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Figure 21: Wiring using a single-stage thermostat
Figure 22: Wiring using a multi-stage thermostat
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 indoor coils. Separate plenums and
supply ducts systems cannot be utilized. A single return
air plenum, common to both furnaces, must be used. It is
suggested that a return platform be utilized, with bottom
air entrance into each furnace.
If a side entrance return 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 taps. See Figure 23 for
a typical application.
If furnace staging is required with two single stage
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, it is best practice to use
an air-mixing device in the plenum to mix the air from
both furnaces. 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
22
Installation Manual: RL18 Single-Stage Standard ECM Residential Gas Furnaces (Non-condensing Multi-position Standard
Low NOx)
mixing device is not used, any ducts that are connected
downstream from the furnace that is not operating in
heat mode supply cold air to the occupied spaces.
Figure 23: Twinned furnaces
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 system
Vent connections
All models are provided with a flue transition that is sized
for 4 in. diameter vent connections. If a larger size vent
connector is required, that connection must be installed
external to the furnace. Figure 24 shows the furnace as it
is shipped from the factory. To convert to a horizontal or
downflow position, remove the four screws that secure
the inducer assembly and rotate 90º, being careful not
to damage the gasket. Reinstall screws. Remove cap
from appropriate vent outlet location on the cabinet, cut
insulation in cabinet to same size as the hole provided,
and reinstall cap in the hole in the top panel.
Important: In downflow applications, do not
block the combustion air inlet. The furnace must
be installed on a coil cabinet or subbase to allow
combustion air to enter the burner compartment.
Johnson Controls Ducted Systems
CAUTION
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