(1.) Obtain average yearly heat value for local gas supply.
(2.) Obtain average yearly specific gravity for local gas
supply.
(3.) Verify the furnace model. Table 8 can only be used for
model 58EJB Furnaces.
(4.) Find natural gas heat value and specific gravity in
Table 8.
(5.) Follow heat value line and specific gravity line to
point of intersection to find orifice size and manifold
pressure settings for proper operation at given natural
gas condition.
(6.) Check and verify orifice size in furnace. NEVER
ASSUME THE ORIFICE SIZE. ALWAYS CHECK
AND VERIFY.
EXAMPLE: Heat value 1025 Btu/cu ft
Specific gravity of 0.62
Therefore, Orifice No. 41,
Manifold pressure 3.5-in. wc
The furnace is shipped with No. 42 orifices. In this example
all main burner orifices must be changed and manifold
pressure must be adjusted.
(7.) Proceed to item c to adjust manifold pressure.
b. Determine natural gas input rate by clocking gas meter and
adjusting manifold pressure per item c.
NOTE:
Be sure all pressure tubing, combustion-air and vent
pipes, and burner enclosure plates are in place when checking
input.
(1.) 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.
(2.) Turn off all other gas appliances and pilots.
(3.) Start furnace and let it run for 3 minutes.
(4.) Measure time (in sec) for gas meter test dial to
complete 1 revolution.
(5.) Refer to Table 9 for cubic ft of gas per hr.
(6.) 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 = 1070 Btu/cu ft
Time for 1 revolution of 2 cu ft dial = 72 sec
Gas rate = 100 cu ft/hr (from Table 9)
Btu heating input = 100 X 1070 = 107,000 Btuh
(7.) 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, item c, and replace the
main burner orifices as needed. Refer to NFGC Appendix F, Table
F-4 for proper orifice sizing at high altitudes.
(8.) Proceed to item c to adjust manifold pressure.
c. Adjust gas input.
NOTE:
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 for natural gas.
(1.) Remove burner enclosure front and cap that conceals
adjustment screw for gas valve regulator. (See Fig.
14.)
(2.) Turn adjusting screw either counterclockwise (out) to
decrease input rate or clockwise (in) to increase rate.
When adjusting input rate, DO NOT set manifold
pressure less than 3.2 or more than 3.8-in. wc for
natural gas. Make any major adjustments by changing
main burner orifices.
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 the burners and heat exchangers, causing
failures.
NOTE:
If orifice hole appears damaged or it is suspected to have
been redrilled, check the orifice hole with the correct size
numbered drill bit. Never redrill an orifice. A burr-free and
squarely aligned orifice hole is essential for proper flame charac-
teristics.
(3.) Replace burner enclosure plates and measure adjusted
gas input rate using method outlined in item b.
(4.) Replace cap that conceals gas valve regulator adjust-
ment screw.
Be sure burner enclosure front is in place after adjustments
have been made.
d. Look through sight glass in burner enclosure and check
burner flame. The main burner flame should be clear blue,
almost transparent. (See Fig. 15.)
2. Set temperature rise.
The furnace must not be operated outside the range of temperature
rise specified on the unit rating plate. Determine the air tempera-
ture rise as follows.
a. Place thermometers in return and supply ducts as near
furnace as possible. Be sure thermometers do not "see" heat
exchanger so that radiant heat does not affect readings. This
practice is particularly important with straight-run ducts.
b. When thermometer readings stabilize, subtract return-air
temperature from supply-air temperature to determine air
temperature rise.
Fig. 14—Redundant Automatic Gas Valve
A91219
REGULATOR
ADJUSTMENT
MANIFOLD
PRESSURE
TAP
INLET
PRESSURE
TAP
MANUAL
ON/OFF
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