Rheem RGRA SERIES, Installation Instructions Manual

Page: 45 / 64

45
ADJUSTING OR CHECKING
FURNACE INPUT
The maximum gas supply pressure
to the furnace should be 7" w.c.
for natural gas. The minimum gas
supply pressure for purposes of input
adjustment to the furnace should be
5" w.c.
A properly calibrated magnehelic
gauge or manometer is required for
accurate gas pressure readings.
The manifold pressure should be set
at 3.5" w.c. for natural gas. Only
small variations in the gas flow
should be made by means of the
pressure regulator adjustment. In no
case should the final manifold
pressure vary more than plus or
minus 0.3" w.c. from the above-
specified pressures. To adjust the
pressure regulator, remove the
regulator cap and turn the adjustment
screw clockwise to increase pressure
or counterclockwise to decrease
pressure. Then replace the regulator
cap securely. Any necessary major
changes in the gas flow rate should
be made by changing the size of the
burner orifices.
To change orifice spuds, shut off the
manual gas valve and remove the
gas manifold. On LP gas furnaces,
the LP gas supply pressure must be
set between 11" and 14" w.c. by
means of the tank or branch supply
regulators. The furnace manifold
pressure should be set at 10" w.c. at
the gas control valve. For elevations
up to 2,000 feet, rating plate input
ratings apply. For high altitudes
(elevations over 2,000 ft.), see
conversion kit index 92-21519-XX for
derating and orifice spud sizes.
Checking furnace input is important
to prevent over firing beyond its
design-rated input. NEVER SET
INPUT ABOVE THAT SHOWN ON
THE RATING PLATE. Use the
following table or formula to
determine input rate. Start the
furnace and measure the time
required to burn one cubic foot of
gas. Prior to checking the furnace
input, make certain that all other gas
appliances are shut off, with the
exception of pilot burners. Time the
meter with only the furnace in
operation.
SETTING INPUT RATE
The furnace is shipped from the
factory with #50 orifices. They are
sized for natural gas having a heating
value of 1075 BTU/cu. ft. and a
specific gravity of .60.
Since heating values vary geo-
graphically, the manifold pressure
and/or gas orifice size may need to
be changed to adjust the furnace to
its nameplate input. Consult the local
gas utility to obtain the yearly
average heating value and orifice
size required to fire each individual
burner at 15,000 BTU/HR.
TABLE 6
METER TIME
METER TIME IN MINUTES AND SECONDS FOR NORMAL INPUT RATING OF FURNACES
EQUIPPED FOR NATURAL OR LP GAS
HEATING VALUE OF GAS BTU PER CU. FT.
METER
900 1000 1040 1100 2500 (LP)
INPUT SIZE
BTU/HR CU. FT. MIN. SEC. MIN. SEC. MIN. SEC. MIN. SEC. MIN. SEC.
ONE 1 12 1 20 1 23 1 28 3 20
45,000 TEN 12 0 13 20 13 50 14 40 33 20
ONE 0 54 1 0 1 3 1 6 2 30
60,000 TEN 9 0 10 0 10 24 11 0 25 0
ONE 0 44 0 48 0 50 0 53 2 0
75,000 TEN 7 12 8 0 8 19 8 48 20 0
ONE 0 36 0 40 0 42 0 44 1 40
90,000 TEN 6 0 6 40 7 0 7 20 16 40
ONE 0 31 0 34 0 36 0 38 1 26
105,000 TEN 5 10 5 40 6 0 6 20 14 20
ONE 0 27 0 30 0 31 0 33 1 15
120,000 TEN 4 30 5 0 5 10 5 30 12 30
Heating Value of Gas (BTU/FT 3 ) x 3600
Formula: Input BTU/HR =
Time in Seconds (for 1 cu. ft.) of Gas