What is the typical moisture load formula used in humidification calculations?

• A. Total moisture load = Space gains + Infiltration rate + Ventilation rate + Indoor-outdoor humidity difference.
• B. Total moisture load = Outdoor temperature difference + Ventilation rate.
• C. Total moisture load = Humidity ratio × Airflow rate.
• D. Total moisture load = Space gains × Infiltration rate.

Answer: (A)

Moisture load in humidification calculations is all the moisture that must be added to the space air to maintain the desired humidity. The main sources are moisture generated inside the space (space gains) and moisture carried in by outdoor air through infiltration and ventilation. The amount carried in depends on how much air is exchanged and the difference in humidity between outdoor and indoor air (the indoor-outdoor humidity difference). So you sum those contributions: space gains plus infiltration load plus ventilation load plus the load associated with bringing in outdoor air (driven by the indoor-outdoor humidity difference). This gives the total moisture load the humidification system must offset.

The other options miss important pieces: outdoor temperature difference doesn’t directly govern moisture content; simply doing humidity ratio times airflow ignores internal generation and the separate effects of infiltration/ventilation; and multiplying space gains by infiltration rate combines two separate concepts in a way that doesn’t reflect how moisture actually enters the space.