50
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. 23 or 32 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. 33.) 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--second prepurge
period.Afterthelow--heatpressure switchre--closes
thefurnacecontrolCPUwill begin a15--secondpre-
purge 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--heatpressure switch relay
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--second prepurge period
after the low--heat pressure switch LPS closes. If the
high--heatpressure switch HPS fails to close and the
low--heat pressure switch LPS closes, the furnace
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 igniter 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,thegasvalveopens,and24 vacpower issupplied
for a field--installed humidifier at the HUM terminal. The
gasvalvesolenoidGV--M permitsgasflowtotheburners
where it is ignited by the HSI. Five sec after the GVR
closes, a 2--sec flame proving period begins.The HSI ig-
niter will remain energized until the flame issensed or un-
til 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 burnerflameis proved atthe
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 continuestohold thegasvalveGV--
M open. If the burner flame is not proved within two
seconds,thecontrolCPU will close thegas valve GV--M,
and the control CPU will repeat the ignition sequence for
up to three more Trials--For--Ignition before going to Ig-
nition-- Lockout. Lockout will be reset automatically
after three hours, by momentarily interrupting 115 vac
power to the furnace, or by interrupting 24 vac power at
SEC1 orSEC2 to thefurnacecontrol CPU (not atW/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
blower--ON delay for low--heat and high--heat are as fol-
lows:
Low--Heat--45 seconds after the gas valve GV--M is en-
ergized the blower motor (BLWM) is energized at LO
HEAT speed.
High--Heat--25 secondsafter thegas valve GV--M is en-
ergized the BLWM is energized at HI HEAT speed. Sim-
ultaneously, the electronic air cleaner (EAC--1) terminal
is energized and remains energized as long as the BLWM
is energized.
f. SwitchingfromLow--toHigh--Heat--Ifthefurnacecon-
trol CPU switches from low--heat to high--heat, the fur-
nace control CPU will switch the inducer motor IDM
speed from low to high. The high--heat pressure switch
relay HPSR is de--energized to close the NC contact.
When sufficient pressure is available the high--heat pres-
sure switch HPS closes, and the high--heat gas valve
solenoid GV--HI isenergized. The blower motor BLWM
will switch to HI HEAT speed five seconds after the fur-
nace control CPU switches 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
whilethethermostat R--to--W circuit is closed when using
a single 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 gasflowtotheburners, andde--energiz-
ing the humidifier terminal HUM. The inducer motor
IDM will remain energized for a 5--second post--purge
period. The blower motorBLWM andair cleanertermin-
al EAC--1 will remain energized for 90, 120, 150, or 180
seconds (depending on selection at blower--OFF delay
switches). The furnace control CPU is factory--set for a
120--second blower--OFF delay.
2. Two--Stage Thermostat and Two--Stage Heating (See Fig.
31 for thermostat connections)
NOTE: In this mode the LHT must be ON to select the low--heat
only operation mode in response to closing the thermostat
R--to--W1 circuit. Closing the thermostat R--to--W1--and--W2
circuits always causes high--heat operation, regardless of the
setting of the low--heat--only switch.
The wall thermostat “calls for heat”, closing the R--to--W1 circuit
for low--heat or closing the R--to--W1 and--W2 circuits for
high--heat. 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.
58CT
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