Overview - Metasys - LIT-12012070 - Field Device - VAV: Variable Air Volume Controller

Air Handling Unit Reset Application Note
Product name
VAV: Variable Air Volume Controller
Document type
Application Note
Document number
LIT-12012070
Revision date
2018-03-01

The LCT AHU Reset application has two distinct modes, summer and winter. When one mode is enabled, the other is disabled. The logic determines the current mode by evaluating whether the outside air temperature is above or below a setpoint (default 65°F with a 5°F differential).

Note: If you already have code that determines the summer and winter modes, you can use it if its output enumeration is State 0=Winter and State 1=Summer.

The application evaluates the VAV box temperature and pressure requests every 5 minutes by default. You can adjust this frequency.

In the summer mode, the discharge air temperature setpoint switches to its fixed temperature setpoint. The application decreases the discharge static pressure setpoint of the AHU until the total number of VAV boxes with zone dampers more than 95% open is greater than the pressure request setpoint. At this point, the application increases the discharge static pressure setpoint of the AHU. When the total number of VAV boxes with zone dampers less than 85% open is less than the pressure request setpoint, the application decreases the discharge static pressure setpoint to maintain the pressure request setpoint. The VAV boxes should be 85 to 95% open to maximize efficiency.

In the winter mode, the discharge air pressure setpoint switches to its fixed pressure setpoint. The application increases the discharge air temperature setpoint of the AHU until the total number of VAV boxes with a cooling output greater than 95% is greater than the temperature request setpoint. At this point, the application decreases the discharge air setpoint of the AHU. When the total number of VAV boxes with a cooling output less than 85% is less than the temperature request setpoint, the application increases the discharge air temperature setpoint to maintain the zone cooling demand at 85 to 95%.

As of CCT 10.1, all single-duct VAV box applications include logic for both the pressure request (PRESS‑REQ SRC) and the cooling request (CLG‑REQ SRC). Each logic module sets a floating point network output value. The pressure and temperature request outputs of each VAV box controller managed by the application must be mapped into the supervisory controller and ultimately into the temperature and pressure sums control system.