Table 12 for cubic ft. of gas per hr.
e. Multiply gas rate (cu ft./hr) by heating value (Btuh/cu
ft.) to obtain input.
