49
TWO-STAGE HEATING WITH SINGLE-STAGE
THERMOSTAT (ADAPTIVE MODE)
(See Fig. 31 for thermostat connections.)
NOTE
:
Low-heat-only switch, SW-1, selects either the
low-heat-only operation mode when ON, or adaptive heating
mode when OFF, in response to a call for heat. (See Fig. 37.)
This furnace can operate as a 2-stage furnace with a single-stage
thermostat because furnace control CPU includes a programmed
adaptive sequence of controlled operation, which selects low-gas-
heat or high-gas-heat operation. This selection is based upon the
stored history of the length of previous gas heating on/off periods
of the single-stage thermostat.
The furnace will start up in either low-or high-gas heat. If the
furnace starts up in low-gas heat, the control CPU determines the
low-gas heat on time (from 0 to 16 minutes) which is permitted
before switching to high-heat.
If power is interrupted, the stored history is erased. When this
happens, the control CPU will initially 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 thermostat cycle times.
When wall thermostat “calls for heat”, R-W/W1 circuit closes.
The furnace control performs a self check, verifies the low-heat
and high-heat pressure switch contacts LPS and HPS are open,
and starts inducer motor IDM in high speed.
1.
Inducer Prepurge Period
-- If the furnace control CPU
selects low-heat operation the inducer motor IDM comes
up to speed, the low-heat pressure switch LPS closes, and
the furnace control CPU begins a 15-second prepurge
period. After the low-heat pressure switch re-closes the
furnace control CPU will begin a 15-second prepurge
period, and continue to run the inducer motor IDM at
high-speed.
If the furnace control CPU selects high-heat operation, the
inducer motor IDM remains running at high-speed, and the
high-heat pressure 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-heat pressure 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.
2.
Igniter Warm-Up
-- At end of the inducer prepurge
period, the Hot Surface Igniter HSI is energized for a
17-sec igniter warm-up period.
3.
Trial-For-Ignition Sequence
-- When the igniter warm-up
period is completed the main gas valve relay contacts
GVR close to energize the gas valve GV, the gas valve
opens. The gas valve GV permits gas flow to the burners
where it is ignited by the Hot Surface Igniter HSI. Five
seconds after the GVR closes, a 2-second flame period
begins. The HIS igniter will remain energized until the
flame is sensed or until the 2-second flame proving
period begins.
4.
Flame-Proving
-- When 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 seconds, the
control CPU will close the gas valve GV-M, and the
control CPU will repeat the ignition sequence for up to
three
more
Trials-For-Ignition
before
going
to
Ignition-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 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.
5.
Blower-On Delay
-- If burner flame is proven the blower
motor BLWM is energized 66 sec after gas valve GV-M is
opened.
Low-heat
-- The blower motor BLWM is energized at LO
HT speed.
High-heat
-- The BLWM is energized at HI HT speed.
Simultaneously, the electronic air cleaner terminal EAC-1
is energized and remains energized as long as the blower
motor BLWM is energized.
6.
Switching From Low- To High-Heat
-- If the furnace
control 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 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 blower motor BLWM will switch
to HI HT speed five seconds after the furnace control CPU
switches from low-heat to high-heat.
7.
Switching From High- To Low-Heat
-- The control CPU
will not switch from high-heat to low-heat while the
thermostat R-to-W circuit is closed when a single-stage
thermostat is used.
8.
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 remain energized for a 15-second post-purge period.
The blower motor BLWM and air cleaner terminal 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.
TWO-STAGE HEATING WITH TWO-STAGE
THERMOSTAT (NON-ADAPTIVE HEATING MODE)
(See Fig. 32 and 50--56 for thermostat connections).
NOTE
: In this mode, the low-heat only switch 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.
The start-up and shutdown functions and delays described in
item 1 above apply to 2-stage heating mode as well, except for
switching from low-to high-heat and vice versa.
1.
Switching From Low- To High-Heat
-- If the thermostat
R to W1 circuit is closed and the R to W2 circuit closes,
the furnace 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
pressure switch HPS closes, and the high-heat gas valve
solenoid GV-HI is energized. The blower motor BLWM
will switch to HI HT speed five seconds after the R to W2
circuit closes.
