California IDEX Practice Exam 2026 - Free IDEX Practice Questions and Study Guide

The device that controls the volume of air and its distribution in an HVAC system is indeed called a register. Registers are essential components within an HVAC system that allow for the adjustment of airflow to different areas of a building. They typically have an adjustable damper that can open or close to increase or decrease the amount of air delivered to a space.

While other options play roles in air circulation, they do not serve the same function as registers. Grilles and diffusers are primarily intended for air distribution but do not have the adjustable features that let users control airflow volume directly. Vents, on the other hand, generally refer to openings that allow air to flow in or out but do not typically include mechanisms for regulating that flow. Thus, the register is uniquely suited for controlling both the volume and distribution of air in an HVAC system.