60
Fig. 92 — Timer Relay Control (TR1)
SEQUENCE OF OPERATION FOR ELECTRONIC
MODULATION WITH INDUCER SPEED CONTROL
When the system is powered up 24 VAC will be applied to the ig
-
nition control (IC) terminals 24VAC/R and to the Timer Relay
Control (TR1). The ignition control will reset, perform a self
check routine, initiate full time flame sensing, flash the diagnostics
LED for up to four seconds and enter the thermostat scan standby
state. The amber light on the TR1 will be lit indicating it is in the
ready position. See Fig. 93.
CALL FOR HEAT
1. Thermostat (heat enable) closes on call for heat.
2. 24 VAC to is supplied to IC terminal TH, provided limit
switch is in closed position.
3. The control will check that pressure switch contacts are open
(IC terminal PSW is not powered).
4. Combustion blower is then energized at high speed.
5. When the Air Switch (APS1) closes, a 15-second pre-purge
period begins.
6. At end of pre-purge period, the spark commences and the first
stage gas valve is energized for the trial for ignition period.
7. TR1 is powered (terminal 7) simultaneously (SR LED lit)
and begins timing a 90-second warm-up period while main
-
taining the combustion blower at high speed (FR LED lit) and
powers the SC30. The SC30 will output 12 to 13 VDC to the
modulating control valve during the timing duration (90 sec
-
onds) of TR1, regardless of the analog input signal to SC30
terminals 7 and 8.
8. Burners ignite and cross light, operating at the adjusted mid-
fire input rate (manifold pressure set at 1.2 - 1.5 in. wg).
9. When flame is detected by flame sensor, the spark is shutoff
immediately and gas valve(s) and combustion blower remain
energized.
10. When the initial timer in TR1 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 building tempera
-
ture controller. The SR LED turns off and the MR LED is lit.
11. If the controller is providing an analog signal between 0.5 and
5.3 VDC to the SC30 control, the system will continue to run
at low speed combustion blower and with only the first stage
valve open. The modulating valve will be powered propor
-
tional to the input voltage signal from the controller, and will
open or close changing the gas manifold pressure. Manifold
pressure will vary from 0.3 to 1.2 in. wg operating in this
mode.
12. If the signal increases above 5.3 VDC, the SC30 relay closes
powering terminal 6 on the TR1, and starts a second time
delay of 15 seconds. At the end of this time delay the fan
switches to high speed (FR LED lit) and the second stage gas
valve opens (CR LED lit) through the TR1 (terminal 9) pro
-
vided the High Air Switch contacts are closed. The manifold
pressure will vary from 1.4 to 3.5 in. wg in this mode.
13. During heating operation, the thermostat, pressure switch and
main burner flame are constantly monitored by the IC to
assure proper system operation.
14. Operation continues in the high fire mode until the controller
input signal to the SC30 control drops to 4.7 VDC. At this
point the SC30 relay circuit opens (SC30 terminal 5 has no
output) de-energizing the second stage valve and the TR1
switches the combustion blower to low speed operation.
Low-fire modulation will continue as in Step 11.
15. When the thermostat (temperature controller) is satisfied and
the demand for heat ends, the heat enable contact opens and
the first 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.
IGNITION AND OPERATIONAL FAILURES DURING A
CALL FOR HEAT RESULT IN “LOCKOUT” OF THE IGNI
-
TION CONTROL
1. If flame is lost during an operational cycle, the control will
respond within 0.8 seconds. The spark will be energized for a
trial for ignition period to attempt to relight burners and prove
flame sensor. If flame is re-established, normal operation
resumes.
2. If the burners fail to light or carryover during a trial for igni
-
tion, 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.
3. 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 IC will be de-energized shutting of the gas
valve immediately and the combustion blower following a
30-second post-purge period.
RECOVERY FROM LOCKOUT
1. 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.
ON
SR
MR
FR
CR
LIT WHEN 24-V POWER TO CONTROL
LIT DURING WARM-UP TIMER DURATION
LIT AFTER WARM-UP PERIOD TIMES OUT
LIT WHEN COMBUSTION BLOWER
OPERATING AT HIGH SPEED
LIT WHEN SECOND STAGE GAS VALVE
IS POWERED
