Strategies for outdoor airflow control from a systems perspective

Controls for variable-air-volume (VAV) systems must ensure a minimum outdoor airflow rate for space pressurization. ASHRAE Standard 62-1989 places additional emphasis on the outdoor air flow to ensure adequate ventilation. Economizer controls also vary outdoor airflow to maintain discharge air temperature. In VAV systems, meeting these multiple outdoor-airflow-related requirements presents some significant control challenges. This paper focuses on VAV central system controls and shows how on-line outdoor airflow measurement and control can be used to ensure ventilation as well as meet space pressurization requirements. A novel method for on-line outdoor airflow measurement is described. The method is based on a tracer gas concentration balance and uses carbon dioxide (CO{sub 2}) as a tracer gas. It is easy to implement as a retrofit on existing systems and provides outdoor airflow measurement accuracy that is better than 10% of full scale. A control strategy that integrates the multiple requirements for providing outdoor air is recommended.