Modulating; Rtu Gas Heating; Min Dat - Daikin MicroTech III Operation Manuals

Modulating

Entering Heating Operating State
The unit enters the Heating operating state from the Fan Only
operating state when the Control Temperature falls below
the Occupied or Unoccupied Heating Set Point by more than
half the Occupied or Unoccupied Heating Dead Band . The
unit transitions from heating to Fan only when the Control
Temperature rises above the Occupied or Unoccupied Heating
Set Point by more than half the Occupied or Unoccupied
Heating Dead Band . The unit will also transition from the
Heating to Fan Only operating state if heating operation
is disabled due to OA ambient lockout . When the unit is in
the Heating operating state, heating capacity is modulated
to maintain the discharge air temperature at the Discharge
Heating Set Point .
There are several different modulating heating types available
with this equipment . There are some differences in the control
sequence depending on the heat type installed . The different
types are described in the following sections .
Steam or Hot Water Heat: Face and Bypass Damper
Control
When a unit is equipped with steam or hot water with face and
bypass damper heating there are two different methods used
for controlling the heating arrangement . These are the "Open
Valve" and "Modulating Valve" methods and are described in
the following sections .
Open Valve
When the unit enters the Heating operating state, the steam
or hot water valve is driven fully open . The face and bypass
dampers are then modulated to maintain the discharge air
temperature at the discharge heating set point .
Modulating Valve
When the outdoor air temperature is below the F&BP
changeover temperature, the Heating valve is driven to 100%
open to protect the coil . The face and bypass dampers are
then modulated to satisfy the heating load . When the outdoor
air temperature rises above the F&B Changeover temperature
by a differential of 2 .0ºF, the face and bypass dampers are set
at 100% open to the face of the coil and the Heating valve is
modulated to satisfy the heating load . The default value for the
changeover temperature is 37°F .
OM 920-6 • MICROTECH UNIT CONTROLLER

RTU Gas Heating

When a unit is equipped with modulating gas heating and is
in the Heating operating state, the gas valve is modulated
to maintain the discharge air temperature at the Discharge
Heating Set Point . Differences in the control of modulating
gas heat are described in the following sections . On units
equipped with modulating gas heat, the Discharge Heating
Set Point is limited according to a maximum heat exchanger
temperature rise limit . This factory set limit varies depending
on the unit burner model and can be found on the gas heat
data plate attached to the unit . The controller does not allow
the Discharge Heating Setpoint to be set above the current
temperature entering the discharge fan (EFT) by more than this
maximum heat exchanger temperature rise limit .

Min DAT

If heating is enabled and there is no heating load (normally Fan
Only operating state), the controller activates the units heating
equipment as required to prevent the discharge air temperature
from becoming too cool if the Min DAT Control Flag (DAT
units only) is set to yes via the Heating menu . The unit enters
the Min DAT operating state during occupied operation when
neither cooling nor heating is required based on the heat/
cool changeover function but the discharge temperature falls
below a minimum discharge temperature limit . If the discharge
air temperature falls below the this minimum discharge
temperature limit by more than half the discharge heating
deadband, the unit operating state changes from Fan Only to
Min DAT .
NOTE: On VAV or CAV discharge control units, the DAT
cooling setpoint parameter in the Cooling menu acts
as the minimum discharge temperature limit . On CAV
zone control units the Min DAT Limit parameter in
the Heating menu acts as the minimum discharge
temperature limit .
Typical Sequence of Operation (20-1 gas burner)
When 120V power is furnished through the system ON/OFF
switch (S1), through the burner ON/OFF switch (S3), and
through the high limit control (FLC), terminal #6 on the flame
safeguard (FSG) is powered on a call for heat . Whenever
power is restored to the flame safeguard, the flame safeguard
will go through a 10 second initiation period before the
prepurge period will begin . The burner air control valve will be
at minimum position during OFF cycles . Upon a call for heat
or any other time that a prepurge cycle occurs, the air control
valve will be repositioned to the maximum position for prepurge
and then returned to the minimum position for low fire start.
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