Which statement about recirculation is correct?

• A. It refers to exhausting all indoor air to the outdoors.
• B. It implies never using outdoor air.
• C. It involves conditioning outdoor air only.
• D. It is the process of reusing air within a space instead of continuously bringing in new outdoor air.

Answer: (D)

Recirculation refers to reusing air that’s already inside the space by returning it to the air handler and conditioning it again before sending it back to the space, rather than continually replacing it with outdoor air. In practice, systems mix a portion of outdoor air to meet ventilation requirements, while the rest of the air is recirculated after filtration and conditioning. This helps save energy because you’re conditioning air that’s already near the desired temperature and humidity instead of always treating fresh outdoor air. The statement that best captures this idea is the one describing reusing air within a space instead of continuously bringing in new outdoor air.