Carrier 58MCA Installation, Start-Up, And Operating Instructions Manual page 37

e. Find closest natural gas heat value and specific gravity on
Table 9 or 10.
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 9)
Heating value = 1050 Btu/cu ft
Specific gravity = 0.62
Therefore: Orifice No. 45
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 cap that conceals adjustment screw for gas valve
regulator. (See Fig. 50.)
Fig. 50—Redundant Automatic Gas Valve
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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.
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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.
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.
e. Replace gas valve regulator adjustment screw cap.
f. Replace burner enclosure front and verify adjusted gas
input rate using method outlined in item 3.
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 burners and heat exchangers causing failures.
(See Fig. 51.)
g. Look through sight glass in burner enclosure and check
burner flame. Burner flame should be clear blue, almost
transparent. (See Fig. 52.)
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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 enclosure 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 approved
for a 2% derate for each 1000 ft above sea level. See Table
11 for derate multiplier factor.
EXAMPLE: 100,000 BTUH INPUT FURNACE INSTALLED AT
4300 FT.
Furnace Input Rate
at Sea Level
100,000
CANADA
At installation altitudes from 2001 to 4500 ft, this furnace
must be derated 5% by an authorized Gas Conversion
Station or Dealer. To determine correct input rate for
altitude, see example above and use 0.95 as derate multi-
plier factor.
A00238
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b. Reinstall burner box cover.
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c. Gas valve regulator seal cap MUST be on.
d. Turn off all other gas appliances and pilots.
e. Start furnace and let operate for 3 minutes.
f. Measure time (in sec) for gas meter test dial to complete 1
revolution.
g. Refer to Table 12 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.
37
Derate Furnace Input Rate
X Multiplier = at Installation
Factor Altitude
X 0.91 = 91,000