(1.) Obtain average yearly heat value for local gas supply.
(2.) Check and verify orifice size in furnace. NEVER ASSUME THE ORIFICE SIZE. ALWAYS CHECK AND VERIFY.
(3.) Turn off all other gas appliances and pilots.
(4.) Start furnace and let run for 3 minutes.
(5.) Measure time (in sec) for gas meter to complete 1 revolution.
(6.) Refer to Table 8 for cu ft of gas per hr.
(7.) Multiply gas rate (cu ft/hr) by heating value (Btu/cu ft).
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 X 1070 = 107,000 Btuh
(8.) Check to make sure that measured gas input does not exceed gas input on unit rating plate.
(9.) Proceed to item c. to adjust manifold pressure.
c. Adjust gas input.
(1.) Remove regulator adjustment seal cap. (See Fig. 15.)
(2.) Turn adjusting screw counterclockwise to decrease input. Turn screw clockwise to increase input. DO NOT set manifold pressure
less than 3.2-in. wc or more than 3.8-in. wc for natural gas. Make any major adjustments by changing main burner orifices.
(3.) When correct input is obtained, replace regulator adjustment seal cap. Main burner flame should be clear blue, almost transparent.
(See Fig. 16.)
d. High altitude—In the United States, gas input on unit 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.
e. Canadian installations only—The Canadian ratings are approved for altitudes up to 2000 ft for natural and propane gases. High-altitude
ratings are from 2000 ft to 4500 ft above sea level. See Table 9 for nominal burner orifice size. High-altitude input ratings include a 10
percent derated as required by Canadian standards.
2. Set temperature rise.
Furnace must operate within range of temperature rise specified on the unit rating plate. Determine the air temperature rise as follows.
a. Place duct thermometers in return and supply ducts as near furnace as possible. Be sure thermometers do not "see" heat exchangers so
that radiant heat does not affect thermometer readings. This is particularly important with straight-run ducts.
b. When thermometer readings stabilize, subtract return-air temperature from supply-air temperature to determine temperature rise.
Fig. 15—Redundant Automatic Gas Control Valve
A90044
OFF
MANIFOLD
PRESSURE TAP
CONTROL
KNOB
GAS
PRESSURE
REGULATOR
ADJUSTMENT
INLET
PRESSURE TAP
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