d. Find installation altitude in Table 13 or 14.
NOTE:
For Canadian altitudes of 2001 to 4500 ft, use U.S.A.
altitudes of 2001 to 3000 ft in Table 13 or 14.
e. Find closest natural gas heat value and specific gravity in
Table 13 or 14.
f. Follow heat value and specific gravity lines to point of
intersection to find orifice size and manifold pressure
settings for proper operation.
EXAMPLE: (0-2000 ft altitude using Table 13)
Heating value = 1050 Btu/cu ft
Specific gravity = 0.62
Therefore: Orifice No. 45 and
Manifold pressure 3.6-in. wc
* Furnace is shipped with No. 45 orifices. In this
example all main burner orifices are the correct size
and do not need to be changed to obtain the proper
input rate.
Check and verify burner orifice size in furnace. NEVER ASSUME
ORIFICE SIZE; ALWAYS CHECK AND VERIFY.
2. Adjust manifold pressure to obtain input rate.
NOTE:
Manifold pressure must always be measured with burner
enclosure front REMOVED. Gas meter must always be clocked
with burner enclosure front INSTALLED.
a. Remove burner enclosure front.
b. Remove seal cap that conceals adjustment screw for gas
valve regulator. (See Fig. 55.)
c. Jumper R and W thermostat connections on control to
start furnace.
d. Turn adjusting screw, counterclockwise (out) to decrease
manifold pressure or clockwise (in) to increase manifold
pressure.
NOTE:
This furnace has been approved for a manifold pressure
of 3.2 in. wc to 3.8 in. wc when installed at altitudes up to 2000 ft.
For altitudes above 2000 ft, the manifold pressure can be adjusted
from 2.0 in. wc to 3.8 in. wc. If manifold pressure is outside this
range, change burner orifices to obtain pressure in this range.
CAUTION:
UNIT DAMAGE HAZARD
Failure to follow this caution may result in reduced
furnace life.
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.
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.
CAUTION:
UNIT DAMAGE HAZARD
Failure to follow this caution may result in component
damage due to flame impingement of burners and heat
exchangers.
DO NOT redrill orifices. Improper drilling (burrs, out-of-
round holes, etc.) can cause excessive burner noise and
misdirection of burner flames. (See Fig. 56.)
e. Replace gas valve regulator adjustment screw seal cap.
f. Replace burner enclosure front and verify adjusted gas
input rate using method outlined in item 3.
g. Look through sight glass in burner enclosure and check
burner flame. Burner flame should be clear blue, almost
transparent. (See Fig. 57.)
h. Remove jumper from R and W.
3. Verify natural gas input rate by clocking gas meter.
NOTE:
Be sure all pressure tubing, combustion-air and vent
pipes, and burner box cover front are in place when checking input
by clocking gas meter.
a. Calculate high-altitude adjustment (if required).
UNITED STATES
At altitudes above 2000 ft, this furnace has been ap-
proved for a 2 percent derate for each 1000 ft above sea
level. See Table 15 for derate multiplier factor.
CANADA
At installation altitudes between 2001 to 4500 ft, this
furnace must be derated 5 percent by an authorized Gas
Conversion Station or Dealer. To determine correct input
rate for altitude, see example above and use 0.95 as
derate multiplier factor.
b. Reinstall burner box cover.
c. Gas valve regulator seal cap MUST be on.
d. Turn off all other gas applications and pilots.
e. Start furnace and let operate for 3 minutes.
f. Measure time (in sec) for gas meter to dial to complete 1
revolution.
g. Refer to Table 16 for cu ft of gas per hr.
h. Multiply gas rate (cu ft/hr) X heating value (Btu/cu ft)
using natural gas heating value from local gas
utility/supplier.
EXAMPLE: (0-2000 ft altitude)
Furnace input from rating plate is 100,000 Btuh.
Btu heating input = Btu/cu ft X cu ft/hr
Heating value of gas = 975 Btu/cu ft
Time for 1 revolution of 2-cu ft dial = 70 sec
Gas rate = 103 cu ft/hr (from Table 16)
Btu heating input = 103 X 975 = 100,425 Btuh.
In this example, the orifice size and manifold
pressure adjustment is within ±2 percent of the
furnace input rate.
B.
Set Temperature Rise
CAUTION:
UNIT DAMAGE HAZARD
Failure to follow this caution may result in overheating
the heat exchangers or condensing flue gases in heat
exchanger areas not designed for condensate.
Temperature rise must be within limits specified on
furnace rating plate. Recommended operation is at mid-
point of rise range or slightly above.
EXAMPLE:
100,000 Btuh input furnace installed at 4300 ft.
Derate
Furnace Input Rate
Furnace Input Rate
X
Multiplier
=
at Installation
at Sea Level
Factor
Altitude
100,000
X
0.91
=
91,000
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