MG3P Installation Manual

29
• Field supplied wires should also connect the Y / Y2 & Y1
signals to the outdoor unit’s Y & Y1 terminals.
• Connect terminal C to the outdoor unit’s C.
Autostaging for Single Stage Thermostats
The Autostaging feature makes it possible to use a single stage
thermostat and still receive some of the benefits of 2-stage
furnace operation. If Autostage is enabled, the furnace will drop
to LOW fire after initially starting in HIGH fire. After a period of
10 minutes, the furnace will then stage up to HIGH fire, until
the heating load is met. See Figure 38 and the Low Voltage
Connection section on page 28.
The autostaging option on the furnace control board is selected
using the autostage jumper located near the thermostat terminal
block. See Figure 50 (page 46). When the autostage jumper
is moved from OFF to ON, the following occurs during a call
for heat:
Setting the Autostage jumper in the ON position signals the
control to utilize a 10 minute delay between LOW fire and HIGH
fire when a call for heat is supplied via 24V signal to W1. This
setting is the same as jumping R to W2.
NOTE: If the autostage jumper is in the ON position and a W1
demand is present when power is applied to the furnace, the
first heat cycle will run in second stage heat with no autostaging.
After the first cycle, the furnace will resume normal autostaging
operation. DO NOT jump W1 & W2 together while the
autostage jumper is in the ON position.
Autostaging for Two-Stage Thermostats
The Autostage setting on the furnace control board (Figure 50
(page 46)) is disabled when shipped from the factory. This
feature will be not used when paired with a two-stage thermostat.
The autostage jumper setting (P7) must be kept on OFF to allow
the thermostat to adjust stages.
Blower Speed Configuration
Two-stage furnaces use high efficiency circulating air motors that
come in two variations and both are controlled differently. The
fixed speed furnace uses a motor control board that maintains
constant motor torque, while the variable speed furnace uses a
motor control board that maintains constant CFM airflow. Both
boards use the same furnace control board.
MG3S ONLY
NOTE: This section applies only to furnaces with model numbers
suffixed with two numbers, followed by a letter, such as 35C or
45D. If your model has suffix VB, VC, or VD, please consult the
Variable Speed Blower Application section below.
The fixed speed motor control board (Figure 49 (page 46))
contains a set of dip switches for setting the blower speed.
Use pins 1 to 4 to set the blower speed for heating and pins 5
to 8 to set the speed for cooling. To determine the appropriate
switch settings for your installation, see the separate blower
table attachment.
For thermostats with a dehumidifier output, use a field supplied
wire to connect the thermostat’s dehumidifier output to the
terminal marked DEHUM. The thermostat should be set so that the
DEHUM output should be high (positive) when dehumidification
is needed. See also Dehumidification Options section.
CAUTION:
The terminal marked “Y1_IN” on the variable
speed motor control board is not an output to
drive the outdoor unit. DO NOT connect Y1_IN
on the motor control board to the outdoor unit.
Figure 38. Wiring Configuration 1
R DHUM
C
Y
G
W
1
ROOM THERMOSTAT
(SINGLE STAGE)
DHUM
A/C CONDENSING UNIT
(SINGLE STAGE)
C
R
C
Y/Y2
Y
W2
G
W1
Y1
HIGH EFFICIENCY
BLOWER MOTOR
CONTROL BOARD
TWO STAGE FURNACE BOARD
Figure 39. Wiring Configuration 2
DHUM
A/C CONDENSING UNIT
(SINGLE STAGE)
HIGH EFFICIENCY
BLOWER MOTOR
CONTROL BOARD
C
Y
R DHUMC
Y
G
ROOM THERMOSTAT
(TWO - STAGE)
W
1
W
2
TWO STAGE FURNACE BOARD
R
C
Y/Y2
W2
G
W1
Y1
Single Stage AC & Two Stage Thermostat
This option uses the full two stage heating capability of the furnace
with a single stage outdoor unit. See Figure 39 (page 29).
• Connect the thermostat’s W1 & W2 outputs to the furnace
control board’s W1 & W2 terminals.
• Always connect the thermostat C & R to the furnace control
board C & R
• For cooling, connect the thermostats Y signal to the furnace
control board Y / Y2 terminal.
• Field supplied wires should also connect Y / Y2 & C to the
outdoor unit’s Y & C terminals.
Two-Stage AC & Two-Stage Thermostat
This option uses the full two stage heating capability of the
furnace with a two stage outdoor unit. See Figure 40 (page 29).
• Connect the thermostat’s W1 & W2 outputs to the furnace
control board’s W1 & W2 terminals.
• Always connect the thermostat C & R to the furnace control
board C & R.
• For cooling, connect the thermostat Y & Y1 to the furnace
control board’s Y / Y2 & Y1 terminal’s.
A/C CONDENSING UNIT
(TWO STAGE)
HIGH EFFICIENC
Y
BLOWER MOTOR
CONTROL BO
ARD
DHUM
C
Y2
Y1
R DHUMC
Y
2
G
ROOM THERMOSTAT
(TWO STAGE)
W
1
W
2
TWO STAGE FURNACE BOARD
Y
1
R
C
Y/Y2
W2
G
W1
Y1
Figure 40. Wiring Configuration 3