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RT-SVX063E-EN
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, 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 34, p. 84
below summarizes
Multi Zone-VAV supply fan operation where
Modulating means Duct Static Pressure Control.
Table 34.
Multi zone-VAV unit – Supply fan operation
Staged Gas
Modulating Gas
Staged Electric
Heat
Modulating
Electric Heat
External Heat
Occupied Heat
Full
Modulate
Full
Modulate
Modulate
Maximum Heat
Full
Full
Full
Full
Full
Morning Warm Up
Full
Full
Full
Full
Modulate
Daytime Warm Up
Full
Full
Full
Full
Modulate
Unoccupied Heat
Full
Full
Full
Full
Modulate
Occupied Cool
Supply Air Tempering
NA – not allowed
Modulate
NA
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
configured; otherwise, the supply fan operates at full
capacity in all other heat operating mode
All heat types, when the control terminates heating
capacity or exists a heating mode; a 5-minute post heat
timer is enforced. This keeps supply fan On to remove
heat from the unit before transitioning to a cooling
mode or cycling the supply fan off.
Gas Heat
There are three types of gas heat available: Staged gas,
Modulating gas and Ultra Modulating gas. Each type of
gas heat is composed of one or two manifolds, each
manifold can have one or more burners. As demand for
heat increases, incremental stages of heat are added.
All burners on a manifold ignite in sequential order
starting with manifold A. When manifold A has staged
up to full capacity, staging starts on manifold B (when
equipped). As the demand for heat decreases, the
highest burner on the highest manifold will turn off
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