JOHNSON CONTROLS
64
FORM100.50-EG12 (918)
Powered Variable Volume Exhaust-Discharge Damper Controlled
This optional variable volume powered exhaust system consists of a fixed speed fan
configured with a proportionally controlled discharge damper. The single package unit
controller monitors the pressure inside the building and controls the exhaust damper and
the exhaust fan. If the building pressure rises, the exhaust damper is proportionally con-
trolled open and the exhaust fan is controlled ON. If the building pressure falls, the ex-
haust damper is proportionally controlled closed and the exhaust fan is controlled OFF.
The position of the exhaust damper in which the exhaust fan is controlled ON and OFF
as well as the building pressure setpoint is user selectable from the single package unit
user interface.
Powered Variable Volume Exhaust-Variable Frequency Drive (VFD) Controlled
This optional variable volume powered exhaust system consist of an exhaust fan driven
by a variable frequency drive (VFD), which is controlled by the single package unit con
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troller. The single package unit controller monitors the pressure within the building. As the
pressure rises, the VFD is controlled to increase exhaust fan speed. As the pressure falls,
the VFD is controlled to decrease exhaust fan speed. The building pressure setpoint is
user selectable from the single package unit user interface. ON/OFF control is maintained
the same as exhaust-discharge damper control stated above.
Return Fan Controlled
This optional variable volume powered return fan system consists of two return fans con-
trolled by one VFD that is controlled by the single package unit control center. The VFD is
controlled to maintain a slightly positive pressure over the mixing box section to prevent
reverse flow. As the return and/or exhaust air dampers open, the return plenum pressure
drops, the fan will speed up to maintain pressure. When the return and/or exhaust air
dampers close, the return plenum pressure increases causing the VFD to slow the fan
speed down.
LOW AMBIENT/HEAD PRESSURE CONTROL OPERATION
The single package unit controller continuously monitors the OAT to determine if me-
chanical cooling should be allowed. As a safety, if the OAT falls to or below the low ambi-
ent lockout temperature, mechanical cooling is prevented from operating. For units with
economizers, the low ambient lockout temperature is typically low enough that mechani-
cal cooling will rarely be required. However, for some applications mechanical cooling is
required when the OAT is lower than the low ambient lockout temperature.
For these applications, the unit must be equipped with optional low ambient controls. For
optional low ambient operation, the single package unit controller monitors the refrig-
eration system discharge pressure and controls the speed of the condenser fans. If the
discharge pressure falls, the speed of the first condenser fan on the refrigeration circuit
is reduced to maintain acceptable condensing pressures in the refrigeration system. With
the optional low ambient controls, mechanical cooling is allowed down to outside air tem-
peratures of 0.0°F.
Controls (Cont'd)
