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RT-SVX063E-EN
Discharge Air Reset
Discharge Air Reset is a method to save energy by
resetting the discharge air temperature as heating and
cooling building loads increase and decrease.
When enabled, the controller can independently reset
the Discharge Air Heating Setpoint Active and/or
Discharge Air Cooling Setpoint Active based on Zone
(space temperature) or Outdoor Air temperature. The
new target setpoints are reported as Discharge Air
Setpoint Heating Status and Discharge Air Cooling
Setpoint Status.
The front panel display provides settings for the reset
temperature range (Zone or Outdoor Air) and discharge
air temperature setpoint reset amount over the
specified range. For example: the discharge air
temperature cooling setpoint shall increase 5°F over an
outdoor air temperature range of 90°F to 70°F. If
Discharge Air Cooling Setpoint Active is 50°F and
Outdoor Air Temperature is 80°F, the reset function
calculates and reports Discharge Air Cooling Setpoint
Status = 52.5°F. The controller will provide discharge
air at 52.5°F.
Duct Static Pressure Control
Multi Zone-VAV equipment have variable speed supply
fans and are often used in Variable Air Volume (VAV)
systems consisting of ductwork serving multiple
building zones and VAV boxes that control space
comfort independent of the rooftop air handler. VAV
boxes modulate air volume by a damper that opens
and closes to maintain space comfort. In response,
duct static pressure increases and decreases. The
controller will modulate supply fan speed to maintain
Duct Static Pressure relative to the Duct Static Pressure
Setpoint
Duct Static Pressure High Limit
The controller operates the supply fan to maintain duct
static pressure below the Duct Static Pressure High
Limit setpoint. In the event that duct static pressure
approaches the high limit setpoint, the controller will
reduce and limit the supply fan speed.
Space Temperature Control
Space Temperature Control uses available heating and
cooling to deliver comfort to a building space. The
control requires a valid space temperature and
discharge air sensor value; if either input to the control
is invalid, the equipment cannot operate.
Space Temperature Control determines the type of
load (heating or cooling) in the space based on active
cooling and heating setpoints and space temperature.
If space temperature is above the cooling setpoint, this
represents a space-cooling load. The control will
calculate a Discharge Air Temperature Setpoint to
satisfy the cooling demand of the space.
The relationship of the calculated discharge air setpoint
and discharge air temperature will drive heating and
cooling capacity available to satisfy the discharge air
temperature setpoint. For Example: As space
temperature cooling load increases, space temperature
control will calculate a lower Discharge Air
Temperature setpoint and therefore will increase
cooling capacity. The calculated discharge air cooling
setpoint is bound by Discharge Air Temperature
Maximum Cool Limit and Discharge Air Temperature
Minimum Cool Limit.
If heat-cool mode input is either not provided or
unsupported the mode input is Auto. The controller will
automatically determine the appropriate heat or cool
mode of operation based on space temperature,
setpoints, and heating/cooling configured for the unit.
Heat or Cool is reported by Heat Cool Mode Status.
Auto Changeover
When the controller input mode of operation is Auto,
the controller will automatically determine space
heating and cooling mode, the unit changes from cool
to heat or from heat to cool, when the integrated error
between the space setpoint and space temperature is
900°F-seconds or greater. The integrated error is
calculated once every 10 seconds. See figure below for
an example of changing from space cooling to space
heating. If Cool is the active mode and space
temperature is less than 1°F below the active heating
setpoint the system will immediately change to Heat
mode. If space temperature is greater than 1°F above
the active cooling setpoint, the mode to change to
Cool.
Space Dehumidification is a Cool mode sequence of
operation if the unit is equipped with hot gas reheat. A
unit that has a dehumidification demand will switch
from Heat mode to Cool mode if heating capacity is off
and space temperature is above the heating setpoint. A
unit operating in dehumidification control will inhibit
the normal mode transition from Cool mode to Heat
mode.
Figure 39.
Auto changeover logic
1. This is the point at which the cooling capacity
equals 0% and the error integrator starts to add up.
Error integration does not begin until the capacity is
0%. The error integrator cannot be less than zero.
2. The Space Temperature must be less than Space
Setpoint before the controller can change to Heat.
The Space Temperature must rise above the active
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