Appendix H
Twinning
Only upflow, PSC, 2-speed furnaces can be twinned. Units that are certified to be twinned are listed in twinning kit Installation Instructions. Both
of the units to be twinned must be the same size and same model furnace. Mis-matching twinned units is not allowed.
NOTE:
When troubleshooting twinned furnaces, it is helpful to disconnect twinning jumper and troubleshoot furnaces separately. If a polarity
status code is flashed, go to page number indicated in Index for Rapid Flashing LED.
WARNING:
Do not use back of furnace for return-air
duct connection as limit cycling will occur. A failure to
follow this warning can cause fire, personal injury, or
death.
CAUTION:
Failure to select identical blower speed taps
in both furnaces can result in overheating of furnace
components, possible loss of furnace operation, insuffi-
cient airflow, and damage to the furnaces.
WARNING:
Do not remove center return-air partitions
between furnaces. Failure to follow this warning could
result in improper auxiliary limit operation, fire, personal
injury, or death.
CAUTION:
When common-venting twinned, Category I
(negative pressure venting) furnaces, excessive conden-
sate may occur as a result of oversized vent systems.
Dedicated vents and/or proper vent sizing per furnace
Installation Instructions will reduce the potential for
condensation.
IMPORTANT TWINNING REQUIREMENTS TO CONSIDER
Please read and follow the entire furnace Installation Instructions and twinning kit Installation Instructions when installing these furnaces.
A combination of 1 full side and bottom inlet plenum or a bottom-only inlet plenum must be used for return air for each furnace. The preferred
method is that all return air be brought into bottom of furnaces through a common bottom plenum.
With 2-speed furnaces, low-speed gas heat in both furnaces is used for single-stage heat. High-gas heat in both furnaces is used for second-stage
heat.
The furnace installed on left-hand (LH) side is considered the MASTER furnace. The right-hand (RH) furnace is considered the SLAVE.
Thermostat connections are made only to MASTER unit.
SEQUENCE OF OPERATION
See Fig. 11 and 12 for 2-speed PSC furnace twinning connection and schematic wiring diagrams while reviewing sequence of operation.
Twinning operation is controlled by LH or MASTER furnace. The TWIN/TEST and C
OM
-24V connection wires ensure the 2 furnaces coordinate
their blower operation. When either furnace requires blower operation, both furnaces operate their blowers at the same speed (the highest speed
required by either furnace). Both furnaces operate simultaneously in the same mode: low-gas heat, high-gas heat, low-cool, high-cool, or
continuous fan. Exceptions can occur if a safety device in either furnace is activated (such as low-gas-heat or high-gas-heat pressure switch, flame
rollout switch, limit switch, draft safeguard switch, twinning auxiliary limit switch, or flame-proving sensor). In such a case, the other furnace
continues to operate unless the safety device is the flame rollout, limit, or twinning auxiliary limit switch, in which case both furnaces respond.
TWO-STAGE HEAT WITH SINGLE-STAGE GAS-HEAT THERMOSTAT
NOTE:
SW-1 and SW-2 in BOTH furnaces in OFF position.
1. LH or MASTER furnace control determines whether furnaces are both operating in low-gas heat or high-gas heat, depending on control’s
adaptive gas heating mode when R-W/W1 circuit is closed in MASTER furnace.
2. Operation in all modes is the same for twinned furnaces as for individual furnaces. See Sequence of Operation section in this guide or in
the furnace Installation, Start-Up, and Operating Instructions.
TWO-STAGE HEAT WITH 2-STAGE GAS-HEAT THERMOSTAT
NOTE:
SW-1 in both furnaces in OFF position, SW-2 in both furnaces in ON position.
1. The 2-stage thermostat (NOT 2-stage furnace control’s adaptive gas heating mode) determines whether furnaces are both operating in
low-gas heat or high-gas heat, depending on whether 1 or both thermostat stages (W/W1 or W/W1-and-W/W2) are calling for heat.
2. Operation in all modes is the same for twinned furnaces as for individual furnaces. See Sequence of Operation section in this guide or in
the furnace Installation, Start-Up, and Operating Instructions.
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