In humidification calculations, the typical infiltration load is usually considered under which condition?

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

Answer: (B)

Infiltration load is the moisture that enters a space through leaks in the building envelope, not through the intended ventilation system. To estimate this latent load in a simple, actionable way, designers commonly isolate it by assuming there is no deliberate ventilation at all. When the ventilation rate is zero, any moisture that enters comes solely from infiltration, so the resulting latent load represents the baseline infiltration that the humidification system would need to handle. Real conditions will vary with outdoor humidity and indoor setpoints, but using zero ventilation provides a clear, worst-case–type estimate of infiltration. The other conditions change the actual moisture balance, but they don’t define the standard way infiltration load is modeled.