Bryant 987MB Installation, Start-Up, Operating And Service And Maintenance Instructions page 72

987MB: Installation, Start-up, Operating and Service and Maintenance Instructions
Two-Stage Thermostat and Two-Stage Intermediate/
Maximum Heating
See Fig. 37and Fig. 39 for thermostat connections
NOTE: In this mode the intermediate-heat only switch SW4-2 must be
ON to select the intermediate-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 maximum-heat operation, regardless
of the setting of the intermediate-heat only switch.
The wall thermostat "calls for heat", closing the R to W1 circuit for
intermediate-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 intermediate/maximum heating mode as well, except for
switching from maximum- to intermediate-heat.
1. Switching from Maximum- to Intermediate-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 reduces pressure sufficiently, the high pressure
switch HPS will open and the gas rate will be changed to
intermediate-heat. When the inducer motor speed gets within 15%
of the required intermediate-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 intermediate-heat airflow.
Cooling Mode
The thermostat "calls for cooling."
1. Single-Speed Cooling
See Fig. 37and Fig. 39 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 airflow. Cooling airflow is based on the
A/C selection shown in Fig. 60.
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
2. Single-Stage Thermostat and Two-Speed Cooling (Adaptive
Mode)
See Fig. 37 and Fig. 39 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. 37. When 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
Manufacturer reserves the right to change, at any time, specifications and designs without notice and without obligations.
Fig. 37.
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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 SW3 selection as shown in
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 selection shown in Fig. 56.
NOTE: When transitioning from low-cooling to high-cooling the
outdoor unit compressor will shut down for 1 minute while the furnace
blower motor BLWM transitions to run at high-cooling airflow.
The electronic air cleaner terminal EAC-1 is energized with 115 vac
whenever the blower motor BLWM is operating.
When the thermostat is satisfied, the R to G-and-Y circuit are opened.
The outdoor unit stops, and the furnace blower BLWM and electronic air
cleaner terminal EAC-1 will remain energized for an additional 90
seconds. Jumper Y1 to DHUM to reduce the cooling off-delay to 5
seconds, see Fig. 37.
3. Two Stage Thermostat and Two-Speed Cooling
See Fig. 37and Fig. 39 for thermostat connections.
NOTE: The air conditioning relay disable jumper ACRDJ must be
disconnected to allow thermostat control of the outdoor unit staging, see
Fig. 37.
The thermostat closes the R to G-and-Y1 circuits for low cooling or
closes the R to G-and-Y1-and-Y2 circuits for high cooling. The R to Y1
circuit starts the outdoor unit on low cooling speed, and the R to
G-and-Y1 circuit starts the furnace blower motor BLWM at low-cooling
airflow which is the SW3 selection as shown in
Y1-and-Y2 circuits start the outdoor unit on high-cooling speed, and the
R to G-and-Y/Y2 circuits start the furnace blower motor BLWM at
high-cooling airflow. High-cooling airflow is based on the A/C selection
shown in Fig. 60.
The electronic air cleaner terminal EAC-1 is energized with 115 vac
whenever the blower motor BLWM is operating.
When the thermostat is satisfied, the R to G-and-Y1 or R to
G-and-Y1-and-Y2 circuits are opened. The outdoor unit stops, and the
furnace blower BLWM and electronic air cleaner terminal EAC-1 will
remain energized for an additional 90 seconds. Jumper Y1 to DHUM to
reduce the cooling off-delay to 5 seconds, see
4. Dehumidification Mode
See Fig. 37and Fig. 39 for thermostat connections.
The dehumidification output, H on the Thermostat should be connected
to the furnace control thermostat terminal DHUM. When there is a
dehumidify demand, the DHUM input is activated, which means 24 vac
signal is removed from the DHUM input terminal. In other words, the
DHUM input logic is reversed. The DHUM input is turned ON when no
dehumidify demand exists. Once 24 vac is detected by the furnace
control, dehumidification capability is activated. If the DHUM input is
removed for more than 48 hours, the furnace control reverts back to
non-dehumidification mode.
The cooling operation described above in the Cooling Mode section also
applies to Dehumidification mode. The exceptions are listed below:
a. Low cooling – When the R to G-and-Y1 circuit is closed and
there is a demand for dehumidification, the furnace blower motor
BLWM will drop the blower airflow to 86% of low cooling
airflow which is the SW3 selection as shown in
Fig. 56.
Fig. 60. The R to
Fig. 60.
Fig. 60.