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In minimum ventilation mode and the damper feedback
position is >10% of the damper commanded value for 5
continuous minutes.
FDD: Outdoor Air Damper Not Modulating
In minimum ventilation mode and the damper feedback is
<10% of the damper commanded value for 5 continuous
minutes.
FDD: Unit Economizing When It Should Not
In economizer cooling mode and the damper feedback is
>10% of the damper commanded value for 5 continuous
minutes.
FDD: Unit Not Economizing When It Should
In economizer cooling mode and the damper feedback is
<10% of the damper commanded value for 5 continuous
minutes.
Dehumidification Control -
Occupied
Dehumidification control is a sub-mode of Cool when hot
gas reheat is configured and the dehumidification feature is
enabled. A valid Evaporator Leaving Air Temperature
sensor(s) are required. Two circuit, split coil units will have
an Evaporator Leaving Air Temperature sensor installed in
each airstream. When a valid space relative humidity value
is above the Space Dehumidification Setpoint,
dehumidification control is invoked. Economizer cooling
disables. The sub-mode Cool – Dehumidification is
reported on the user interface Operating Modes screen.
Dehumidification control disables when space relative
humidity is less than Space Dehumidification Setpoint
minus the Occupied Dehumidification Offset, Heat Cool
Mode Status is not Cool or a sensor required for
dehumidification control is invalid.
Discharge Air Control Units
When Dehumidification is active, cooling capacity is
controlled by Evaporator Leaving Air Temperature relative
to Evaporator Leaving Air Temperature Setpoint. This
setpoint is suitable to satisfy the space humidity load.
Cooling capacity will first load on circuit 1 to provide hot gas
reheat capacity. The Hot Gas Reheat valve modulates to
manage reheat capacity to satisfy the Discharge Air
Temperature Setpoint.
Optionally, a building automation system can set the
Discharge Air Reheat Setpoint. This then becomes the
target setpoint for hot gas reheat control, when
dehumidification is active. Likewise, Evaporator Leaving Air
Temperature Setpoint can be written for dehumidification
control.
RT-SVX063G-EN
Space Temperature Control Units
Space temperature units internally use dew point to
manage space humidity control. When hot gas reheat is
inactive, the cooling control calculates a suitable setpoint to
manage both space temperature and space humidity. If
space temperature falls below the space cooling setpoint
minus a fixed differential, the hot gas reheat valve
modulates to maintain space temperature control. The
evaporator leaving air temperature setpoint is calculated to
control humidity independent of space temperature.
Optionally, a building automation system can set the
Evaporator Leaving Air Temperature Setpoint to control DX
cooling capacity.
Dehumidification Control -
Unoccupied
Unoccupied Dehumidification control sequence of
operation is the same for all unit types when the controller
is in unoccupied mode, hot gas reheat is configured and
the unoccupied dehumidification feature is enabled.
Typically, the unit is off when in unoccupied mode;
however, when a valid space relative humidity value rises
above the Space Dehumidification Unoccupied Setpoint
and Space Temperature is above a fixed 64°F threshold,
dehumidification control will start unit operation.
Economizer cooling disables and the sub-mode Cool –
Dehumidification is reported on the user interface
Operating Modes screen.
When Dehumidification is active, cooling capacity is
controlled by Evaporator Leaving Air Temperature relative
to Evaporator Leaving Air Temperature Setpoint. This
setpoint is suitable to satisfy the space humidity load.
Cooling capacity will stage to satisfy the evaporator cooling
demand. The hot gas reheat valve modulates to maintain a
space temperature setpoint.
Unoccupied Dehumidification control disables when space
relative humidity is less than Space Dehumidification
Unoccupied Setpoint minus Unoccupied Dehumidification
Offset or Space Temperature falls below a fixed 60°F
threshold.
Heat Types
Variable speed supply fan operation is different depending
on the heat type installed. Some heat types require the
supply fan to be at full speed. Other heat types allow
modulating supply fan speeds in specific operating modes.
Table 35, p. 84
below summarizes Multi Zone-VAV supply
fan operation where Modulating means Duct Static
Pressure Control.
Sequence of Operation
83
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