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igniter will remain energized until flame is sensed or until
the 2-second flame proving period begins.
4. Flame-Proving - When the burner flame is proved at the
flame-proving sensor electrode FSE, the furnace control
CPU begins the blower-ON delay period and continues to
hold the gas valve GV (pin 5) open. If the burner flame is
not proved within two seconds, the furnace control CPU
will close the gas valve GV (pin 5), 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 fur-
nace 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 intermedi-
ate-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 for intermediate-heat and maximum-heat
are as follows:
Intermediate-heat - 45 seconds after the gas valve GV
(pin 5) is opened the blower motor BLWM is turned ON at
the modulating heat airflow.
Maximum-heat - 25 seconds after the gas valve GV (pin
5) is opened the BLWM is turned ON at maximum-heat air-
flow.
Simultaneously, the humidifier terminal HUM and elec-
tronic air cleaner terminal EAC-1 are energized and remain
energized throughout the heating cycle.
7. Switching from Intermediate-Heat to a Low Range In-
put - If the furnace control CPU switches from intermedi-
ate-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 mod-
ulating 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 (medi-
um 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 re-
quired.
Switching from a Intermediate-Heat to a High Range
Input - If the furnace control CPU switches from interme-
diate-heat to a high range input (high range input is an in-
put rate greater than or equal to 72% of full rate), the fur-
nace 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 in-
put to maximum-heat, the furnace control CPU will de-en-
ergize the PSR relay to close the NC contact and slowly in-
crease 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 re-
sistance. This evaluation is then used to determine the re-
quired RPM necessary to operate the inducer motor at
maximum-heat. The blower motor BLWM will transition
to maximum-heat airflow five seconds after the furnace
control CPU switches from a low range input to maxi-
mum-heat. As the inducer RPM increases, the high pres-
sure switch HPS should close.
Switching from Medium Range Input to Maxi-
mum-Heat - If the furnace control CPU switches from
medium range input to maximum-heat, the furnace control
CPU will increase the inducer motor speed to the maxi-
mum-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 in-
creases, 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
(pin 5), stopping gas flow to the burners, and humidifier
terminal HUM. The inducer motor IDM will remain ener-
gized for a 15-second post-purge period. The blower mo-
tor BLWM and air cleaner terminal EAC-1 will remain en-
ergized 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- -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 with transition back to the lower heating capacity if
the two- -stage thermostat opens the R- -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/Max-
imum Heating
See Fig. 36 and 38 for thermostat connections.
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.
93
circuits
always
causes
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