Trane Tracer Uc page 49

All units have a modulating air valve that is used to control how much air is flowing through the
diffusers and into the space. By modulating the volume of airflow, the unit heating capacity is
modulated.
Reheat Enable Setpoint
Units may have local or remote reheat, which may be hydronic or electric. Reheat is turned on when
the space temperature is below the active space temperature setpoint, and the supply air
temperature is below the reheat enable setpoint. Reheat is used to maintain the space temperature
at the heating setpoint. Heating operation and Reheat are two different entities.
Auto-changeover
The auto-changeover feature is used if the RTU has heat and the UC400 has a valid local or
communicated Supply Air Temperature. If the supply air temperature reaches the Auto-changeover
Point, then the air valve control changes from cooling control action to heating control action.
In cooling control action, the air valve modulates between the Airflow Setpoint Minimum and
Airflow Setpoint Maximum to meet the required cooling capacity. In heating control action, the air
valve modulates between Airflow Setpoint Minimum Heat and Airflow Setpoint Maximum Heat to
meet the required heating capacity. Essentially, the RTU is providing heat to the space. If the
heating capacity increases, the air valve modulates further open to provide more hot air to the
space. In reheat mode, the supply air is assumed to be cool and the air valve closes to the Airflow
Setpoint Minimum Local Heat to reduce the volume of cold air the VAV box heating element or coil
has to heat.
Air Valve Control in Space Temperature Control Operation
Air delivered to the space is controlled with a three-wire, floating-point actuator that modulates the
air valve. The controller positions the modulating air valve to deliver the desired airflow (cooling
or heating capacity).
The desired airflow is called the active flow setpoint. The controller positions the modulating air
valve to deliver the desired airflow (cooling and heating capacity) to within +/-2% of nominal
airflow. For example, if the nominal airflow of the box is 900 CFM, the air valve is controlled to
maintain flow within +/- 18 CFM of the airflow setpoint.
The airflow control algorithm compares the active airflow setpoint with the measured airflow and
calculates the necessary air valve movement to minimize error. The airflow setpoint is limited by
applicable minimum and maximum flow setpoints.
Reheat Control
There are two (2) types of reheat control:
•
•
Reheat is allowed to turn ON if the zone temperature is below the heating setpoint. Space
temperature control can use reheat if the following conditions are all TRUE:
VAV-SVX07B-EN
Hydronic heat
Electric heat
– The unit is not calibrating
– Heat Cool Mode Status is heat
– For local reheat only, the fan, if present, is not being overridden to Off (communicated fan
override)
– The supply air temperature is less than the configured reheat enable setpoint
Calibration, Operation Modes, and Control
49