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CNT-SVX05A-EN
Sequence of operation
The cooling outputs are controlled based on the unit configuration and the
required machine cooling capacity. At 0% machine cooling capacity, the cooling
valve is closed and the outdoor air damper is at its minimum position. As the
required machine cooling capacity increases, the cooling valve and/or the
outdoor air damper opens above their minimum positions.
The constant volume control algorithm calculates a desired discharge air
temperature to maintain the space cooling setpoint. Cool capacity is controlled
to achieve the desired discharge air setpoint. Heat capacity can also be used to
temper cold outdoor air conditions to maintain ventilation and the discharge air
setpoint.
The outdoor air damper is used for cooling whenever economizing is possible
and there is a need for cooling. If economizing is not possible, it will not be
used in cooling. If economizing is possible, it is always the first stage of cooling.
See the
Outdoor air damper
section for more information.
Heating operation
In the heating mode, the Tracer
™
AH.540 controller attempts to maintain the
space temperature at the active heating setpoint. Based on the controller's
occupancy mode, the active space heating setpoint is one of the following:
• Occupied heating setpoint
• Occupied standby heating setpoint
• Unoccupied heating setpoint
The outputs are controlled based on the unit configuration and the required
machine heating capacity. At 0% machine heating capacity, the heating capacity
is at its minimum position. As the required machine heating capacity increases,
the heating capacity is opened above its minimum position. At 100% machine
heating capacity, the heating capacity is open to its maximum position.
The economizer outdoor air damper is never used as a source of heating.
The economizer damper is only used for ventilation when the unit is heating.
For more information about outdoor air damper operation, see the
Outdoor air
damper
operation
section.
Constant volume discharge air temperature control
Constant volume discharge air temperature control maintains a discharge air
temperature at a desired discharge air temperature setpoint regardless of the
entering air conditions of the air handling unit.
Figure 9 shows the steps the Tracer AH.540 controller takes to control discharge
air. First the controller determines if a communicated discharge air heating
setpoint and discharge air cooling setpoint are present. The communicated
setpoint has precedence over the configured (default) setpoint. If no
communicated value is present, the controller uses the configured discharge air
temperature setpoint.
Discharge air temperature setpoint minimum and maximum limits are placed
on the discharge air setpoint depending on the effective heat or cool mode.
If the effective heat/cool mode is cool, the maximum discharge air cooling
setpoint limit and minimum discharge air cooling setpoint limit the discharge
air cooling setpoint. See the
Configuration
section for more information about
discharge air heating and cooling setpoint limits.
