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gas reheat (if installed) controls to the discharge air
temperature setpoint active.
Multi Zone VAV with Outdoor Air Damper
Dehumidification mode is enabled when the outdoor air
dewpoint rises above the outdoor air dewpoint setpoint (adj.)
and is terminated when the outdoor air dewpoint falls 3deg
below setpoint. During dehumidification mode, cooling is
controlled to the dehumidification temperature setpoint active,
and hot gas reheat (if installed) controls to the discharge air
temperature setpoint active.
Multi Zone VAV without Outdoor Air Damper
Dehumidification mode is enabled when the return air
dewpoint rises above the return air dewpoint setpoint (adj.) and
is terminated when the return air dewpoint falls 3° below
setpoint. During dehumidification mode, cooling is controlled
to the dehumidification temperature setpoint active, and hot
gas reheat (if installed) controls to the discharge air
temperature setpoint active.
Heating and Cooling Mode
Heating and Cooling modes are determined using a series of
time-delay latches that vary based on distance from setpoint
and a variable deadband. Generally, the mode of operation will
be changed from cooling to heating whenever the cooling
capacity is at 0% and the temperature is below the setpoint
minus the deadband, occupied offset. Vice-versa when
switching from heating to cooling mode.
Heating Mode
During heating mode, the entire range of heating capacity is
done in "stages" of each component, with each "stage"
stacking on top of the previous one to achieve the total heating
capacity. The stages of heat, in order from first to last, are: ERV,
heat pump, primary heat, and finally secondary heat. The heat
types installed on a unit can be all, none, or any combination
of those. This section describes normal heating operation, but
each component has a dedicated operation for specifics on
how they're controlled.
During heating mode, each of the various heat capacities are
controlled to the discharge air temperature setpoint active. To
see a detailed explanation of how each component is
controlled, refer to the section that describes each component
in detail.
Energy Recover Wheel Variable Effectiveness
First, the energy recovery wheel is used as variable
effectiveness by modulating exhaust airflow across the ERV
using the bypass damper. Once energy recovery is at full
capacity (exhaust air bypass fully closed), the next stage of
heat is engaged after a delay, and the exhaust air bypass
remains fully closed while the next stage of heat is in operation.
Heat Pump
Heat pump is engaged, and compressor staging begins by
modulating the compressor heating capacity to the discharge
air temperature setpoint active. As the compressor heating
OAU-SVX007A-EN
Sequence of Operation v12.0
reaches 100% capacity (or disabled under some other
function), the next stage of heating (if available) is engaged,
and the heat pump capacity remains locked at 100%.
Primary Heat
The primary heater is engaged, and heating capacity is
modulated to the discharge air temperature setpoint active.
Secondary Heat (Pre-Heat)
Once all other stages of heat have been applied and operating
at 100% capacity, the secondary electric heater is used as
additional heat capacity.
Cooling Mode
During cooling mode, cooling capacity controls to discharge air
temperature setpoint active. For more information on
compressor staging, see section [Additional Features].
Exhaust Fan Starting Sequence
Starting Sequence with Isolation (Actuated)
Dampers
Isolation dampers are actuator-controlled dampers with end
switches. During initial startup, the isolation damper(s) are
energized, and the exhaust fan is started after the end
switch(es) are made on the actuators. On initial startup, the
exhaust fan speed is set to 50% signal for the first 30-seconds
of operation.
Starting Sequence with Gravity or Barometric
Dampers
Gravity dampers are either a weighted (barometric) or non-
weighted (gravity) damper. The powered exhaust fan starts
immediately on a call for exhaust and the dampers are opened
using the airflow from the exhaust. On initial startup, the
exhaust fan speed is set to 50% signal for the first 30-seconds
of operation.
Exhaust Fan Operation
Return Duct Static Pressure Control
Standard on units equipped with exhaust fan(s) and
modulating outdoor/return air dampers with economizer. A
differential return duct static pressure transducer is factory
provided, field installed.
After completing the exhaust fan startup sequence, the
exhaust fan controls to the space static pressure setpoint
(0.25" WC default, adj.).
When there is no demand for the exhaust fan, the fan will
operate at minimum speed for 5-minutes before disabling the
fan. The isolation exhaust dampers (if installed) will be closed
after the exhaust fan is disabled.
Constant Volume Control
Standard on units equipped with exhaust fan(s), and with two-
position outdoor/return air dampers (not modulating with
economizer, and with airflow monitoring (piezo) on the exhaust
fan.
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