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SERVICE AND TECHNICAL SUPPORT MANUAL
close the gas valve GV, and the furnace 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, or
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 at full speed until
flame is no longer proved.
5. Inducer Speed Change - If the cycle starts in
intermediate-heat, the furnace control CPU reduces the
inducer speed slightly after flame sense. If cycle starts in
maximum-heat, the furnace control CPU increases the
inducer speed after flame sense.
6. Blower-On delay — If the burner flame is proven, the
blower-ON
delay
maximum-heat are as follows:
Intermediate-heat - 45 seconds after the gas valve GV
is opened the blower motor BLWM is turned ON at the
modulating heat airflow.
Maximum-heat - 25 seconds after the gas valve GV is
opened the BLWM is turned ON at maximum-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 Intermediate-Heat to a Low Range
Input - If the furnace control CPU switches from
intermediate-heat to a low range input (low range input
is an input rate less than or equal to 51% of full rate),
the furnace control CPU will turn the blower ON or switch
to the modulating heat airflow, energize the PSR relay to
open the NC contact, and slowly decrease the inducer
motor speed to the desired inducer RPM.
Switching from a Intermediate-Heat to a Medium
Range Input - If the furnace control CPU switches from
intermediate-heat to a different medium range input
(medium range input is an input rate between 52% and
71% of full rate), the furnace control CPU will turn the
blower ON or switch to the modulating heat airflow, and
continue to maintain the inducer motor speed or change
it further if required.
Switching from a Intermediate-Heat to a High Range
Input - If the furnace control CPU switches from
intermediate-heat to a high range input (high range input
is an input rate greater than or equal to 72% of full
rate), the furnace control CPU will turn the blower ON or
switch to the modulating heat airflow, and increase the
inducer motor speed to the desired inducer motor RPM.
Switching from Low Range Input to Maximum-Heat -
If the furnace control CPU switches from a low range
input to maximum-heat, the furnace control CPU will
de-energize the PSR relay to close the NC contact and
slowly increase the inducer motor speed until the
medium pressure switch MPS closes. When the medium
pressure switch MPS closes 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 at maximum-heat.
blower motor BLWM will transition to maximum-heat
airflow five seconds after the furnace control CPU
switches from a low range input to maximum-heat. As
the inducer RPM increases, the high pressure switch
HPS should close.
Switching
from
Medium
Maximum-Heat - If the furnace control CPU switches
440 04 4201 01
for
intermediate-heat
and
The
Range
Input
Specifications subject to change without notice.
from medium range input to maximum-heat, the furnace
control CPU will increase the inducer motor speed to the
maximum-heat inducer motor RPM. The blower motor
BLWM will transition to the maximum-heat airflow five
seconds after the furnace control CPU switches from a
medium range input to maximum-heat. As the inducer
RPM increases, the high pressure switch HPS should
close.
8. Blower-Off Delay — When the thermostat is satisfied,
the R to W circuit is opened, de-energizing the gas
valve GV, stopping gas flow to the burners, 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 minimum-heat airflow or
transition to minimum-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 Modulating Heating
(Adaptive Mode)
Operation of Modulating Heating (Adaptive Mode; SW1-2 and
SW4-2 are both OFF) with a two-stage thermostat is the same
as with a single-stage thermostat, EXCEPT for the following:
If the two-stage thermostat R to W2 circuit closes along with the
R to W/W1 circuit, while the furnace is operating at any
capacity less than maximum, the furnace will switch to the
maximum heating capacity. At this point, the furnace control
heating algorithm is overridden and the two-stage thermostat
takes control of staging until BOTH R to W/W1 and R to W2
circuits are opened.
The furnace will transition back to the lower heating capacity if
the two-stage thermostat opens the R to W2 circuit, but leaves
R to W/W1 circuit closed. The furnace control will continue to
follow the commands from the two-stage thermostat for W/W1
and W2, until the R to W/W1 circuit is opened.
After the two-stage thermostat opens both the R to W/W1 and
R to W2 circuits, the adaptive algorithm will set the beginning
capacity of the next heating cycle to the same value as the
heating cycle that most recently completed.
Two-Stage Thermostat and Two-Stage
Minimum/Maximum Heating
NOTE: In this mode, the minimum-heat only switch SW1-2
must be ON to select the minimum-heat only operation mode in
response to closing the thermostat R to W1 circuit. Closing the
thermostat R
to
W1-and-W2
maximum-heat operation, regardless of the setting of the
minimum-heat only switch.
The furnace will start up in either intermediate-, or
maximum-heat. The furnace will operate in minimum-heat after
starting and operating for 1 minute at intermediate-heat before
transitioning to minimum-heat.
The wall thermostat "calls for heat", closing the R to W1 circuit
for minimum-heat or closing the R to W1-and-W2 circuits for
maximum-heat. The furnace control performs a self-check,
and verifies the low, medium and high pressure switch
contacts LPS, MPS and HPS are open, then de-energizes the
PSR relay to close the NC contact.
The start up and shut down functions and delays described
above apply to the two-stage minimum/maximum heating mode
as well, except for switching from maximum- to minimum-heat.
1. Switching from Maximum- to Minimum-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
intermediate-heat RPM. When the inducer motor IDM
to
reduces pressure sufficiently, the high pressure switch
Gas Furnace: (F/G)9MAC
circuits
always
causes
29
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