McQuay OM 920-1 79
Operator’s Guide
Dehumidification Mechanical Cooling Control
During Dehumidification, control of Mechanical Cooling is based on the following two
editable values of the Leaving Coil Temperature setpoint. The Leaving Coil Temperature
setpoint parameter is adjustable via the following path: Extended Menus/Dehum Setup/Mn
Lvg Coil T or Mx Lvg Coil T.
• Mx Lvg Coil T (Default = 52°F)
• Mn Lvg Coil T (Default = 48°F)
For compressorized units, the number of Compressor Stages increases when:
• The time since the last stage change exceeds the Clg Stage Time AND
• Leaving Coil Temperature is greater than the Mx Lvg Coil T AND
• The current cooling stage is less than the available number of stages
During the dehumidification mode, the number of Compressor Stages decreases when:
• The time since the last stage change exceeds the Clg Stage Time AND
• Leaving Coil Temperature is less than Mn Lvg Coil T Spt AND
• The current cooling stage is greater than zero
Units with modulated cooling are controlled by a PI Loop to maintain the temperature leaving
the coil at the Mn Lvg Coil T. The normal DAT Cooling Deadband and PI Loop parameters
are used in this PI Loop.
Dehumidification Reheat Control
During Dehumidification control either an analog or digital Hot Gas Reheat (HGRH) output or
the standard modulating heating output is controlled to maintain the current Reheat Setpoint.
If Reheat Type software configuration parameter is set to Staged Hot Gas, then a digital
HGRH output is controlled to maintain the Reheat Setpoint. If the Reheat Type software
configuration parameter is set to Modulating Hot Gas, then an analog HGRH output is
controlled to maintain the Reheat Setpoint. If the Reheat Type software configuration
parameter is set to Standard Heat, then the normal heating analog output is controlled to
maintain the Reheat Setpoint.
• Cooling/Fan Only: In the Cooling and Fan Only states, a PI Loop is used to control the
HGRH valve to maintain the Discharge Air Temperature at the Dehumidification Reheat
Setpoint. The deadband for this loop is the same one used for other control of heating to
maintain the DAT Heating setpoint.
• Cooling: In the Cooling state, the Dehumidification Reheat Setpoint equals the DAT
Cooling Setpoint. For DAT units, this is the normal DAT setpoint resulting from any reset.
For Zone Control units, this setpoint is the result of a PI Loop based on the Control
Temperature as is done in the Economizer state or with modulated cooling in the Cooling
state.
• Fan Only: In the Fan Only state, the Dehumidification Reheat Setpoint equals an editable
MaxReheatSpt (Default = 65°F) when the Control Temperature drops to the Zone Heating
setpoint and equals an editable MinReheatSpt (Default = 55°F) when the Control
Temperature rises to the Zone Cooling setpoint. The Dehumidification Reheat Setpoint
varies linearly between these two points.
This means that no matter what the Control temperature is, including OAT, the temperature
leaving the unit is at a maximum just before the unit needs to go into heating and at a
minimum just before the unit needs to go into Cooling. This reduces opportunity for unit to go
into heating or cooling during Dehumidification. The Dehumidification Reheat Setpoints are
editable via the path, Extended Menus/Dehum Setup/Min Rheat Spt or Max Rheat Spt.
Staged HGRH Control
In the Cooling and Fan Only states, a staged output is turned on and off to control the HGRH
valve to maintain the Discharge Air Temperature at the Dehumidification Reheat Setpoint.
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