986TC: Installation, Start-up, Operating, Service and Maintenance Instructions
Manufacturer reserves the right to change, at any time, specifications and designs without notice and without obligations.
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circuits always causes high-heat operation, regardless of the setting of
the low-heat only switch.
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, verifies the low-heat and
high-heat pressure switch contacts LPS and HPS are open, and starts the
inducer motor IDM in high-speed.
The start up and shut down functions and delays described in item 1.
above apply to the 2-stage heating mode as well, except for switching
from low- to high-Heat and vice versa.
a. Switching from Low- to High-Heat - If the thermostat R-to-W1
circuit is closed and the R-to-W2 circuit closes, the furnace
control CPU will switch the inducer motor IDM speed from low
to high. The high-heat pressure switch relay HPSR is
de-energized to close the NC contact. When sufficient pressure is
available the high-heat pressure switch HPS closes, and the
high-heat gas valve solenoid GV-HI is energized. The blower
motor BLWM will transition to high-heat airflow five sec after
the R-to-W2 circuit closes.
b. 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 switch the inducer motor IDM speed
from high to low. The high-heat pressure switch relay HPSR is
energized to open the NC contact and de-energize the high-heat
gas valve solenoid GV-HI. When the inducer motor IDM reduces
pressure sufficiently, the high-heat pressure switch HPS will
open. The gas valve solenoid GV-M will remain energized as
long as the low-heat pressure switch LPS remains closed. The
blower motor BLWM will transition to low-heat airflow five sec
after the R-to-W2 circuit opens.
3. Cooling mode
The thermostat “calls for cooling”.
a. Single-Speed Cooling-
See Fig. 39 and Fig. 40 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. 61.
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 sec. Jumper Y/Y2 to DHUM to reduce the cooling
off-delay to 5 sec. See Fig. 39.
b. Single-Stage Thermostat and Two-Speed Cooling (Adaptive
Mode) -
See Fig. 39 and Fig. 40 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. 39. When ACRDJ is 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 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 true on-board CF selection
as shown in Fig. 39.
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. 39.
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 sec.
Jumper Y1 to DHUM to reduce the cooling off-delay to 5 sec. See
Fig. 39.
c. Two-Stage Thermostat and Two-Speed Cooling
See Fig. 39 and Fig. 40 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. 39.
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 true on-board CF (continuous fan)
selection as shown in Fig. 61. The R-to-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 (air conditioning) selection
shown in Fig. 61.
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 sec. Jumper Y1 to DHUM to
reduce the cooling off-delay to 5 sec. See Fig. 39.
4. Dehumidification Mode
See Fig. 39 and Fig. 40 for thermostat connections.
The dehumidification output, D or DHUM 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 on the
DHUM input, 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 in item 3. above also applies to
operation with a dehumidification thermostat. The exceptions are
listed below:
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