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a 30 second post-purge period. If the thermostat
(controller) is still calling for heat one hour after a
lockout occurs, the control will automatically reset and
initiate a call for heat sequence.)
7. Burners ignite and cross light. Timer relay control
is powered (terminal 7) simultaneously and begins
timing a 90 second warm-up period while maintaining
the combustion blower at high speed. The timer relay
control will maintain this mode of operation, regardless
of status of thermostat second stage.
8. When flame is detected by flame sensor, the spark is
shut-off immediately and gas valve(s) and combustion
blower remain energized.
9. When the initial timer in timer relay control times out,
it defaults the gas valve to low fire and the combustion
blower to low speed and returns control of the
operating mode to the temperature controller.
10. If the controller is calling for second stage heat timer
relay control terminal 6 is powered. After a short time
delay (approximately 15 seconds), the system switches
the combustion blower to high speed and the 2nd stage
gas valve @ 3.5” w.c. (8.7 mbar) manifold pressure,
provided the high air pressure switch is proved.
11. During heating operation, the thermostat, pres-
sure switch and main burner flame are constantly
monitored by the ignition control to assure proper
system operation.
12. Operation continues on high fire until the 2nd stage
thermostat is satisfied, opening the 2nd stage contact
and de-energizes terminal 6 on the timer relay control,
turning off the 2nd stage gas valve and returning the
combustion blower to low speed.
13. When the thermostat (controller) is satisfied and the
demand for heat ends, the 1st stage valve is de-
energized immediately, the control senses loss of
flame and a 30 second post- purge occurs (at high
speed) before de-energizing the combustion blower.
11.5.2 Sequence of Operation for Two-Stage
Furnace with 250-400 MBH (73.3-117.2 kW) Input
When system is powered up 24VAC will be applied to the
ignition control (ignition control) terminals 24VAC / R and
to the timer relay control.
The ignition control will reset, perform a self check
routine, flash the diagnostic LED for up to four seconds
and enter the thermostat scan standby state. The amber
light on the timer relay control will be lit indicating it is in
the ready position.
Call for Heat:
1. Controller provides contact closure (1st and 2nd
stage) on call for heat. 24VAC to is supplied to
ignition control terminal TH, provided limit switch is
in closed position.
2. The control will check that pressure switch con-
tacts are open (ignition control terminal PSW is not
powered). Combustion blower is then energized at
high speed. When the airflow switch closes, a 15
second pre-purge period begins.
3. At end of pre-purge period, the spark commences
and the 1st stage gas valve is energized for the trial for
ignition period. (If the burners fail to light or carryover
during a trial for ignition, the control will attempt two
additional ignition trials. If no flame is present at the
flame sensor within 10 seconds, the spark and gas
valve will be de-energized. A 15 second inter- purge
period begins and the combustion blower continues to
run. After the inter-purge period another ignition trial
will take place. If burner fails to light or prove the flame
sensor following the two additional trials the control
will go into lockout. The valve relay in the ignition
control will be de-energized shutting off the gas valve
immediately and the combustion blower following
a 30 second post-purge period. If the thermostat
(controller) is still calling for heat one hour after a
lockout occurs, the control will automatically reset and
initiate a call for heat sequence.)
4. Burners ignite and cross light. Timer relay control
is powered (terminal 7) simultaneously and begins
timing a 90 second warm-up period while maintaining
the combustion blower at high speed. The timer relay
control will maintain this mode of operation, regardless
of status of thermostat 2nd stage.
5. When flame is detected by flame sensor, the spark is
shut-off immediately and gas valve(s) and combustion
blower remain energized.
6. When the initial timer in timer relay control times out,
it defaults the gas valve to low fire and the combustion
blower to low speed and returns control of the
operating mode to the temperature controller.
7. If the controller is calling for 2nd stage heat timer relay
control terminal 6 is powered. After a short time delay
(approximately 15 seconds), the system switches the
combustion blower to high speed and the 2nd stage
gas valve @ 3.5” w.c. (8.7 mbar) manifold pressure,
provided the high air pressure switch is proved.
8. During heating operation, the thermostat, pres-
sure switch and main burner flame are constantly
monitored by the ignition control to assure proper
system operation.
9. Operation continues on high fire until the 2nd stage
thermostat is satisfied, opening the 2nd stage contact
and de-energizes terminal 6 on the timer relay control,
turning off the 2nd stage gas valve and returning the
combustion blower to low speed.
10. When the thermostat (controller) is satisfied and the
demand for heat ends, the 1st stage valve is de-
energized immediately, the control senses loss of
flame and a 30 second post- purge occurs (at high
speed) before de-energizing the combustion blower.
PR Series Installation, Operation, and Maintenance Manual
