blockDampers

Information

This sequence sets the dampers for dual-duct terminal unit using mixing control with discharge flow sensor. The implementation is according to Section 5.13.5 of ASHRAE Guideline 36, May 2020. The calculation is done following the steps below.

1. When the zone state is cooling (uCoo > 0), then the cooling loop output uCoo shall reset the supply airflow setpoint from the minimum VActMin_flow to cooling maximum setpoint VActCooMax_flow. The cooling damper shall be modulated by a control loop to maintain the measured aiflow VDis_flow at setpoint. The heating damper shall be closed yHeaDam=0.
   • If the cold-deck supply air temperature TColSup from the cooling air handler is greater than room temperature TZon, cooling supply airflow setpoint shall be no higher than the minimum.
2. When the zone state is deadband (uCoo=0 and uHea=0), the cooling airflow set point VColDucDis_flow_Set shall be the minimum set point VActMin_flow. The cooling damper shall be modulated by a control loop to maintain the measured cooling airflow at set point. The heating damper shall be closed.
3. When the zone state is heating (uHea > 0), then the heating loop output uHea shall be mapped to the heating damper position. The cooling damper is modulated to maintain measured discharge airflow at the minimum airflow setpoint VActMin_flow.
   • If the hot-deck supply air temperature THotSup from the heating air handler is less than room temperature TZon, the heating damper shall be closed.
   • Maximum heating airflow shall be limited by a reverse-acting P-only loop whose setpoint is the heating maximum VActHeaMax_flow and whose output is maximum heating damper position rangingfrom 0% to 100%.
4. Overriding above controls:
   • If heating air handler is not proven ON (u1HeaAHU=false), the heating damper shall be closed (yHeaDam=0).
   • If cooling air handler is not proven ON (u1CooAHU=false), the cooling damper shall be closed (yCooDam=0).

The sequences of controlling dampers position for dual-duct terminal unit using mixing control with inlet flow sensor are described in the following figure below.

Note that when the zone is in heating state, the wordings in Section 5.13.5.1.c does not specify the flow setpoint. This causes confusion when implementing system request sequences that is specified in Section 5.13.8.4, which requires the flow setpoint. To avoid the confusion, the sequence was then implemented according to the plot above, which is Figure 5.13.5 in Guideline 36.

Parameters

Connectors

Components

Revisions

• January 12, 2023, by Jianjun Hu:
  Removed the parameter have_preIndDam to exclude the option of using pressure independant damper.
  This is for issue 3139.
• August 1, 2020, by Jianjun Hu:
  First implementation.