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PKG-PRC002-EN 75
Sequence of
Operation
Supply Air Temperature
Control Unit Sequence Of
Operation
1
Occupied Supply Air Temperature
Control
Cooling/Waterside Economizer
During occupied cooling mode, the
waterside economizer option and
mechanical cooling are used to control
the supply air temperature. The supply
air temperature setpoint and deadband
are user defined at the human interface
panel. Waterside economizing enables
when the units entering water
temperature is below the units entering
mixed air temperature by 4 F plus the
user adjustable economizer approach
temperature. The approach temperature
default is 4 F and is adjustable from 0-9 F
at the HI. Waterside economizing disables
when the units entering water
temperature is not below the units
entering mixed air temperature by at
least the water economizer approach
temperature.
The economizer acts as the first stage of
cooling. If the economizer is unable to
maintain the supply air setpoint, the
compressor
module will bring on
compressors as required to meet the
setpoint. If the unit does not include an
economizer, only mechanical cooling will
satisfy cooling requirements.
Cooling/Airside Economizer
During occupied cooling mode of
operation, the airside economizer option
and mechanical cooling are used to
control the supply air temperature. The
supply air temperature setpoint and
deadband are user-defined at the human
interface panel. If the temperature of the
mixed air is appropriate to use “free
cooling,” the economizer initiates to
satisfy the supply air setpoint. Then if
required, the mechanical cooling stages
on to maintain supply air temperature
setpoint. Minimum on/off timing of the
mechanical cooling prevents rapid
cycling.
On units with an airside economizer, a call
for cooling will modulate the fresh air
dampers open. The rate of economizer
modulation is based on deviation of the
discharge temperature from setpoint, i.e.,
the further away from setpoint, the faster
the fresh air damper will open. First stage
of cooling initiates after the economizer
reaches full open.
Note that the airside economizer is only
allowed to function freely if ambient
conditions are below the setpoint
control
settings or below the return air enthalpy,
if unit has the comparative enthalpy
option. If outside air is not suitable for
“economizing,” the fresh air dampers
drive to the minimum open position. A
field adjustable, factory default setting in
the human interface panel or Tracer
®
can
provide the input to establish the
minimum damper position.
At outdoor air conditions above the
setpoint or comparative enthalpy control
setting, only mechanical cooling is used
and the fresh air dampers remain at
minimum position.
If the unit does not include an
economizer, only mechanical cooling
operates to satisfy cooling requirements.
Heating: Hot Water or Steam
On units with hot water or steam heating,
the supply air temperature can be
controlled to a heating setpoint during the
occupied mode. The supply air
temperature heating setpoint and
deadband are user defined at the human
interface panel. VAV occupied heating on
hot water and steam heat units is initiated
by closing a field-supplied switch or
contacts connected to a changeover input
on the unit’s RTM module board .
Supply Air Setpoint Reset
Supply air reset can be used to adjust the
supply air temperature setpoint on the
basis of a zone temperature or outdoor
air temperature. Supply air reset
adjustment is available from the human
interface panel for supply air heating and
supply air cooling control.
Reset based on outdoor air temperature
Outdoor air cooling reset is sometimes
used in applications where the outdoor
temperature has a large effect on
building load. When the outside air
temperature is low and the building
cooling load is low, the supply air setpoint
can be raised, thereby preventing
subcooling of critical zones. This reset can
lower usage of mechanical cooling, thus
savings in compressor kW, but an
increase in supply fan kW may occur.
Outdoor air heating reset is the inverse of
cooling, with the same principles applied.
For both outdoor air cooling reset and
heating reset, there are three user
defined parameters that are adjustable
through the human interface panel.
•Beginning reset temperature
•Ending reset temperature
•Maximum amount of temperature reset
Reset based on zone temperature
Zone reset is applied to the zone(s) in a
building that tends to overcool or
overheat. The supply air temperature
setpoint is adjusted based on the
temperature of the critical zone(s). This
can have the effect of improving comfort
and/or lowering energy usage. The user-
defined parameters are the same as for
outdoor air reset.
Supply Air Tempering (Hot Water and
Steam Units Only)
When supply air temperature falls below
the supply air temperature deadband low
end, the heating valve modulates open to
maintain the minimum supply air
temperature setpoint.
2
Unoccupied Supply Air Temperature
Control Zone Heating and Cooling
During unoccupied mode, the unit
operates to maintain zone temperature
with fan cycling as needed for building
load. VAV boxes drive full open. However,
unit airflow modulation control operates
to maintain duct static setpoint. The unit
controls zone temperature within the
unoccupied zone cooling and heating
(heating units only) deadbands.
Daytime Warmup
During occupied mode, if the zone
temperature falls to a preset, user-
defined zone low limit temperature
setpoint, the unit is put into daytime
warmup. The system changes over to CV
heating, the VAV boxes drive full open.
However, unit airflow modulation control
operates to maintain duct static setpoint,
and full heating capacity is provided until
the daytime warmup setpoint is reached.
The unit is then returned to normal
occupied mode.