Controls for variable-air-volume (VAV) systems must ensure a minimum outdoor air flow rate for space pressurisation. ASHRAE Standard 62-1989 places additional emphasis on the outdoor air flow to ensure adequate ventilation. Economiser controls also vary outdoor air flow to maintain discharge air temperature. In VAV systems, meeting these multiple outdoor-airflow-related requirements presents some significant control challenges. Focuses on VAV central system controls and shows how on-line outdoor air flow measurement and control can be used to ensure ventilation as well as meet space pressurisation requirements. A novel method for on-line outdoor air flow measurements is described. The method is based on a tracer gas concentration balance and uses carbon dioxide (CO2) as a tracer gas. It is easy to implement as a retrofit on existing systems and provides outdoor air flow measurement accuracy that is better than 10% of full scale. A control strategy that integrates the multiple requirements for providing outdoor air is recommended.

KEYWORDS: year 1995, outdoor air, air flow, air flow rate, controls, ventilation, pressurisation, variable volume air conditioning, economisers, air temperature, measuring, tracers, content, carbon dioxide, accuracy