CSLB C-20 HVAC ContractorVentilation and Air DistributionMedium
A technician is selecting a supply diffuser for a large open room and needs the air to travel across the space without creating a noticeable draft in the occupied zone. Which factor has the greatest direct effect on how far the diffuser will throw air?
- AThe color and finish of the diffuser face
- BThe static pressure reading measured at the return grille
- CThe discharge (outlet) velocity produced by the CFM and the diffuser's effective free area
- DThe thickness of duct insulation upstream of the diffuser
Show answer & explanationAnswer & explanation
Correct answer: C. The discharge (outlet) velocity produced by the CFM and the diffuser's effective free area
Throw distance is primarily governed by the discharge velocity leaving the diffuser, which depends on the CFM delivered and the diffuser's effective (free) area. Higher discharge velocity increases throw, while insulation thickness and return static pressure have no direct effect on how far the air jet travels.
Why the other options are wrong
- A. Cosmetic finish has no bearing on airflow performance.
- B. Return static pressure relates to system resistance, not diffuser throw distance.
- D. Duct insulation affects heat loss/gain, not diffuser throw.
Diffuser Throw
The distance air travels from a diffuser before its velocity decays to a specified terminal velocity, primarily controlled by discharge velocity from CFM and effective outlet area.
- Throw increases with higher discharge velocity
- Discharge velocity = CFM / effective free area
- Manufacturer throw/terminal velocity charts guide diffuser selection
Memory trick: Faster jet, farther yet — velocity drives the throw.