84
RT-SVX073A-EN
Mode Status is not Cool, or a sensor required for
dehumidification control is invalid.
Dew Point
If space dew point is greater than the space dew point
setpoint (default = 60F) and outdoor air dew point is greater
than the outdoor air dew point threshold (default = 60F),
the unit enters dehumidification.
In addition to a space humidity sensor, an outdoor air
humidity sensor is also required.
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.
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, dehumidification control will start unit operation
under the following conditions:
• When a valid space relative humidity value rises above
the Space Dehumidification Unoccupied Setpoint, and
Space Temperature is above a fixed 64°F threshold.
• Or when Space Dew Point is greater than Space Dew
Point Unoccupied Setpoint AND Outdoor Air Dew Point
is greater than Outdoor Air Dew Point Threshold.
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
• Or Space Dew Point is less than Space Dew Point
Unoccupied Setpoint minus Space Dew Point offset.
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.
The table below summarizes Multi Zone-VAV supply fan
operation where Modulating means Duct Static Pressure
Control.
Table 38. Multi zone-VAV unit – Supply fan operation
Staged Gas Modulating Gas Staged Electric Heat External Heat
Occupied Heat Full Modulate Modulate Modulate
Maximum Heat Full Full Full Full
Morning Warm Up Full Full Full Modulate
Daytime Warm Up Full Full Full Modulate
Unoccupied Heat Full Full Full Modulate
Occupied Cool Supply Air
Tempering
NA – not allowed Modulate Modulate NA
Single Zone-VAV units have limited supply fan speed
modulation in Occupied Heat modes of operation when
modulating gas heat or modulating electric heat are
Unit Startup
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