Which duct system allows for demand-based heating and cooling?

A variable-air-volume system is designed to provide demand-based heating and cooling by adjusting the amount of air distributed to various spaces based on the specific heating or cooling requirements at any given time. This system modulates the airflow rather than maintaining a constant flow, allowing for greater energy efficiency and comfort.

In practical application, as the occupancy levels or temperature needs change in different areas of a building, the variable-air-volume system can respond by increasing or reducing airflow to those zones. This flexibility helps to maintain optimal conditions while minimizing energy use, making it an ideal choice for environments where fluctuating heating and cooling demands are common.

In contrast, a constant air-volume system delivers a fixed amount of air regardless of the actual heating or cooling loads, which can lead to inefficiencies and discomfort. The all-water system primarily uses water as the heat transfer medium and typically does not offer the same immediate responsiveness to demand as an air-based system. Lastly, a two-pipe system, while effective for certain applications, does not inherently provide the ability to adjust airflow on demand in the same manner as a variable-air-volume system.