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Registers, Grilles and Diffusers

Registers, grilles and diffusers are the air terminals at the end of the duct. A grille has no damper, a register adds a damper, and a diffuser spreads air in several directions, and each is selected from rated throw, noise and pressure drop data.

Ref HV-0024AHRI 880ASHRAE 70

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Air terminals deliver supply air into a room or return it to the system. The main types are:

Grille: a face with fixed or adjustable bars and no damper. Common on returns.
Register: a grille with a built-in damper to adjust or shut off airflow. Common on supply in homes.
Diffuser: a supply outlet that spreads air in several directions, usually in a ceiling. Square, round, louvered, plaque and slot styles are common.
Linear slot diffuser: long narrow outlet used along windows and in modern ceilings.

How to identify

Look for a damper lever or blades behind the face. If present, it is a register. Ceiling outlets with cones or louvers that throw air sideways are diffusers. Sizes are given by the duct opening (the neck or wall opening), not the outer face, which is larger.

Terminal types and uses

Type	Damper	Air pattern	Typical location
Return grille	No	Inlet only	Walls, ceilings, doors
Filter grille	No	Inlet with filter slot	Return at air handler
Supply register	Yes	One or two way	Floors, walls, low sidewall
Ceiling diffuser	Optional	Two to four way, or round	Commercial and residential ceilings
Linear slot diffuser	Optional	One or two way along length	Perimeter and architectural ceilings
Floor register	Yes	Upward	Under windows on older homes

Selection terms

Throw: the distance the air travels before slowing to a set terminal velocity. Too short leaves dead spots; too long causes drafts.
Noise criterion (NC): the rated sound level. Lower is quieter.
Pressure drop: the static pressure lost through the outlet at a given airflow.
Face velocity and neck velocity: higher velocity raises noise and throw.

These values come from the maker's catalog, tested to ASHRAE 70. AHRI 880 is the certification program for air terminals.

Key points

Size outlets from the airflow for each run, then check throw and NC in the maker's data.
A damper at the register is for fine balancing. Major balancing belongs at branch dampers.
Match the outlet pattern to the room. Ceiling diffusers near walls may need a blank-off on one side.
Return grilles need enough free area to keep noise and pressure drop down.

Common mistakes

Ordering by face size instead of duct opening size.
Using a register damper to fix a badly sized system.
Placing a high-throw outlet where it blows directly on people.
Ignoring free area on filter grilles.

Governing standards: ASHRAE 70 (method of testing air outlets and inlets); AHRI 880 (air terminals); maker's performance data.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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