Specifications

IM 685-2 11
Typical Sequence of Operation
When the rooftop unit is energized 120 volt power is sup-
plied to the system on/off switch (S1), to burner on/off
switch (S3) and 24 volts to the (BO#11) contacts on the main
control board (MCB).
Note: On field supplied controls, 120V power is supplied
through the system on/off switch (S1) to burner on/off
switch (S3) and to the field supplied gas heat on/off
contacts.
Burner on/off switch (S3) will power the modulating gas
valve actuator (VM1) and terminal #5 (L1) on the flame
safeguard (FSG). Upon a call for heat, the control system
will close (BO#11) on the main control board (MCB), thus
energizing relay (R20). When 120V power is furnished
through the system on/off switch (S1), through the burner
on/off switch (S3), through relay (R20) contacts, through the
high limit control (FLC) and through the optional automatic
reset low gas pressure switch (LP5) and the optional manual
reset high gas pressure switch (HP5), terminal #6 on the
flame safeguard (FSG) is powered. The flame safeguard then
energizes its terminal #4, which powers the burner combus-
tion air blower motor (BM). Whenever power is restored to
the flame safeguard, the flame safeguard will go through a
10 second initiation period before the prepurge period will
begin.
The burner air control valve will be at minimum position
during off cycles. Upon a call for heat or any other time that
a prepurge cycle occurs the air control valve will be re-posi-
tioned to the maximum position for the prepurge and then
returned to the minimum position for low fire start.
(VM1), through the N/C contacts of (R20) and (R23), posi-
tions the burner air and gas control valves to minimum after
run cycle. When (R20) is energized for a new call for heat,
(VM1) through the N/O contacts of (R20) and the N/C con-
tacts of (R21), will re-position the burner air valve to its
maximum open position for pre-purge. When the air control
valve reaches the full open position switch (LS2) is 'made',
powering (FSG) terminal #7 through the burner air switch
(AS). This initiates the 60 second prepurge cycle. Concur-
rently, (LS2) powers timer (TD10) which will energize relay
(R21) after 20 seconds. When (R21) is energized (VM1) will
start the air control valve on its way toward the minimum air
valve position through the N/O contact of (R21) and the N/C
contact of (R23). At the completion of the 60 second
prepurge cycle the valve will be at the minimum open posi-
tion and the minimum position switch (LS1) will be
“MADE”. If (LS1) is not “MADE” the combination gas con-
trol start valves (GV1) will not open and the burner will go
out on safety lockout.
After completion of the 60 second prepurge period there will
be a 10 second trial for ignition during which terminal #8
(combination gas valve - GV1) and terminal #10 (ignition
transformer - IT) will be energized. If flame is being
detected through the flame rod (FD) at the completion of the
10 second trial for ignition period terminal #10 (ignition
transformer - IT) will be de-energized and terminal #9 (relay
R23 coil and main gas valves - GV4 and GV5) will be ener-
gized and the control system will be allowed to control the
firing rate. The flame safeguard contains 'LEDS' (lower left
corner) that will glow to indicated operation.
After the flame has lit and been proven, relay (R23) is ener-
gized allowing (VM1), as controlled by (BO#9) and
(BO#10) on the main control board (MCB) to position the
burner air and gas valves for the required firing rate. When
the main control system closes (BO#10) the gas valve actua-
tor will re-position toward a higher firing rate until (BO#10)
opens or the actuator reaches its maximum position. When
the main control system closes (BO#9), the actuator will re-
position toward a lower firing rate. If neither (BO#9) or
(BO#10) on the main control board (MCB) are closed, the
actuator will remain at its present position. The heating
capacity is monitored by the main control board (MCB)
through (AI #10) via a position feedback potentiometer on
the actuator.
In the event the flame fails to ignite or the flame safeguard
fails to detect its flame within 10 seconds, terminals #4, 8, 9,
and 10 will be de-energized, thus de-energizing the burner
and terminal #3 will become energized. The flame safe-
guard would then be on safety lockout and would require
manual resetting. Terminal #3 will energize the heat alarm
relay (R24), which would then energize the remote 'HEAT
FAIL' indicator light and send a fail signal to binary input #5
in MicroTech II main control board (MCB).
If an attempt is made to restart the burner by resetting the
flame safeguard, or if an automatic restart is initiated after
flame failure the earlier described prepurge cycle with the
wide open air valve will be repeated.
If the unit overheats, the high limit control (FLC) will cycle
the burner, limiting furnace temperature to the limit control
set point.
TEST
CONN.
BURNER
BLOWER
AIR
SWITCH
MODULATING
ACTUATOR
V
v
TEST
COCK
VENT
(OPT.)
HP5
HI PRESS.
SWITCH
COMBINATION GAS CONTROLS
GV1B
GV1A
GV4B
GV4A
GV5B GV5A
(OPT.)
CAPPED LEAK
TEST COCK
GV1 - HT 020 TO 200
GV4 - HT 040 TO 200
GV5 - HT 079 TO 200
GV6 - HT 110 TO 200
GV7 - HT 140 TO 200
GV8 - HT 200 ONLY
VENT TO
ATMOSPHERE
VENT
(OPT.)
GV3
SAFETY
VALVE
(OPT.)
GV2
SAFETY
VALVE
(OPT.)
N.O. VENT
VALVE
(OPT.)
LP5
LO PRESS.
SWITCH
(REQ'D FOR
OVER .50 PSI)
(OPT.)
HIGH PRESS.
REG.
SHUTOFF
COCK
VENT
Figure 6b. Typical Piping Schematic