Bryant 987MA Installation, Start-Up, Operating And Service And Maintenance Instructions page 91

CPU will repeat the ignition sequence for up to three more
Trials-For-Ignition before going to Ignition-Lockout. Lock-
out will be reset automatically after three hours, or by mo-
mentarily 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 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 intermedi-
ate-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 mod-
ulating 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-ener-
gize 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 res-
istance. 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 maxim-
um-heat. As the inducer RPM increases, the high pressure
switch HPS should close.
Switching from Medium Range Input to Maxim-
um-Heat - If the furnace control CPU switches from medi-
um 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 trans-
ition 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
(pin 5), stopping gas flow to the burners, and humidifier ter-
minal HUM. The inducer motor IDM will remain energized
for a 15-second post-purge period. The blower motor BL-
WM and air cleaner terminal EAC-1 will remain energized
at minimum-heat airflow or transition to minimum-heat air-
flow for 90, 120, 150, or 180 seconds (depending on selec-
tion 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. 34 and 35 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.
The start up and shut down functions and delays described above
apply to the 2-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 de-
crease the inducer motor speed to the required intermedi-
ate-heat RPM. When the inducer motor IDM reduces pres-
sure sufficiently, the high pressure switch HPS will open
and the gas rate will be changed to intermediate-heat. The
gas valve solenoid GV (pin 5) will remain energized as long
as the low pressure switch LPS remains closed. When the
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circuits always causes