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PKG-PRC002-EN
74
Sequence of
Operation
Zone Temperature Control
Unit Sequence Of Operation
1
Occupied Zone Temperature Control
Cooling/Waterside Economizer
During occupied cooling mode, the
waterside economizer option and
mechanical cooling function to control
zone temperature. If the entering
condenser water temperature is
appropriate to use “free cooling,” the
economizer initiates to attempt to satisfy
the cooling zone temperature setpoint
with the compressors staging on as
necessary. Minimum on/off timing of
compressors prevents rapid cycling.
Waterside economizing enables when
the unit’s entering water temperature is
below the unit’s 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 human interface (HI) panel.
Waterside economizing disables when
the unit’s entering water temperature is
not below the unit’s entering mixed air
temperature by at least the water
economizer approach temperature. The
approach temperature defaults to 4 F and
is adjustable from 0-9 F at the HI. The
economizer acts as the first stage of
cooling. If the economizer is unable to
maintain the zone temperature 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 operate to
satisfy cooling requirements.
Cooling/Airside Economizer
During occupied cooling mode, the
economizer option and mechanical
cooling operate to control zone
temperature. If the outside air enthalpy is
appropriate for airside economizing or
“free cooling,” the economizer initiates to
satisfy the cooling zone temperature
setpoint with the compressors staging on
as necessary. Minimum on/off timing of
compressors will prevent 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
zone temperature from setpoint; i.e., the
further away from setpoint, the faster the
fresh air damper will open. First stage of
cooling will start after the economizer
reaches full open.
Note that the airside economizer will only
function freely if ambient conditions are
below the enthalpy control settings or
below the return air enthalpy if unit has
comparative enthalpy installed. 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
enthalpy control setting, only mechanical
cooling is used and the fresh air dampers
remain at minimum position.
If the unit does not include an airside
economizer, only mechanical cooling will
operate to satisfy cooling requirements.
Heating: Electric
On units with electric heating, the zone
temperature can be controlled to a
heating setpoint during the occupied
mode by cycling a single stage electric
heater. Interface is provided for field
supplied single stage electric heat. The
zone temperature heating setpoint and
deadband are user defined at the human
interface panel.
Heating: Hot Water or Steam
On units with hot water or steam heating,
the zone temperature can be controlled
to a heating setpoint during the occupied
mode. The zone temperature heating
setpoint and deadband are user defined
at the human interface panel.
Supply Air Tempering
For hot water, steam, or electric heat
units in the heat mode but not actively
heating, if the supply air temperature
drops to 10 F (5.5 C) below the occupied
zone heating temperature setpoint, one
stage of heat will be brought on to
maintain a minimum supply air
temperature. The unit transitions out of
heat mode if the supply air temperature
rises to 10 F above the occupied zone
heating temperature setpoint.
Auto Changeover
When the system mode is auto, the mode
will change to cooling or heating as
necessary to satisfy the zone cooling and
heating setpoints. The zone cooling and
heating setpoints can be as close as 2 F
(1.1 C) apart.
2
Unoccupied Zone Temperature Control
Cooling and Heating
Both cooling and heating modes can be
selected to maintain unoccupied zone
temperature deadbands. For unoccupied
periods, heating, economizer operation,
or compressor operation can be
selectively locked out at the human
interface panel.