How to Select Supply-Air Diffusers Using Throw, Drop and Spread

nexoradesign.net
Jul 24
2 min read

Selecting a supply-air diffuser is not simply a matter of matching the diffuser neck size to the duct connection. A diffuser must deliver the required airflow, distribute conditioned air throughout the occupied zone, avoid uncomfortable drafts, operate within the permitted noise level, and maintain an acceptable pressure loss.

Three of the most important performance characteristics used during diffuser selection are:

Throw
Drop
Spread

These characteristics describe how the supply-air jet travels after leaving the diffuser. Understanding them allows designers to predict whether conditioned air will reach the required parts of a room without causing drafts, stagnant zones, short-circuiting, or excessive noise. (How to Select Supply-Air Diffusers Using Throw, Drop and Spread)

This article explains these terms, how they interact, how to read diffuser performance data, and how to apply them in an actual selection.

1. Why Diffuser Selection Matters

A well-designed HVAC system can still provide poor comfort if the air terminals are incorrectly selected or located. The cooling coil may produce the correct supply-air temperature, the fan may deliver the design airflow, and the ductwork may be properly sized—but the occupants can still experience: