30
Twinning
Single stage M4 furnaces are not supplied with a
built-in twinning capability. Other valuable features and
enhancements were made to the new control that made it
necessary to remove the twinning capability. All standard
two-stage furnaces equipped with the fi xed speed iSEER™
motor include a twinning terminal on the blower control
board.
However, if both single stage furnaces are upgraded to
the fi xed speed iSEER ™ blower, the twin terminal on the
blower control boards (Figure 39) may be used to twin the
single stage furnaces only if the following criteria are met:
• Both furnaces and motors must be the same size.
• Both motors must be on the same speed for cooling
and heating.
• Both furnaces must have a common return duct and
common supply plenum.
• Both furnaces must be on the same leg of power.
Figure 38. Line Voltage Field Wiring
Field Supplied
Disconnect w/in
Sight of Furnace
Field Supplied
Panel Connector
Field Supplied
Fused Service
Panel
Black (Hot)
White (Neutral)
Green or Bare
(Ground)
Black
White
Black
White
Black
White
Field Line Voltage Wiring
Factory Line Voltage Wiring
Ground
Ground
Junction Box (may be int. or ext.
to the furnace). These connections
can be made in the field supplied
disconnect at the furnace.
Ground
Furnace
Model Number
*RC/*RL
Furnace
Input
(Btuh)
Cabinet
Width
(in.)
Nominal
Electrical
Supply
Maximum
Operating
Voltage
Minimum
Operating
Voltage
Maximum
Furnace
Amperes
Minimum
Wire
Gauge
Maximum
Fuse or Circuit
Breaker Amps**
054D-24B
54,000
17
½
115-60-1
127
103
10.0
14
20
072D-35C
72,000
21
115-60-1
127
103
9.3
14
15
090D-35C
90,000
21
115-60-1
127
103
15.6
14
30
108D-45D
108,000
24
½
115-60-1
127
103
15.6
14
30
** Time-delay fuses or HACR-type circuit breakers are required.
Thermostat Wire Gauge
Recommended Thermostat Wire Length
2 - wire
(Heating)
4 or 5 wire
(Cooling)
24
55 ft.
25 ft.
22
90 ft.
45 ft.
20
140 ft.
70 ft.
18
225 ft.
110 ft.
Table 6. Wire Length and Voltage Specifi cations
IMPORTANT NOTE: M4 furnaces equipped with
variable speed iSEER ™ motors may not be twinned
under any circumstances. Please contact your furnace
distributor for technical details.
FURNACE
W
G
Y
C
R
THERMOSTAT
W
G
Y
R
A/C
FSHE
TWIN TERMINAL
BOARD
UNIT
BOARD
FURNACE
BOARD
FSHE
TWIN TERMINAL
BOARD
FURNACE 1
FURNACE 2
W
G
Y
C
R
Figure 39. Single Stage Twinning