1083287-UIM-A-0114
Johnson Controls Unitary Products
15
HUMIDIFIER CONNECTION
Two 1/4” (6.4 mm) spade terminals (HUM and NEUTRAL) for humidifier
connections are located on the control board. The terminals provide 115
VAC (1.0 amp maximum) during heating system operation.
A mounting hole is provided on the control panel next to the furnace
control board for mounting a humidifier transformer if required.
SECTION VI: TWINNING AND STAGING
In applications where more heating capacity or more airflow capacity is
needed than what one furnace can deliver, twinning can be used to
make two furnaces operate in tandem (as long as the two furnaces
have the same model number). When two identical furnaces are
installed using the same duct system, it is very important that the two
furnace circulating air blowers operate in unison. If one blower starts
before the second blower, the duct system will become pressurized and
the blower on the second furnace will turn backwards causing the sec-
ond furnace to overheat, resulting in damage to the furnace. Twinning is
used to make two furnaces operate in tandem, using one duct system,
one room thermostat and causing both furnaces to turn on and off
simultaneously.
TWINNING DUCT SYSTEM
Twinned furnaces must only be applied on a common duct system. A
single air supply plenum must be used for both furnaces and coil(s).
Separate plenums and supply ducts systems cannot be utilized. A sin-
gle return air plenum, common to both furnaces must be used. It is sug-
gested that a return platform be utilized, with bottom air entrance into
each furnace. If a side entrance returns system is used, the common
return duct must be divided equally so as to supply each furnace with
an equal amount of return air.
Both furnaces must be identical models in both heating capacity and
CFM capacity. Both furnaces must be operated on the same motor
speed tap. See typical application, Figure 20.
If furnace staging is desired with two single stage furnaces on a com-
mon duct, where the gas burner on the first furnace operates on W1
and the gas burner on the second furnace operates on W2, then the
use of an air-mixing device in the plenum to mix the air from both fur-
naces is strongly recommended. The mixing device must be installed
before any ducts that supply air to occupied spaces. Twinning causes
both indoor fans to operate simultaneously. If a mixing device is not
used, any ducts that are connected down stream from the furnace that
operates on W2, will be supplying cold air in the Heating mode to the
occupied spaces unless W2 is energized.
GAS PIPING
Furnace gas supplies must be provided as specified with these instruc-
tions. Since the furnaces are side by side, with no space between, gas
supplies must enter on the right and left respectively. All gas piping
must be in accordance with the national fuel gas code, ANSI Z223.1,
latest edition, and/or all local code or utility requirements.
TWINNING
In applications where more heating capacity or more airflow capacity is
needed than what one furnace can deliver, twinning can be used to
make two furnaces operate in tandem, using one duct system and one
room thermostat. When one duct system is used for two furnaces, it is
necessary that the two blowers operate in unison. The twinning function
of the board in this furnace ensures that both blowers turn on and off
simultaneously, and operate on the same blower speed.
The control in the furnace has the single-wire twinning feature. With this
feature, a wire is connected between the TWIN terminal on one furnace
board to the TWIN terminal on the second furnace board. The board
then communicates the blower status from one furnace to the other
along this wire. This communication makes the second furnace blower
come on at the same time, and on the same speed, as the first furnace
blower. To ensure stable communication, the common terminal of each
control must be connected.
Twinning Instructions
Connect the control wiring as shown in Figure 21.
1.
Connect the low voltage wiring from the wall thermostat to the ter-
minal strip on the control board of Furnace #1.
2.
Connect a wire from the TWIN terminal of Furnace #1 to the TWIN
terminal of Furnace #2.
3.
Install a separate 24V relay as shown in the diagram below. Use of
this relay is required, as it ensures that the transformers of the two
furnaces are isolated, thus preventing the possibility of any safety
devices being bypassed.
4.
Connect the 24V common wires of furnace #1 to the 24V common
terminal of furnace #2.
Before installing the relay and wiring, disconnect electrical power to
both furnaces. Failure to cut power could result in electrical shock or
equipment damage.
The relay must not be installed in any location where it could be
exposed to water. If the relay has been exposed to water in any way,
it must not be used.
Twinned furnaces are to be on the same power source or line leg
(phase). Notify the job electrician early so they can properly balance
the electrical load. Failure to do this may result in control voltage
polarity issues. Refer to Rapid Red Flash Fault Code.
FIGURE 20:
Typical Twinned Furnace Application
When two furnaces are twinned, typical system total airflow will be
approximately 85% of additive individual furnaces, i.e., two 2000 CFM
units will yield a total 3400 CFM.
If a return duct is connected to only one furnace (with a connection
between the two furnaces) an imbalance in the airflow will occur and
the furnace furthest from the return plenum will overheat.
Vent Pipe
Electrical
Supply
Gas Supply
(Both sides)
1 Coil for
Each Furnace
COMMON
SUPPL
Y
PLENUM
Supply
Air
