What is the minimum zone supply outdoor air requirement for ventilation systems?

What is the minimum zone supply outdoor air requirement for ventilation systems?

• A. Zone supply must be at least equal to the outdoor air requirement.
• B. Zone supply must be less than the outdoor air requirement.
• C. Zone supply must be at least 2 times the outdoor air requirement.
• D. Zone supply must be at least 1.5 times the outdoor air requirement (Vpz-min = 1.5Voz).

Answer: (D)

The key idea is that the outdoor air required to meet IAQ must actually reach the space, but the air that ends up in the zone is a mix of outdoor and recirculated air, with distribution losses in the system. To guarantee the zone gets the necessary amount of outdoor air, the zone supply must be larger than the outdoor air requirement. The standard sets the minimum zone supply outdoor-air rate as Vpz-min = 1.5 × Voz, meaning you must design the zone’s supply to deliver at least 50% more outdoor air than the basic outdoor-air requirement. For example, if the outdoor-air requirement Voz is 100 cfm, you design the zone supply to provide at least 150 cfm of outdoor air, so after mixing with recirculated air the zone effectively receives the required 100 cfm.

The key idea is that the outdoor air required to meet IAQ must actually reach the space, but the air that ends up in the zone is a mix of outdoor and recirculated air, with distribution losses in the system. To guarantee the zone gets the necessary amount of outdoor air, the zone supply must be larger than the outdoor air requirement. The standard sets the minimum zone supply outdoor-air rate as Vpz-min = 1.5 × Voz, meaning you must design the zone’s supply to deliver at least 50% more outdoor air than the basic outdoor-air requirement. For example, if the outdoor-air requirement Voz is 100 cfm, you design the zone supply to provide at least 150 cfm of outdoor air, so after mixing with recirculated air the zone effectively receives the required 100 cfm.