9
Connecting transformers that are not phased correctly may
result in a direct short, which could damage transformers and/
or the thermostat. Phasing problems are likely if the units share
a common ground with secondary grounded transformers.
To avoid this problem, if possible, phase all HVAC units to-
gether. If phasing is impractical, isolation relays may be used to
isolate the transformers. To isolate the auxiliary input, use a
separate transformer for the auxiliary control device (usually a
timeclock). Connect the device to an isolation relay coil. Con-
nect one set of isolated contacts to each thermostat at CK1 and
GND.
To prevent phasing problems when controlling multiple
HVAC units with one thermostat, phase the HVAC units or use
isolation relays to isolate the unit transformers. One unit will be
connected directly to the thermostat and the unit will power the
thermostat. All other units will require one set of isolated con-
tacts for each of the following: Heat, Cool, Fan, and Reversing
Valve (if required).
Units that are phased will have the high voltage legs to the
24 VAC power transformer in common. This requires identify-
ing which terminals from the high voltage input lugs are pro-
viding power to the transformer. Do not assume that the same
terminals will always provide power to the transformer. For
each unit, check the unit wiring diagram. See Fig. 10.
SOFT START — When multiple thermostats are controlled
by the same external device, a Soft Start option can be used to
stagger the turn on times of the HVAC equipment. This feature
may cause a delay in operation after entering Unoccupied
mode.
10-Watt Resistor —
A 10-watt, 270-ohm resistor is pro-
vided with the thermostat. This resistor should only be used in
applications where it is necessary to use 4 wires in a power-
stealing mode when the common wire (C) is not available. It is
recommended that a common wire be used whenever possible.
Refer to the Troubleshooting section for more information on
resistor use and wiring.
Outside
Temperature
Sensor
(33CS250-FS
Only) (Optional) —
An optional outside temperature sen-
sor may be provided to sense outdoor-air temperature. Press
and hold the Mode button for 5 seconds to display outdoor-air
temperature. The “outside” temperature annunciator will be
displayed. Press the Mode button again to exit the Outside
Temperature display.
Determining proper phasing can be difficult without the
help of an electrician (especially across different building
transformers). Damage to equipment can result if phasing
is improperly done. The use of isolation relays is recom-
mended before attempting phasing.
Do not attempt to rewire equipment internally to phase
units. Damage to unit components may occur.
L
L
2
3
L
1
L
1
L
3
L
2
A
B
C
SITE
POWER
24 VAC
UNIT WITH
TRANSFORMER
ON L AND L
3
2
24 VAC
UNIT WITH
TRANSFORMER
ON L AND L
2
1
Fig. 10 — Example of Unit Phasing
105
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Summary of Contents for DEBONAIR 33CS
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