In ventilation calculations, infiltration loads are typically considered when the ventilation rate is which condition?

• A. When the ventilation rate is zero.
• B. When the ventilation rate is at maximum.
• C. When outdoor humidity is high.
• D. When indoor humidity is at setpoint.

Answer: (A)

Infiltration is the unintentional exchange of outdoor air through a building’s envelope. In ventilation calculations, you account for infiltration particularly when there is no mechanical or intentional ventilation supplying outdoor air—the ventilation rate is effectively zero. When nothing else is introducing outdoor air, infiltration becomes the sole source of outdoor air and drives the outdoor-air load, so it must be included to avoid underestimating ventilation and related energy and IAQ impacts.

If the ventilation rate were at or near a designed or maximum supply, the system’s controlled outdoor-air delivery dominates the calculation, and infiltration is not the primary driver. Outdoor humidity or indoor humidity setpoints don’t determine whether infiltration is considered; they affect latent loads but not the decision about relying on ventilation rate to define outdoor-air supply.