Classrooms and other school spaces must be ventilated to remove odors and pollutants.
A typical classroom using a dedicated outdoor air system (DOAS) distributes outside air (OA) to the rooms through a dedicated diffuser or introduced through the return air duct of the sensible cooling system.
To absorb internal latent loads properly designers must design the DOAS to deliver air at a lower dew point than the required dew point condition. The dry supply air will help absorb additional internal moisture, keeping the space at desired dew point conditions and preventing moisture-related problems.
Typically, engineers wanting to use a DOAS best design practice (divide and conquer), would plan to use a system that would deliver OA at a "neutral" condition, 75°F/24°C and 55°F/13°C dew point. This delivered condition is commonly associated with maintaining 75°F/24°C and 50% RH.
A simple DOAS can accomplish these conditions during the cooling season by processing OA through a cold coil down to a 55°F/13°C saturated condition and then reheating the air back up to 75°F/24°C, ensuring the air be delivered at 55°F/13° dew point. However, introducing OA at a 55°F/13°C dew point will not hold the space at 50% RH. A classroom will generate its own additional moisture from the students, space processes (such as in a lab), and infiltration. Schools that mop or steam clean carpets on a regular basis will introduce an incredible amount of additional moisture to the space. All of this additional moisture must be removed from the space in order to control RH within the recommended 40-60% range. Without this moisture control, structural damage and occupant health risks can happen.