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ACCESSORY CONNECTIONS
The furnace control will allow power-switching control of various
accessories.
ELECTRONIC AIR CLEANER CONNECTION
Two 1/4" (6.4 mm) spade terminals (EAC and NEUTRAL) for electronic
air cleaner connections are located on the control board. The terminals
provide 115 VAC (1.0 amp maximum) during circulating blower
operation.
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.
SINGLE STAGE THERMOSTAT
If a single stage thermostat is used, the high/low input staging will be
controlled by the furnace control board, using the staging jumper. If the
staging jumper is set to the 10, 15, or 20 position, the furnace will switch
from low fire to high fire after 10, 15, or 20 minutes and will stay in high
fire until the thermostat is satisfied. If the staging jumper is left in the
OFF position and a single stage thermostat is used, the furnace will
only operate at low fire.
SECTION VI: 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. When two furnaces are installed
using the same duct system, it is very important that the two furnace cir-
culating 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 second 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 simulta-
neously.
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.
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.
Johnson Controls Unitary Products
If furnace staging is desired with two furnaces on a common 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 furnaces 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.
Vent Pipe
Electrical
Supply
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.
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 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 com-
municates 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.
505309-UIM-D-0511
Gas Supply
(Both sides)
1 Coil for
Each Furnace
Supply
Air
15
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