Twinning The Ml193Df Furnace - Lennox MERIT ML193DF Installation Instructions Manual

Twinning the ML193DF
The control board in this furnace is equipped with a provi-
sion to "twin" (interconnect) two(2) adjacent furnaces with a
common plenum such that they operate as one (1) large
unit.
When twinned, the circulating blower speeds are synchro-
nized between the furnaces. If either furnace has a need to
run the blower, both furnaces will run the blower on the
same speed. The cooling speed has highest priority, fol-
lowed by heating speed and fan speed.
Field installation of twinning consists of connecting wires
TWO−STAGE THERMOSTAT
Call For Cool
Call For Fan
Call For 1st
Stage Heat
Call For 2nd
StageHeat
ISOLATION
RELAY
FIELD WIRING FOR TWINNING
TWIN
R
R
TWIN 1
Y
Y
G
G
W1
W
W2
C
TWIN
R
TWIN 2
Y
G
W
C
between the "C" and "Twin" terminals of the two controls.
The 24 VAC secondary of the two systems must be in
phase. All thermostat connections are made to one control
only. Figure 61 show wiring for two−stage and single stage
thermostats.
The twinned furnace without thermostat connections is to
have the call for heat supplied by an external 24VAC isola-
tion relay to prevent its rollout switch from being bypassed
by the other twinned furnace. The coil of the isolation relay
connects from the thermostat "W" to 24 VAC common. The
contacts of the relay connect "R" to "W" on the non−thermo-
stat twin.
SINGLE STAGE THERMOSTAT
Call For Cool
Call For Fan
Call For Heat
FIGURE 61
Page 42
TWIN
R
R
TWIN 1
Y
Y
G
G
W
W
C
TWIN
ISOLATION
RELAY
R
TWIN 2
Y
G
W
C