49
5. Inducer Speed Change — If the cycle starts in medium heat,
the furnace control CPU reduces the inducer speed slightly
after flame sense. If cycle starts in high--heat, the furnace
control CPU increases the inducer speed after flame sense.
The reduction in speed in medium--heat is to optimize
combustion for maximum efficiency.
6. Blower--On delay — If the burner flame is proven the
blower--ON delay for medium--heat and high--heat are as
follows:
Medium--heat — 60 seconds after the gas valve GV--M is
opened the blower motor BLWM is turned ON at low-- or
medium--heat airflow.
High--heat — 35 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.
7.
Switching from Medium-- to Low--Heat
— If the furnace
control switches from medium--heat to low--heat, the
furnace control will turn the blower ON at low--heat airflow,
energize the HPSR relay to open the NC contact, and slowly
decrease the inducer motor speed. When the HPSR relay is
energized and the NC contact opens the throttling valve TV
is deenergized and the gas flow reduces to low--heat rate.
Switching from Low-- to Medium--Heat
— If the furnace
control CPU switches from low--heat to medium--heat, the
furnace control CPU will de--energize the HPSR relay to
close the NC contact and slowly increase the inducer motor
speed until the medium--heat pressure switch MPS closes.
When the medium--heat pressure switch MPS closes, the
throttling valve solenoid TV is energized and the inducer
motor RPM is noted by the furnace control CPU. The RPM
is used to evaluate vent system resistance. This evaluation is
then used to determine the required RPM necessary to
operate the inducer motor in medium--heat and high--heat
mode. The blower motor BLWM will transition to
medium--heat airflow five seconds after the furnace control
CPU switches from low--heat to medium--heat.
Switching from Low-- to High--Heat
— If the furnace
control CPU switches from low--heat to high--heat, the
furnace control CPU will de--energize the HPSR relay to
close the NC contact and slowly increase the inducer motor
speed until the medium--heat pressure switch MPS closes.
When the medium--heat pressure switch MPS closes, the
throttling valve solenoid TV is energized and the inducer
motor RPM is noted by the furnace control CPU. The RPM
is used to evaluate vent system resistance. This evaluation is
then used to determine the required RPM necessary to
operate the inducer motor in medium-- and high--heat mode.
The blower motor BLWM will transition to high--heat
airflow five seconds after the furnace control CPU switches
from low--heat to high--heat. As the inducer RPM gradually
increases the high--heat pressure switch HPS closes and the
gas valve solenoid GV--HI is energized.
Switching from Medium-- to High--Heat
— If the furnace
control CPU switches from medium--heat to high--heat, the
furnace control CPU will gradually increase the inducer
motor speed to the required high--heat RPM. The blower
motor BLWM will transition to high--heat airflow five
seconds after the furnace control CPU switches from
medium--heat to high heat. As the inducer RPM gradually
increases the high--heat pressure switch HPS closes and the
gas valve solenoid GV--HI is energized.
Switching from High-- to Medium-- or Low--Heat
— The
furnace control CPU will not switch from high--heat to
medium-- or low--heat while the thermostat R to W circuit is
closed when using a single--stage thermostat.
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 throttling valve TV, and 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 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.
Two--Stage Thermostat and Two--Stage Low / High
Heating
See Fig. 58 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 furnace will start up in either medium--, or high--heat. The
furnace will operate in low--heat after starting and operating for 1
minute at medium--heat before transitioning to low--heat.
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, and verifies the
low--heat and medium--heat pressure switch contacts LPS and MPS
are open, then de--energizes the HPSR relay to close the NC
contact.
The start up and shut down functions and delays described above
apply to the 2--stage low/high heating mode as well, except for
switching from high-- to low--heat.
1.
Switching from High-- to Low--Heat
— If the thermostat
R to W2 circuit opens, and the R to W1 circuit remains
closed, the furnace control CPU will gradually decrease the
inducer motor speed to the required medium--heat RPM.
When the inducer motor IDM reduces pressure sufficiently,
the high heat pressure switch HPS will open and the
high--heat gas valve solenoid GV--HI will be de--energized.
The gas valve solenoid GV--M will remain energized as
long as the low--heat pressure switch LPS remains closed.
When the inducer motor speed gets within 15% of the
required medium--heat RPM the furnace control CPU will
start a 5 second blower airflow change delay. After the 5
second blower airflow change delay is completed the
blower airflow will transition to low--heat airflow. At this
point the furnace control CPU will energize the HPSR relay
to open the NC contact and slowly decrease the inducer
motor speed to the required low--heat RPM. When the
HPSR relay is energized and the NC contact opens the
throttling valve TV is de--energized and the gas flow
reduces to low--heat rate. When the inducer motor IDM
reduces pressure sufficiently, the medium--heat pressure
switch MPS will open.
Two--Stage Thermostat and Two--Stage Medium/High
Heating
See Fig. 58 for thermostat connections.
NOTE
: In this mode the medium--heat only switch SW4--2 must
be ON to select the medium--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 medium--heat only switch.
The wall thermostat ”calls for heat”, closing the R to W1 circuit for
medium--heat or closing the R to W1--and--W2 circuits for
high--heat. The furnace control performs a self--check, and verifies
the low--heat and medium--heat pressure switch contacts LPS and
MPS are open, then de--energizes the HPSR relay to close the NC
contact.
58MV
C
