Rheem Gra Series Installation Instructions Manual page 49

Sequence if the system doesn't
light or doesn't sense flame:
1. On a call for heat, the control
runs the inducer for 30 seconds
to pre-purge.
2. After the 30-second pre-purge,
the spark igniter energizes. The
inducer continues to run.
3. After the spark igniter energizes,
the gas valve opens for an 8-
second trial for ignition. The
inducer continues and the igniter
stays energized.
4. If flame is not sensed within 8
seconds after the gas valve
opens, the gas valve closes, the
igniter de-energizes and:
5. The inducer completes a 10-
second post-purge, the inducer
stops, and the control verifies that
the pressure switch has opened.
Once the open pressure switch is
confirmed, the control begins the
next ignition cycle by energizing
the inducer for a pre-purge of 30
seconds. After the pre-purge, the
igniter energizes and the gas
valve opens (inducer continues to
run). If no flame is sensed on the
second attempt, the control goes
into a "self-healing" mode, in
which the blower and the inducer
run for 3 minutes before another
ignition attempt is made.
NOTE: The following applies
only to units manufactured
before third Quarter 2003: After
a 30-second inter-purge (inducer
runs continuously), the igniter is
energized, and the gas valve
opens for 8 seconds. If flame is
not sensed in 8 seconds, the gas
valve is closed and the igniter de-
energizes. If no flame is sensed
on this second attempt, the
control goes into a "self-healing"
mode, in which the blower and
inducer run for 3 minutes before
another ignition attempt is made.
6. The control attempts to ignite up
to four times (first attempt
followed by three retries). After
the fourth failure to ignite, the
control goes into a one-hour
"soft-lockout" during which the
control will not respond to the
thermostat heat call (W). The
lockout can be reset by shutting
off main power to the furnace for
five seconds, or by turning the
heat call (W) from the thermostat
off and then back on.
7. The above sequence will repeat
after a one hour delay. It will
continue repeating until ignition is
successful or the call for heat is
terminated.
8. To reset the lock out, make and
break power either at the
thermostat or at the unit
disconnect switch for 5 to 10
seconds. It then goes through
another set of trials for ignition.
TABLE 9
METER TIME
METER TIME IN MINUTES AND SECONDS FOR NORMAL INPUT RATING OF FURNACES
METER
INPUT SIZE
BTU/HR CU. FT.
ONE
45,000 TEN
ONE
60,000 TEN
ONE
75,000 TEN
ONE
90,000 TEN
ONE
105,000 TEN
ONE
120,000 TEN
Formula: Input BTU/HR =
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
EQUIPPED FOR NATURAL OR LP GAS
HEATING VALUE OF GAS BTU PER CU. FT.
900 1000 1040
MIN. SEC. MIN. SEC. MIN. SEC. MIN. SEC. MIN. SEC.
1 12 1 20 1
12 0 13 20 13
0 54 1 0 1
9 0 10 0 10
0 44 0 48 0
7 12 8 0 8
0 36 0 40 0
6 0 6 40 7
0 31 0 34 0
5 10 5 40 6
0 27 0 30 0
4 30 5 0 5
Heating Value of Gas (BTU/FT
Time in Seconds (for 1 cu. ft.) of Gas
ratings apply. For high altitudes
(elevations over 2,000 ft.), see
conversion kit index 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.
NOTE: Refer to the National Fuel
Gas Code for high altitude rate
adjustment above 2,000 ft.
1100 2500 (LP)
23 1 28 3 20
50 14 40 33 20
3 1 6 2 30
24 11 0 25 0
50 0 53 2 0
19 8 48 20 0
42 0 44 1 40
0 7 20 16 40
36 0 38 1 26
0 6 20 14 20
31 0 33 1 15
10 5 30 12 30
) x 3600 x correction factor
3
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