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Normal Start-Up Pre-Purge
The R7120 provides a pre-purge timing selectable with the
ST7800 Purge Timer cards, from two seconds to 30 minutes
with power applied and the operating control is indicating a
call for heat:
a. Running Interlocks, Pre-Ignition Interlocks, Burner
Switch, Run/Test Switch, Lockout Interlocks and all
microcomputer monitored circuits must be in the
correct operating state.
b. The blower motor output, terminal M, is powered to
start the PREPURGE sequence. The PREPURGE
timing for the R7120D1002 does not begin until the
High Fire Switch is closed.
NOTE: A jumpered High Fire Switch adds a 30-second hold
before the purge time starts.
c. The Pre-Ignition Interlock input must remain closed
throughout PREPURGE; otherwise, control returns to
the STANDBY state and holds (30 seconds) for the
R7120D2000, R7120D3007 or safety shutdown for the
R7120D1002 occurs.
d. The Lockout Interlock or Running Interlock inputs
(interlock circuit including Airflow Switch) must close
by ten seconds into PREPURGE; otherwise, a
recycle to the beginning of PREPURGE for the
R7120D2000, R7120D3007 will happen or a safety
shutdown for the R7120D1002 occurs.
e. When PREPURGE timing is complete, the firing rate
motor drives to the low fire position.
f.
When the firing rate motor reaches low fire position,
the Low Fire Switch, terminal M to D, is energized to
enter the Ignition Trial state.
NOTE: A 30-second hold occurs for a jumpered Low Fire
Switch before the ignition trial period begins.
Ignition Trials
1. Pilot Flame Establishing Period (PFEP):
a. With the firing rate motor at the low fire position:
(1) The pilot valve and ignition transformer,
terminals 5 and 6 are energized.
(a)The R7120D2000 has an interrupted or
intermittent pilot valve, terminal 6, depending
on the selection of configuration jumper 2.
(b) The R7120D1002 has a 15-second inter-
rupted pilot valve, terminal 6.
(c)The R7120D3007 has an intermittent pilot
valve, terminal 6.
NOTE: All of the R7120s have a ten-second interrupted pilot
valve/ignition, terminal 5.
(2) During PFEP, the Low Fire Switch must remain
closed. If it opens, a safety shutdown occurs.
(3) The Pre-Ignition Interlock input is ignored
throughout the Ignition Trial state.
b. Flame must be proven by the end of the 10-second
PFEP (four if JR1 is clipped) to allow the sequence
to continue. If flame is not proven by the end of
PFEP, a safety shutdown occurs.
2. Main Flame Establishing Period (MFEP):
a. Terminal 7 is energized when the presence of flame is
verified at the end of the Pilot Flame Establishing
Period (PFEP).
b. Terminal 5 is turned off 10 seconds after terminal 7
is energized.
c. Terminal 6 action:
(1) R7120D1002: De-energized 15 seconds after
terminal 7 is energized.
(2) R7120D2000:
(a)Not turned off, or
(b)15 seconds after terminal 7 is energized and
JR2 is clipped, or
(c)30 seconds after terminal 7 is energized and
terminals 8 and M are jumpered and jumper
JR2 is clipped.
(3) R7120D3007: Remain energized as long as call
for heat is present.
Run
1. Fifteen seconds after the main valve terminal 7 is
energized and flame is maintained, the R7120 goes to
Run.
2. The firing rate motor releases to modulation
(R7120D1002,2000). Damper motor is energized
(R7120D3007).
3. The R7120 is now in RUN and remains in RUN until the
controller input, terminal 13 opens, indicating that the
demand is satisfied or a limit opened.
Postpurge
The R7120 provides a 15-second POSTPURGE following the
completion of the RUN period (call for heat ends). The blower
motor output remains powered to drive all products of
combustion and any unburned fuel from the combustion
chamber. It also supplies combustion air to burn fuel being
purged from the fuel line downstream of the fuel shutoff valve.
1. The main fuel valve and intermittent pilot valve,
terminals 7 and 6, are de-energized and the firing rate
motor is commanded to the low fire position to begin the
POSTPURGE period.
2. After the 15-second POSTPURGE period is completed,
the blower motor (terminal 8) is de-energized and the
R7120 reenters Standby.
Run/Test Switch
WARNING
Explosion Hazard.
Can cause serious injury or death.
Do not use the Run/Test switch during the Pilot Flame
Establishing Period for the R7120 when using Direct
Spark Function, because it turns on the main gas
valve, causing an accumulation of fuel in the burner.
The Run/Test Switch is located on the top side of the R7120,
see Fig. 1. The Run/Test Switch allows the burner sequence
to be altered as follows:
11
R7120D BURNER CONTROL MODULES
66-1196EF—01
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