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29RT-PRC007-EN
Controls
reset temperature setpoint the amount
of reset is equal to the reset amount
setpoint.
b
Reset Based On Zone Or Return
Temperature
Zone or return reset is applied to the
zone(s) in a building that tend to
overcool or overheat. The supply air
temperature setpoint is adjusted based
on the temperature of the critical zone(s)
or the return air temperature. 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.
Logic for zone or return reset control is
the same except that the origins of the
temperature inputs are the zone sensor
or return sensor respectively. The
amount of reset applied is dependent
upon how far the zone or return air
temperature is below the supply air reset
setpoint. The amount is zero where they
are equal and increases linearly toward
the value set at the reset amount
potentiometer on the VAV Setpoint
panel. The maximum value is 3 F. If the
return or zone temperature is more than
3 F below the reset temperature setpoint
the amount of reset is equal to the reset
amount setpoint.
3
Zone Temperature Control
Unoccupied Zone Heating and Cooling
During Unoccupied mode, the unit is
operated as a CV unit. Inlet guide vanes
and VAV boxes are driven full open. The
unit controls zone temperature to the
Unoccupied zone cooling and heating
(heating units only) setpoints.
Daytime Warm-up
During occupied mode, if the zone
temperature falls to a temperature three
degrees below the Morning Warm-up
setpoint, Daytime Warm-up is initiated.
The system changes to CV heating (full
unit airflow), the VAV boxes are fully
opened and the CV heating algorithm is
in control until the Morning Warm-up
setpoint is reached. The unit is then
returned to VAV cooling mode. The
Morning Warm-up setpoint is set at the
unit mounted VAV Setpoint panel or at a
remote panel.
Morning Warm-up (MWU)
Morning warm-up control (MWU) is
activated whenever the unit switches
from unoccupied to occupied and the
zone temperature is at least 1.5 F below
the MWU setpoint. When MWU is
activated the VAV box output will be
energized for at least 6 minutes to drive
all boxes open, the inlet guide vanes are
driven full open, and all stages of heat
(gas or electric) are energized. When
MWU is activated the economizer
damper is driven fully closed. When the
zone temperature meets or exceeds the
MWU setpoint minus 1.5 F, the heat will
be staged down. When the zone
temperature meets or exceeds the MWU
setpoint then MWU will be terminated
and the unit will switch over to VAV
cooling.
CV Units Only
Sequence of Operation
1
Occupied Zone Temperature Control
Cooling/Economizer
During occupied cooling mode, the
economizer (if provided) and primary
cooling are used to control zone
temperature. If the enthalpy of outside
air is appropriate to use “free cooling”,
the economizer will be used first to
attempt to satisfy the cooling zone
temperature setpoint; then primary
cooling will be staged up as necessary.
On units with 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 be allowed to start after the
economizer reaches full open.
Note that the economizer is allowed to
function freely only if ambient conditions
are below the enthalpy control setting or
below the return air enthalpy if unit has
comparative enthalpy. If outside air is
not suitable for “economizing,” the fresh
air dampers drive to the minimum open
position. A field adjustable
potentiometer on the Unitary
Economizer Module (UEM), Tracer or a
remote potentiometer can provide the
input to establish the minimum damper
position.
At outdoor air temperatures above the
enthalpy control setting, primary cooling
only is used and the outdoor air
dampers remain at minimum position.
If the unit does not include an
economizer, primary cooling only is used
to satisfy cooling requirements.
Heating
Gas Heating
When heating is required the UCP
initiates the heating cycle by energizing
the K5 relay, heating relay(s), and the
ignition control module(s). The K5 relay
brings on the combustion fan motor. The
ignition control module(s) begin the
ignition process by preheating the hot
surface ignitor(s). After the hot surface
ignitor is preheated the gas valve is
opened to ignite first stage. If ignition
does not take place the ignition control
module(s) will attempt to ignite 2 more
times before locking out. When ignition
does occur the hot surface ignitor is
deenergized and then functions as a
flame sensor. The UCP will energize the
supply fan contactor 45 seconds after the
initiation of the heat cycle. If more
capacity is needed to satisfy the heating
setpoint, the UCP will call for the second
stage of heat by driving the combustion
blower motor to high speed.
When the space temperature rises above
the heating setpoint, the UCP
deenergizes the K5 relay, the heating
relays, and the ignition control module,
terminating the heat cycle.
Electric Heating
When heat is required, the UCP initiates
first stage heating by energizing the first
stage electric heat contactor. The first
stage electric heater bank(s) will be
energized if the appropriate limits are
closed. The UCP will cycle first stage heat
on and off as required to maintain zone
temperature. If first stage cannot satisfy