What is the role of system ventilation efficiency Ev?

• A. It measures the energy use of fans.
• B. It accounts for the effects of recirculation in ventilation systems.
• C. It sets limits for outdoor air.
• D. It determines filter maintenance frequency.

Answer: (B)

System ventilation efficiency Ev is about how effectively the outdoor air that a system brings in actually reaches and ventilates the occupied space, taking recirculation and distribution into account. In practice, Ev adjusts the amount of outdoor air to reflect real delivery: if a lot of outdoor air is mixed back with return air or poorly distributed, Ev is lower, meaning the outdoor air isn’t contributing as much to ventilation. If outdoor air is delivered and dispersed efficiently with minimal unwanted mixing, Ev is higher, approaching ideal delivery. This is why Ev’s role is to account for the effects of recirculation in ventilation systems. It’s not a measure of energy use, it doesn’t set outdoor air limits, and it doesn’t drive filter maintenance frequency. The concept helps explain why the effective outdoor air delivered to occupants may differ from the nominal outdoor air flow.