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Sequence of Operation
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
74
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 space
Cool Setpoint before the controller can change to Cool.
3. The controller switches to Heat (from cool) after the
error integrator exceeds 900°F seconds or Space
Temperature <= (Heat Setpoint – 1°F).
RT-SVX063G-EN
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