EXAMPLE:
Heat value = 1010 Btu/cu ft
Specific gravity = 0.62
Therefore: Orifice No. 44* with manifold pressure 3.3-in.
wc for high heat and 1.4-in. wc for low heat.
* Furnace is shipped with No. 45 orifices. In this example
all main burner orifices must be changed and the manifold
pressure adjusted.
f. Check and verify orifice size in furnace. NEVER AS-
SUME THE ORIFICE SIZE; ALWAYS CHECK AND
VERIFY.
g. Proceed to item 3 to adjust manifold pressure.
h. Check input rate by clocking meter as described in item
2.
2. Determine natural gas input rate by clocking gas meter and
adjusting manifold pressure per item 3.
NOTE:
Be sure all pressure tubing, combustion-air and vent
pipes, and burner enclosure front are in place when checking input
by clocking the gas meter.
a. Obtain average yearly heat value for local gas supply.
NOTE:
Be sure heating value of gas used for calculations is
correct for your altitude. Consult local gas utility for altitude
adjustments of gas heating value.
b. Turn off all other gas appliances and pilots.
c. Start furnace and let run for 3 minutes.
d. Measure time (in sec) for gas meter test dial to complete
1 revolution.
e. Refer to Table 12 for cubic ft of gas per hr.
f. Multiply gas rate (cu ft/hr) by heating value (Btu/cu ft).
Obtain heating value from local gas utility.
EXAMPLE:
Btu heating input = Btu/cu ft X cu ft/hr
Heating value of gas = 1010 Btu/cu ft
Time for 1 revolution of 2-cu-ft dial = 72 sec
Gas rate = 100 cu ft/hr (from Table 12)
Btu heating input = 100 X 1010 = 101,000 Btuh
In this example, the orifice size and manifold pressure
adjustment is within
±
2 percent of the furnace input rate.
g. Measured gas input must not exceed gas input on unit
rating plate.
NOTE:
High altitude—Gas input on rating plate is for altitudes
up to 2000 ft. Ratings for altitudes over 2000 ft must be 4 percent
less for each 1000 ft above sea level. To obtain the adjusted
altitude rating, adjust the manifold pressure (see item 3) and
replace the main burner orifices as needed. Refer to NFGC
Appendix F, Table F-4 for proper orifice sizing at high altitudes.
NOTE:
In Canada, from 2000 to 4500 ft above sea level, derate
the unit 10 percent from the Conversion Station.
h. Proceed to item 3 to adjust manifold pressure.
3. Adjust gas input.
NOTE:
Manifold pressure must always be measured with the
burner enclosure front REMOVED. The gas meter must always be
clocked with the burner enclosure front INSTALLED. The gas
valve has been nominally set at 3.5-in. wc high heat and 1.5-in. wc
low heat for natural gas.
a. Remove burner enclosure front and caps that conceal
adjustment screws for gas valve regulator. (See Fig. 46.)
b. Turn setup switch SW-2 on control center to ON
position. (See Fig. 25.) This will keep furnace locked in
low-heat operation.
c. Turn low-heat adjusting screw (5/64 hex Allen wrench)
counterclockwise (out) to decrease input rate or clock-
wise (in) to increase rate. When adjusting input rate, DO
NOT set manifold pressure less than 1.3-in. or more than
1.7-in. wc for natural gas. Make any major adjustments
by changing main burner orifices.
d. Turn setup switch SW-2 to OFF position after low fire
adjustment.
e. Jumper R-W2 on control center thermostat connections.
(See Fig. 25.) This will keep furnace locked in high-heat
operations.
f. Turn high-heat adjusting screw (5/64 hex Allen wrench)
counterclockwise (out) to decrease input rate or clock-
wise (in) to increase rate. When adjusting input rate, DO
NOT set manifold pressure less than 3.2-in. or more than
3.8-in. wc for natural gas. Make any major adjustment by
changing main burner orifices.
NOTE:
If orifices are changed, both high-heat and low-heat input
rate must be readjusted.
NOTE:
If orifice hole appears damaged or it is suspected to have
been redrilled, check the orifice hole with a numbered drill bit of
the correct size. Never redrill an orifice. A burr-free and squarely
aligned orifice hole is essential for proper flame characteristics.
g. Remove jumper R-W2 after high-heat adjustment.
h. Replace burner enclosure front and measure adjusted gas
input rate using method outlined in item 2.
i. Replace caps that conceal gas valve regulator adjustment
screws.
TABLE 9—SETUP SWITCH DESCRIPTION
SETUP
SWITCH NO.
NORMAL
POSITION
DESCRIPTION OF USE
SW-1
(FLT)
OFF
Turn switch to ON for fault history dis-
play. No thermostat signal can be present
for fault history display.
SW-2
(LOW)
OFF
Turn switch to ON to lock furnace in low-
heat mode only.
SW-3
(BPH)
OFF
Turn switch to ON when a bypass hu-
midifier is used. This will compensate for
higher return-air temperature and provide
5 percent more airflow in low-heat mode
only.
SW-4
(EMER HEAT)
OFF
Turn switch to ON to bypass micropro-
cessor control. Furnace will operate at
high heat only with main blower and in-
ducer motor operating at maximum RPM.
NO safeties are bypassed.
SW-5
(MZ)
OFF
Turn switch to ON when modulating
dampers are used. In this mode, main
blower speed is recalculated once every
minute while furnace is in low-heat or
continuous fan mode.
SW-6
(COMP TEST)
OFF
Turn switch to ON to initiate component
test. Furnace will operate inducer motor
20 sec low speed, 20 sec high speed,
HSI 15 sec or IID 0 sec, blower 20 sec
low speed, and blower 20 sec high
speed. SW-1 must be in OFF position.
No thermostat signal can be present for
component test to be initiated.
SW-7 and -8
(Blower
Off Delay)
See
Table
10
Adjust switches to provide desired heat-
ing mode blower off delay time, 90, 135,
180, or 225 sec.
TABLE 10—BLOWER OFF DELAY SETUP SWITCH
POSITION
DESIRED HEATING
MODE BLOWER
OFF DELAY (SEC)
SETUP SWITCH (SW-7 AND -8) POSITION
SW-7
SW-8
90
OFF
OFF
135
ON
OFF
180
OFF
ON
225
ON
ON
—35—
