Which equation represents zone outdoor air flow Voz?

• A. Voz = Vbz / Ez
• B. Voz = Ez / Vbz
• C. Voz = Vbz * Ez
• D. Voz = Vbz + Ez

Answer: (A)

The main idea is that zone outdoor air flow is tied to how much outdoor air the system actually brings into the building and how much of that outdoor air ends up serving the zone. Ez represents the fraction of the outdoor air that effectively reaches the zone, a dimensionless factor between 0 and 1. To achieve a desired zone outdoor air flow Voz, you must supply enough outdoor air into the system (Vbz) so that, after accounting for Ez, the zone gets Voz. This leads to Voz = Vbz / Ez. If Ez is less than 1, dividing by Ez increases the result, which makes sense: a portion of the outdoor air may not reach the zone, so you need a larger supply to meet the zone’s requirement.

Example: if Voz is 400 CFM and Ez is 0.8, then Vbz = Voz × Ez = 400 × 0.8 = 320 CFM, and Voz = 320 / 0.8 = 400 CFM.

The other forms either mix incompatible quantities or place the ratio in the wrong position, so they don’t correctly represent the relationship between Voz, Vbz, and Ez.