Sequence Of Operation; Setting Input Rate - Rheem RGFG Installation Instructions Manual

SEQUENCE OF
OPERATION
Heating Cycle Initiation
The heating cycle is always initiated by
a 24 volt signal on W of the thermostat
or, for communicating systems, a mes-
sage is transmitted from the thermostat
to the IFC. When the controller senses
24 volts on W or the communicated
message for heat call, the following
sequence occurs:
• High and low pressure switches are
checked to insure contacts are open.
• Inducer is powered on high speed for
a thirty (30) second prepurge.
• Pre-Purge:
There are two different types of pre-
purge; a normal pre-purge and a
learning-sequence pre-purge. During a
learning-sequence pre-purge, the
inducer motor will incrementally
increase in RPM (stepping) until the
low and high pressure switches are
both closed. After both switches are
sensed to be closed, the inducer
motor will continue to run for an addi-
tional 30 seconds before the ignition
trial.
A learning sequence pre-purge will be
initiated under the following conditions:
- First heat call after power reset.
th
- Every 25 heat call.
- Next heat attempt after a failed pre-
purge (pressure switch(es) does not
close).
- Next heat attempt after a pressure
switch has opened unexpectedly
during normal heating operation.
A normal pre-purge will not go through
the incremental stepping process of
the inducer motor and should be much
quicker. The inducer will start at a pre-
determined RPM (determined during
learning-sequence pre-purge) and this
should close both pressure switches
quickly. Once both pressure switches
are sensed to be closed, the inducer
will run for 30 seconds before the igni-
tion trial starts.
• Hot - surface igniter is energized dur-
ing the pre-purge period.
• The modulating gas valve is set to the
highest possible rate (no flow yet).
• The main solenoids on the gas valve
are energized allowing gas to flow to
the burners.
• When flame is proven, the ignition
control is de-energized - 8 second
maximum trial time.
• The gas valve maintains 100% rate
through the warm-up period - 20 sec-
onds (aka Blower Off Delay).
Heating Cycle Response
MODULATING FUNCTION:
("W" and "V" signal inputs, refer to dip
switch set SW2 on IFC)
After the warm-up period, the furnace
will respond to the thermostat demand
by adjusting the gas valve pressure
and blower speed anywhere between
40% to 100% heating capacity.
® TWO-STAGE FUNCTION – NON-
COMMUNICATING SYSTEMS ONLY:
(Two-stage function only applies when
68
TABLE 21
METER TIME
METER TIME IN MINUTES AND SECONDS FOR NORMAL INPUT RATING OF FURNACES
EQUIPPED FOR NATURAL OR LP GAS
METER
INPUT SIZE
BTU/HR CU. FT.
ONE
60,000 TEN
ONE
75,000 TEN
ONE
90,000 TEN
ONE
105,000 TEN
ONE
120,000 TEN
Formula: Input BTU/HR =
Where C • F =
both S4-2 and S4-3 are in the "ON"
position and a two-stage thermostat is
installed as shown in Figure 57.)
After the warm-up period, the furnace
will respond to the thermostat demand
by adjusting the gas valve pressure and
blower heating speeds to the "W" signal
values. "W" only = 40% gas valve pres-
sure and blower heating speed. "W2" =
65% gas valve pressure and blower
heating speed for first five minutes and
100% thereafter. Also, if the call for heat
ends, the furnace terminates at the pre-
sent rate.
® SINGLE-STAGE FUNCTION – NON-
COMMUNICATING SYSTEMS ONLY:
(SW2-2 and SW2-3 must both be turned
"off" for this operation.)
("W" signal only)
After the warm-up period, the furnace
will respond to the thermostat demand
by altering the gas valve pressure and
blower speed as follows:
Phase 1: 0 to 5 minutes = 40% of furnace
capacity (gas valve output and blower
speed)
Phase 2: 5 to 12 minutes = 65% of fur-
nace capacity (gas valve output and
blower speed)
Phase 3: After 12 minutes = 100% of fur-
nace capacity (gas valve output and
blower speed)
NOTE: If the call for heat ends during any
phase, the furnace will terminate imme-
diately at the firing rate of that phase.
Heating Cycle Termination
("W" signal only, refer to dip switch set
SW2 on IFC)
When the 24 volt signal is removed from
W1 or, for communicatiing systems, a
message is transmitted from the thermo-
stat to the furnace to "end the heat call",
the heating cycle will end and the furnace
will shut down and return to the proper off
cycle operation.
HEATING VALUE OF GAS BTU PER CU. FT.
900 1000 1040
MIN. SEC. MIN. SEC. MIN. SEC. MIN. SEC. MIN. SEC.
0 54 1 0 1
9 0 10 0 10 24
0 44 0 48 0 50
7 12 8 0 8 19
0 36 0 40 0 42
6 0 6 40 7
0 31 0 34 0 36
5 10 5 40 6
0 27 0 30 0 31
4 30 5 0 5 10
DRY Heating Value of Gas (BTU/FT
Time in Seconds (for 1 cu. ft.) of Gas
Gas Pressure (inch • Hg) x 520 (˚F)
Gas Temperature (˚F) x 30 (inches • Hg)

SETTING INPUT RATE

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 follow-
ing table or formula to determine
input rate. Prior to checking the fur-
nace 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. Start the furnace, in
Furnace Test Mode, 100% rate, and
measure the time required to burn
one cubic foot of gas.
The furnace is shipped from the fac-
tory with #50 orifices. They are sized
for natural gas having a heating
value of 1075 BTU/cu. ft. and a spe-
cific gravity of .60. For high-altitude
models (option 278) the furnace
comes equipped with #51 orifices
installed for elevations 5,000 to
5,999 ft. These orifices may still need
to be changed based on both eleva-
tion and gas heating value. Consult
the section of this book titled "High
Altitude Installation" for details.
Since heating values vary geographi-
cally, the manifold pressure and/or
gas orifice size may need to be
changed to adjust the furnace to its
nameplate input. The rate will also
vary with altitude. Consult the local
gas utility to obtain the yearly aver-
age heating value and orifice size
required to fire each individual burner
at 15,000 BTUH. For high altitude
installations, also consult the section
of this manual titled "High Altitude
Installations" for details on how to
calculate the correct orifice size.
1100 2500 (LP)
3 1 6 2 30
11 0 25 0
0 53 2 0
8 48 20 0
0 44 1 40
0 7 20 16 40
0 38 1 26
0 6 20 14 20
0 33 1 15
5 30 12 30
) x 3600
3
x C • F