Daikin DPS - Gas Heating; Minimum Discharge Air Temperature (Min DAT)

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. 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.
Minimum Discharge Air
Temperature (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.
The unit reverts to normal modulation of the gas heating valve
when the Hold Period has elapsed since the unit entered
Heating or MinDAT.
Sequence of Operation (Modulating
Burner)
Low Heat Option with Modulation
The following details the sequence of operation for the low
heat option.
• Unit DDC controller calls for heat.
• Furnace DDC control module receives a call for heat.
• Furnace safety switches and DDC control are checked for
safe conditions.
• 45 second prepurge cycle starts. Proof of airow switch is
checked for combustion airow.
• Spark ignitor is activated.
• Gas valve receives a signal to open fully.
• Burner is ignited and runs for 20 seconds in high re.
• Note: if call for heat is interrupted during this timing, the
furnace will be locked in for the 20 seconds cycle.
• Gas valve and induction blower motor receives a signal
to modulate burner output to match the unit discharge air
temperature setting.
operator’s guIde
www.DaikinApplied.com 109 OM 1141-3 • MICROTECH UNIT CONTROLLER