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Sequence of Operation v12.0
After completing the exhaust fan startup sequence, the
exhaust fan will control to a constant airflow based on the
exhaust airflow setpoint (preset from the factory).
Care must be taken to not increase the exhaust airflow setpoint
above the maximum limit of the energy recovery wheel (if
installed), which will cause premature failure of the motor and/
or bearings.
Constant Speed Control
Standard on units equipped with exhaust fan(s), and with two-
position outdoor/return air dampers (not modulating with
economizer, and without exhaust airflow monitoring).
After completing the startup sequence, the exhaust fan(s) will
operate at a constant speed setting, exhaust fan speed
setpoint (adj., 80% default), which will operate the exhaust
fan(s) at that percentage of the maximum hz setting in the VFD.
Energy Recovery Wheel (ERV)
The energy recovery wheel is used to pre-condition the
outdoor air using energy recovered from the exhaust air. All
units equipped with an ERV will be provided with modulating
bypass dampers on both the outdoor and exhaust air paths.
During occupied operation, ERV is typically on/off, with
variable speed via an optional VFD. The ERV operates during
occupied operation except during ventilation or economizer
mode.
Stop Jog
During ventilation or economizer mode, there is a stop/jog
sequence to prevent stagnant air from causing a musty smell
on the ERV. After 30-continuous minutes of economizer or
ventilation operation, the outdoor air damper is closed to its
minimum position, the bypass dampers are closed, and the
ERV is enabled for 2 minutes.
Exhaust Air Bypass Damper Control
As described in the heating mode section, the exhaust air
bypass damper is used to modulate heat recovery as the first
stage of heating. During heating mode, the exhaust bypass
damper is modulated to maintain the discharge air
temperature setpoint. The bypass damper is set to fully open
whenever the ERV is disabled.
If the bypass damper is fully open and the unit is still
overheating the discharge air temperature, the ERV will be
cycled on/off to maintain the discharge air temperature
setpoint active, with extended minimum on/off times to prevent
short cycling.
Outdoor Air Bypass Damper Control (without
VFD on ERV)
The outdoor air bypass on the energy recovery wheel is used
as frost control for the ERV during low ambient conditions. The
damper is modulated open whenever the exhaust leaving
temperature (located downstream of the ERV) falls below 15°.
The bypass damper is set to fully open whenever the ERV is
disabled.
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If the unit is equipped with an electric pre-heater, the heater is
first used as the frost prevention method before using the
outdoor air bypass damper. See pre-heat section for additional
information.
If the bypass damper is open to 100% and the discharge air
temperature is still above setpoint, the wheel may perform a
start/stop sequence to reduce the capacity further. The wheel
has a 10-minute minimum on/off time during this sequence to
prevent short-cycling.
Outdoor Air Bypass Damper (with VFD on
ERV)
The outdoor air bypass and VFD on the ERV is used as frost
control for the ERV during low ambient conditions. First, the
ERV speed is reduced whenever the exhaust leaving
temperature (after ERV) falls below 15°. Once the ERV
reaches minimum speed and the exhaust temperature is still
below 15°, outdoor air bypass damper is modulated open and
the ERV remains at minimum speed. The bypass damper is set
to fully open whenever the ERV is disabled.
If the unit is equipped with an electric pre-heater, the heater is
first used as the frost prevention method before using the
wheel speed or the outdoor air bypass damper. See (preheat
section) for additional information.
Additional Details on Operation
Evaporator Coil Frost Protection
All units equipped with compressors will have a suction
pressure transducer on at least the first circuit. Since the
evaporator coils are generally interlaced for dual circuit units,
circuit 1 suction pressure is generally a good indication of both
circuits. But, in some cases, the second circuit may also have
a suction transducer for expanded frost protection, depending
on configuration, such as dual digital scroll.
During compressor operation, the frost control first attempts to
limit the modulating capacity (variable or digital scroll, either
circuit) before disabling compressors. The expectation is that
if there's a demand for cooling below the point at which the unit
will freeze, then the unit will actively control to the point just
above that point. Generally, this frost point is 95-100psi (29-32°
saturated) at the compressor but may vary slightly depending
on operating conditions and unit configuration.
Hot Gas Reheat
Hot gas reheat is fully modulating from 0-100%, utilizes waste
energy absorbed from the evaporator coil on circuit 1, and is
used to temper the discharge air temperature during
dehumidification or some cases, during cooling mode.
Because it uses waste heat that would have been rejected
through the condenser, it requires the refrigerant circuit to be
operational to provide heat. The hot gas reheat coil is located
downstream of the evaporator before the supply fan.
When enabled, the hot gas reheat valve command is adjusted
to maintain the discharge air temperature setpoint active and
is always enabled during dehumidification mode.
OAU-SVX007A-EN
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