_ Installation Instructions Combination Units I
Filters
All return air, including economizer air and outside damper air,
MUST pass through a filter before entering the evaporator. An
electronic air cleaner or other accessible filter arrangement
MUST be installed in the return air ductwork. This can ONLY be
done when the internal filters are removed and NO outdoor air ac-
cessories are used. Mimimum recommended filter areas are
based on a velocity of 300 ft/min for disposable filters and 500 ft/
min for high velocity filters (washable).
CAUTION
DO NOT OPERATE THE UNIT WITHOUT A FILTER.
7. Economizer
The purpose of an economizer is to:
• Provide cool outside air to the conditioned space during the
cooling cycle to minimize the use of the compressors.
• Bring outside air into the conditioned space to meet mini-
mum fresh air requirements whenever the circulation blow-
er is running.
Downflow Economizer
I
The economizer is controlled by a logic module which field con-
nects to the unit controls through a plug harness. The logic mod-
ule also controls the compressor staging based on the thermostat
input.
The minimum opening position of the fresh air dampers is field
settable. It is set on the logic module of modulating economizers
or on the actuator motor of three-position economizers.
The enthalpy sensor is factory installed on the fresh air dampers
of the economizer. The enthalpy change-over point is adjustable
on the logic module.
A discharge air sensor is field installed in the blower inlet. The dis-
charge air sensor keeps the discharge air above 56°F (13.3°C).
A positive pressure is created when the fresh air dampers open
and the return dampers close. This pressure must be vented or
the air will not circulate properly. This is the function of the baro-
metric relief dampers. A positive pressure forces the passive ex-
haust dampers to swing open.
The fresh air dampers open to the minimum position for outside air
whenever the circulation blower is ON.
When the thermostat is in the FAN AUTO position, the fresh air
dampers will close completely whenever heating or cooling is not
called for.
When the thermostat is in the FAN ON position:
1. The fresh air dampers will open to the minimum position for
fresh air.
If Using a 2-Stage Thermostat
2. On a call for cooling, if the economizer outdoor air sensor
determines that the outside air is cool enough (and in the
case of an enthalpy sensor, dry enough), the fresh air
dampers will modulate open and the return air dampers wil!
modulate closed.
If the conditioned space is not being maintained at the se-
lected temperature, the thermostat will make Y2 and call for
second stage cooling by energizing the economizer logic
module at low voltage terminal board terminal Y4. This en-
ergizes the Y1 anti-cycle delay and contactor which ener-
gizes the condenser fan and compressor for cooling to
assist the economizer.
Theory of Operation
The economizer has two sets of dampers that are mechanically
linked together. The fresh air dampers regulate the intake of out-
side air and the return air dampers regulate the flow of return air
(see FIGURE 14). When the fresh air dampers modulate open,
the linkage causes the return air dampers to modulate closed.
If Using a Single- Stage Thermostat
When using a single stage thermostat, all steps as outlined above
will be followed with the exception of the following:
The compressor will only be energized when the outside air
reaches a temperature above the preset limit of the economizer.
When ambient conditions are not suitable for economizer opera-
tion, ONLY the compressor will run, and the economizer will NOT
be energized.
E_ 50901 110202
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