80
SEQUENCE OF OPERATION
NOTE
: Furnace control must be grounded for proper operation or
control will lockout. Control is grounded through green wire
connected to gas valve and burner bracket screw. Using the
schematic diagram in Fig. 71, follow the sequence of operation
through the different modes. Read and follow the wiring diagram
very carefully.
NOTE
: If a power interruption occurs during a call for heat
(W/W1 or W/W1--and--W2), the control run the blower for the
selected blower off--delay period two sec after power is restored
and if the thermostat is still calling for gas heating. The LED light
will flash code 12 during this period, after which the LED will be
ON continuous, as long as no faults are detected. After this period,
the furnace will respond to the thermostat normally.
The blower door must be installed for power to be conducted
through the blower door interlock switch ILK to the furnace
control CPU, transformer TRAN, inducer motor IDM, blower
motor BLWM, hot--surface igniter HSI, and gas valve GV..
1.
Two--Stage Heating (Adaptive mode) with Single--Stage
Thermostat
See Fig. 38 and 39 for thermostat connections)
NOTE
:
The low--heat only switch (LHT) selects either the
low--heat only operation mode when ON, (see item 2. below) or
the adaptive heating mode when OFF in response to a call for heat.
See Fig. 38. When the W2 thermostat terminal is energized it will
always cause high--heat operation when the R to W circuit is
closed, regardless of the setting of the low--heat only switch. This
furnace can operate as a two--stage furnace with a single--stage
thermostat because the furnace control CPU includes a
programmed adaptive sequence of controlled operation, which
selects low--heat or high--heat operation. This selection is based
upon the stored history of the length of previous gas--heating
periods of the single--stage thermostat.
The furnace will start up in either low-- or high--heat. If the furnace
starts up in low--heat, the control CPU determines the low--heat
on--time (from 0 to 16 minutes) which is permitted before
switching to high--heat.
If the power is interrupted, the stored history is erased and the
control CPU will select low--heat for up to 16 minutes and then
switch to high--heat, as long as the thermostat continues to call for
heat. Subsequent selection is based on stored history of the
thermostat cycle times.
The wall thermostat “calls for heat”, closing the R to W circuit. The
furnace control performs a self--check, verifies the low--heat and
high--heat pressure switch contacts LPS and HPS are open, and
starts the inducer motor IDM in high--speed.
a. Inducer Prepurge Period
(1.) If the furnace control CPU selects low--heat opera-
tion the inducer motor IDM comes up to speed, the
low--heat pressure switch LPS closes, and the fur-
nace control CPU begins a 15--sec prepurge period.
After the low--heat pressure switch re--closes the
furnace control CPU will begin a 15--sec prepurge
period, and continue to run the inducer motor IDM
at high--speed.
(2.) If the furnace control CPU selects high--heat opera-
tion, the inducer motor IDM remains running at
high--speed, and the high--heat pressure switch re-
lay HPSR is de--energized to close the NC contact.
When sufficient pressure is available the high--heat
pressure switch HPS closes, and the high--heat gas
valve solenoid GV--HI is energized. The furnace
control CPU begins a 15--sec prepurge period after
the low--heat pressure switch LPS closes. If the
high--heat pressure switch HPS fails to close and
the low--heat pressure switch LPS closes, the fur-
nace will operate at low--heat gas flow rate until the
high--heat pressure switch closes for a maximum of
2 minutes after ignition.
b.
Igniter Warm--Up
--At the end of the prepurge period, the
(Hot Surface Igniter) HSI is energized for a 17--second ig-
niter warm--up period.
c.
Trial--for--Ignition Sequence
--When the igniter warm--up
period is completed the main gas valve relay contacts
GVR--1 and --2 close to energize the gas valve solenoid
GV--M, the gas valve opens, and 24 vac power is supplied
for a field--installed humidifier at the HUM terminal. The
gas valve solenoid GV--M permits gas flow to the burners
where it is ignited by the HSI. Five sec after the GVR clos-
es, a 2--sec flame proving period begins. The HSI igniter
will remain energized until the flame is sensed or until the
2--sec flame proving period begins. If the furnace control
CPU selects high--heat operation, the high--heat gas valve
solenoid GV--HI is also energized.
d.
Flame--Proving
--When the burner flame is proved at the
flame--proving sensor electrode FSE, the inducer motor
IDM switches to low--speed unless running at high--speed,
and the furnace control CPU begins the blower--ON delay
period and continues to hold the gas valve GV--M open.
If the burner flame is not proved within two sec, the control
CPU will close the gas valve GV--M, and the control CPU
will repeat the ignition sequence for up to three more Tri-
als--For--Ignition before going to Ignition-- Lockout. Lock-
out will be reset automatically after three hours, by momen-
tarily interrupting 115 vac power to the furnace, or by
interrupting 24 vac power at SEC1 or SEC2 to the furnace
control CPU (not at W/W1, G, R, etc.). If flame is proved
when flame should not be present, the furnace control CPU
will lock out of Gas--Heating mode and operate the inducer
motor IDM on high speed until flame is no longer proved.
e.
Blower--ON Delay
--If the burner flame is proven the blow-
er--ON delay for low--heat and high--heat are as follows:
Low--Heat
--45 sec after the gas valve GV--M is energized
the blower motor (BLWM) is energized at LO HEAT
speed.
High--Heat
--25 sec after the gas valve GV--M is energized
the BLWM is energized at HI HEAT speed. Simultaneous-
ly, the electronic air cleaner (EAC--1) terminal is energized
and remains energized as long as the BLWM is energized.
f.
Switching from Low--to High--Heat
--If the furnace con-
trol CPU switches from low--heat to high--heat, the furnace
control CPU will switch the inducer motor IDM speed
from low to high. The high--heat pressure switch relay HP-
SR is de--energized to close the NC contact. When suffi-
cient pressure is available the high--heat pressure switch
HPS closes, and the high--heat gas valve solenoid GV--HI
is energized. The blower motor BLWM will switch to HI
HEAT speed five sec after the furnace control CPU switch-
es from low--heat to high--heat.
g.
Switching from High-- to Low--Heat
--The furnace con-
trol CPU will not switch from high--heat to low--heat while
the thermostat R--to--W circuit is closed when using a sin-
gle stage thermostat.
h.
Blower--OFF Delay
--When the thermostat is satisfied, the
R to W circuit is opened, de--energizing the gas valve GV--
M, stopping gas flow to the burners, and de--energizing the
humidifier terminal HUM. The inducer motor IDM will re-
main energized for a 15--sec post--purge period. The blow-
er motor BLWM and air cleaner terminal EAC--1 will re-
main energized for 90, 120, 150, or 180 sec (depending on
selection at blower--OFF delay switches). The furnace con-
trol CPU is factory--set for a 120--sec blower--OFF delay.
PG95X
AT