Trane TRG-TRC014-EN Ventilation Reset, Application Considerations, period five, outdoor airflow

period five

Application Considerations

notes

Ventilation Reset

outdoor airflow	RA
		central air handler
measurement
and control		with controls
OA				PA
			VAV terminal units
			with DDC controls
		communicating BAS

Figure 97

With all three components, DDC controls on the VAV terminal units and air handler, a method to measure outdoor airflow at the air handler, and a communicating building automation system (BAS), a VAV system can use the multiple-spaces equation (Equation 6-1) from Standard 62 to dynamically operate the VAV system.

Each VAV terminal unit continuously monitors primary airflow. The DDC controller also knows the ventilation air requirement. With this information, the controller can continuously calculate the space ventilation fraction of the space, or (the ventilation airflow required divided by the primary airflow being delivered). The BAS periodically gathers this data from all the VAV terminal units and solves Equation 6-1. This determines the amount of outdoor airflow that must be introduced at the system level, at the central air handler, to satisfy the ventilation requirements. The BAS then resets the outdoor airflow set point, and the outdoor airflow measuring device controls the damper accordingly.

This method, called ventilation reset, ensures that all spaces are properly ventilated at all load conditions while minimizing wasteful overventilation.

70 TRG-TRC014-EN