80 McQuay OM 920-1
Operator’s Guide
The deadband for this setpoint is an adjustable. When the DAT is above the Dehumidification
Reheat Setpoint by more than half the deadband, cooling capacity is greater than 0% and the
Dehumidification Reheat Timer has expired, the reheat output is turned on. When the DAT is
below the Dehumidification Reheat Setpoint by more than half the deadband and the
Dehumidification Reheat Timer has expired, or if the cooling capacity is 0%, the reheat output
is turned off.
Modulating HGRH Control
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 Loop is enabled
when the unit is in the Cooling or Fan Only operating state and dehumidification becomes
active.
Note – Note: When configured for modulating hot gas reheat the reheat valve is stroked open and
then closed every day at 2:00 AM as long as dehumidification is inactive and the unit is not
in the Cooling operating state at the time.
Reheat Setpoint Control
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.
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.
Energy Recovery
Energy recovery is provided by drawing outside air across half of an Enthalpy Wheel and
drawing exhaust air across the other half. Latent and sensible heat is transferred from the
hotter, moister exhaust air to the colder dryer outside air in winter. Latent and sensible heat is
transferred from the hotter more moist outside air to the cooler dryer air exhaust in summer.
Control consists of starting and stopping an exhaust fan, modulating the speed of the exhaust
fan, starting and stopping the Enthalpy Wheel, and optionally controlling the speed of the
Enthalpy Wheel. The outdoor dampers are controlled in the normal manner. The current
statuses as well as editable parameters associated with Energy Recovery are located in the
Standard Menus/Energy Recovery menus.
Enthalpy Wheel
The Enthalpy Wheel is turned on when all of the following are true.
• The Exhaust Fan is on
• The OA Dampers are at the minimum position
• The unit is not in the Economizer State
• The wheel has not been turned off due to Frosting/Condensation
Note – When turned on the speed of the enthalpy wheel is set to the maximum speed for units
with a VFD controlling the speed of the Enthalpy Wheel.
The Enthalpy Wheel is turned off when any of the following is true
• The Exhaust Fan is off OR
• The OA Damper Position is driven above the Minimum OA Damper Position by more than
3%
To Next Page
Loading...
Need help?
General
