Optional Modulating Hot Gas Reheat - Daikin MPS015 F Installation And Maintenance Manual

Dehumidification Initiation
An analog sensor is mounted in the return duct, the space, or
outdoors to sense Relative Humidity. The location is selected
by setting the Sensor Location value on the keypad to Return,
Space, or OAT. OAT can only be selected for units with DAT
control. Dehumidification is disabled when the unit is in either
the Heating or Minimum DAT state. When Dehumidification is
enabled, Dehumidification operation is initiated when Humidity
Control is set to either Relative Humidity or Dew Point and
that value rises above the appropriate setpoint by more than
half its deadband. Economizer operation is disabled in the
Dehumidification mode so the unit immediately transitions to
Cooling if Dehumidification is initiated in Economizer state.
Dehumidification Termination
Dehumidification is terminated if the selected variable, Relative
Humidity or Dew Point, drops below the appropriate humidity
setpoint by more than half its deadband. Dehumidification is
also terminated if cooling is disabled for any reason or the unit
enters either the Heating or Minimum DAT state. For units with
compressors, the number of cooling stages is reduced by one
and control reverts to normal control when dehumidification
is terminated in the Cooling state. Another compressor stage
change could then occur after one Cooling Stage Time has
elapsed .
IM 1058-8 • MAVERICK II ROOFTOP SYSTEMS
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Control & Arrangement
In conjunction with dehumidification, MHGRH is used to raise
the temperature of the cooled air to a desirable value. MHGRH
is comprised of a parallel coil arrangement, with both the
condenser and reheat coils of the micro channel type, dual
reheat valves (which operate in concert with one another) and
a check valve. MHGRH components will always be installed in
circuit #2.
During Dehumidification control w/ modulating Hot Gas
Reheat (MHGRH) an analog signal (0-10Vdc) is controlled as
described below.
• A PI Loop is used to control the HGRH valves to maintain
the Discharge Air Temperature from the reheat coil.
• Compressor staging during reheat (or dehumidification)
will be controlled by the Leaving DX Coil Temperature.
For increased dehumidification during reheat, the
standard default compressor staging range is 45 - 52°F.
• When dehumidification is active in the Cooling state, the
reheat set point equals the DAT Cooling Setpoint. For
DAT units, this is the normal DAT set point resulting from
any reset. For Zone Control units, this set point is the
result of a PI Loop based on the Control Temperature.
• Communication with the reheat control valves is
accomplished by providing a 0-10Vdc signal to a pair of
interface boards which in turn supply the control signal to
the reheat valves (step type).
• In the Fan Only state, no sensible cooling is required,
but dehumidification mode will still be enabled if the
dew point or humidity sensor is not satisfied. Reheat
set point varies from a maximum value (default 65°F)
when the Control Temperature is at or below the heating
changeover setpoint to a minimum value (default 55°F)
when the Control Temperature is at or above the cooling
changeover setpoint.
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