57
follows:
Low-- heat -- 45 seconds after the gas valve GV --M is
opened the blower motor BLWM is turned ON at
low--heat airflow.
High-- heat -- 25 seconds after the gas valve GV--M is
opened the BLWM is turned ON at high --heat airflow.
Simultaneously, the humidifier terminal HUM and
electronic air cleaner terminal EAC--1 are energized
and remain energized throughout the heating cycle.
f. 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 transition to high--heat
airflow five seconds after the furnace control CPU
switches from low--heat to high --heat.
g. Switching fr om High-- to Low--Heat --The furnace
control CPU will not switch from high--heat to
low--heat while the thermostat R--to--W circuit is
closed when using a singlestage 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 remain energized for a
5-- second post-- purge period. The blower motor
BLWM and air cleaner terminal EAC--1 will remain
energized at low--heat airflow or transition to low--heat
airflow 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. 28 -- 35 for thermostat connections.
NOTE: In this mode the low--heat only switch SW1--2 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 shut down functions and delays described in
item 1. above apply to the 2 --stage heating mode as well, except
for switching from low-- to high-- Heat and vice versa.
a. Switching fro m Low-- to High-- Heat -- I f t h e
thermostat Rto-- W1 circuit is closed and the R--to--W2
circuit c loses, 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 transition to
high-- heat airflow five seconds after the R--to-- W2
circuit closes.
b. Switching fr om High-- to Low--Heat -- I f t h e
thermostat Rto-- W2 circuit opens, and the R--to--W1
circuit remains closed, the furnace control CPU will
switch the inducer motor IDM speed from high to low.
The high--heat pressure switch relay HPSR is
energized to open the NC contact and de--energize the
high-- heat gas valve solenoid GV-- HI.When the
inducer motor IDM reduces pressure sufficiently, the
high--heat pressure switch HPS will open. The gas
valve solenoid GV--M will remain energized as long as
the low--heat pressure switch LPS remains closed. The
blower motor BL WM will transition to low--heat
airflow five seconds after the R--to --W2 circuit opens.
3. Cooling mode
The thermostat “calls for cooling”.
a. Single--Speed Cooling --
See Fig. 28 for thermostat connections
The thermostat closes the R--to-- G--and --Y circuits. The
R--to-- Y circuit starts the outdoor unit, and the
R--to--G--and--Y/Y2 circuits start the furnace blower
motor BLWM on cooling a irflow. Cooling airflow is
basedontheA/CselectionshowninFig.63.The
electronic air cleaner terminal EAC--1 is energized
with 115 vac when the blower motor BLWM is
operating.
When the thermostat is satisfied, the R--to--G--and--Y
circuits are opened. The outdoor unit will stop, and the
furnace blower motor BLWM will continue operating
at cooling airflow for an additional 90 seconds.
Jumper Y/Y2 to DHUM to reduce the cooling
off--delay to 5 seconds. (See Fig. 27.)
b. Single--Stage Thermostat and Two--Speed Cooling
(Adaptive Mode) --
See Fig. 35 for thermostat connections.
This furnace can operate a two--speed cooling unit
with a single--stage thermostat because the furnace
control CPU includes a programmed adaptive
sequence of controlled operation, which selects
low--cooling or high--cooling operation. This selection
is based upon the stored history of the length of
previous cooling period of the single-- stage thermostat.
NOTE: The air conditioning relay disable jumper ACRDJ must
be connected to enable the adaptive cooling mode in response to
a call for cooling. (See Fig. 27.) When ACRDJ is in place the
furnace control CPU can turn on the air conditioning relay ACR
to energize the Y/Y2 terminal and switch the outdoor unit to
high-- cooling.
The furnace control CPU can start up the cooling unit in either
low-- or high--cooling. If starting up in low-- cooling, the furnace
control CPU determines the low--cooling on--time (from 0 to 20
minutes) which is permitted before switching to high--cooling. If
the power is interrupted, the stored history is erased and the
furnace control CPU will select low-- cooling for up to 20 minutes
and then energize the air conditioning relay ACR to energize the
Y/Y2 terminal and switch the outdoor unit to high-- cooling, as
long as the thermostat continues to call for cooling. Subsequent
selection is based on stored history of the thermostat cycle times.
The wall thermostat “calls for cooling”, closing the
R--to-- G--and-- Y circuits. The R--to --Y1 circuit starts the outdoor
unit on low--cooling speed, and the R--to --G--and--Y1 circuits
starts the furnace blower motor BLWM at low--cooling airflow
which is the true on-- board CF selection as shown in Fig. 63.
If the furnace control CPU switches from low--cooling to
high--cooling, the furnace control CPU will energize the air
conditioning relay ACR. When the air conditioning relay ACR is
energized the R--to--Y1--and-- Y2 circuits switch the outdoor unit
to high-- cooling speed, and the R--to --G--and--Y1--and-- Y/Y2
circuits transition the furnace blower motor BLWM to
high-- cooling airflow. High-- cooling airflow is based on the A/C
selectionshowninFig.63.
NOTE: When transitioning from low--cooling to high --cooling
the outdoor unit compressor will shut down for 1 minute while
315A
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