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Figure 17: Standard ECM furnace and single-stage HP
Figure 18: Standard ECM furnace and multi-stage HP
Figure 19: Standard ECM furnace and single-stage AC
Figure 20: Standard ECM furnace and premium multi-
stage AC
Installation Manual: RL18 Single-Stage Standard ECM Residential Gas Furnaces (Non-condensing Multi-position Standard
Accessory connections
The furnace control allows power-switching control of
various accessories.
Electronic air cleaner connection
Two 1/4 in. (6.4 mm) spade terminals (EAC and EAC-N)
for electronic air cleaner connections are located on
the control board. The terminals provide 115 VAC (1 A
maximum) during circulating blower operation.
Humidifier connection
Two 1/4 in. (6.4 mm) spade terminals for humidifier
connections, HUM and HUM N, are located on the control
board. The terminals provide 115 VAC (1 A maximum)
during heating system operation.
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). Both furnaces must have separate
power circuits but the circuits must be from the same
phase or leg of power.
When two identical furnaces are installed using the same
duct system, it is very important that the two furnace
circulating air blowers operate simultaneously at all
times. If one blower starts before the second blower,
the duct system becomes pressurized and the blower
on the second furnace may turn backwards, causing the
second furnace to overheat and resulting in damage
to the furnace. Twinning is used to make two furnaces
operate in tandem, using one duct system and one room
thermostat and causing both furnaces blowers to turn on
and off simultaneously.
When furnaces use the twinning capability, the two
controls do not trade on and off during a W1 call. Only
the control that is connected to the thermostat conditions
while both blowers continue to operate at the same time
and speed.
Furnace control twinning wiring
Twinned furnaces must be on the same power source
or line leg (phase). Notify the job electrician early so
they can correctly balance the electrical load. Failure
to do this may result in control voltage polarity issues.
See the Rapid Red Flash fault code in
diagnostics.
Johnson Controls Ducted Systems
CAUTION
Furnace control
Low NOx)
21
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