Question

how will adding a VAV control the air flow volume in main duct

Answer 1

hello friend here is your answer.
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heating, ventilating, and/or air-conditioning (HVAC) system. Unlike constant air volume(CAV) systems, which supply a constant airflow at a variable temperature, VAV systems vary the airflow at a constant temperature. 
i hope it will help you.
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Answer 2

heating, ventilating, and/or air-conditioning (HVAC) system. Unlike constant air volume(CAV) systems, which supply a constant airflow at a variable temperature, VAV systems vary the airflow at a constant temperature. 
i hope it will help you.
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