HVAC diffusers are outlet ventilation points for heating, ventilation, and air conditioning (HVAC) systems. While they are occasionally used as return air vents, their primary purpose is to introduce conditioned air into a space for even distribution and mixing with minimal noise. HVAC diffusers are produced in a variety of standardized sizes, usually square, rectangular or round; they usually have a set of louvers or fins that direct airflow in a preset pattern. Larger industrial diffusers may include adjustable louvers or dampers that can be adjusted manually or automatically to control airflow. Although diffusers are often thought of as decorative grilles for air intakes, they play a vital role in the efficacy of an HVAC system and should be properly matched to any given application.

A register is a grille with moving parts that open and close to direct airflow as part of a building's heating, ventilation, and air conditioning (HVAC) system. Register location and size are critical to HVAC efficiency. The damper damper is also very important and can play a safety role.

The size of the register varies with the heating and cooling requirements of the room. If the register is too small, the HVAC system will need to push air through the ducts at a faster rate to achieve the desired heating or cooling. This can create a rattling sound that disturbs occupants or interferes with conversation or work (such as recording). The air velocity through the register is usually kept low enough to be masked by background noise. (Higher ambient background noise levels, such as in a restaurant, allow for higher air velocities.) On the other hand, the air velocity must be high enough to achieve the desired temperature. Registers are a critical part of the HVAC system. If not installed properly and connected tightly to the ductwork, air can escape around the regulator and greatly reduce the efficiency of the HVAC system. Ideally, a room will have both heating and cooling registers. In practice, cost considerations often require heating and cooling to be provided by the same register. In this case, heating usually takes precedence over cooling, and the registers are usually close to the floor.

Most diffusers are square or round with vents designed to match the shape of the diffuser, facing in different directions. There are some unique diffuser designs, such as eyeball diffusers, where the diffuser consists of a sphere within a holder that can be rolled to point the sphere air opening in any direction. Diffusers are very effective when air needs to be distributed evenly around a room or in a specific path at the same time. However, the pattern and airflow path depend on the design and location of the damper.

Most of the time, many air conditioning ducts and mechanical ventilation inlets mounted on walls or floors registers to control where the air goes. Some homes, hotels, or malls choose grilles with angled slots that serve as primary airflow guidance but don't offer the control flexibility of regular air vents.

Hvac diffusers are perforated covers for air ducts (for heating, cooling or ventilation, or a combination thereof). Hvac diffusers sometimes have louvers that direct the flow of air. Registers differ from Hvac diffuser by the inclusion of dampers. In practice, however, the terms "diffuser", "register", and "return" are often used interchangeably, and care must be taken to determine the meaning of the terms used.