CSLB C-20 HVAC ContractorPlanning and LayoutMedium
A Manual T register selection process determines a supply diffuser has a rated throw of 12 feet at a terminal velocity of 50 fpm. The diffuser is installed in a room where the opposite wall is 18 feet away. What is the most likely consequence of this selection?
- AIncreasing duct static pressure will automatically extend the register's throw to 18 feet
- BSupply air will lose velocity before reaching the far wall, resulting in poor air mixing and possible stagnant zones
- CThrow ratings are irrelevant once terminal velocity drops below 100 fpm
- DThe register throw is more than sufficient to reach the opposite wall
Show answer & explanationAnswer & explanation
Correct answer: B. Supply air will lose velocity before reaching the far wall, resulting in poor air mixing and possible stagnant zones
Since the diffuser's rated throw of 12 feet falls short of the 18-foot distance to the opposite wall, the supply air stream will decelerate and drop before adequately mixing with room air near that wall, creating comfort complaints and temperature stratification.
Why the other options are wrong
- A. Throw is a diffuser design characteristic and is not significantly extended simply by increasing static pressure.
- C. Terminal velocity of 50 fpm is the standard reference point used specifically to define usable throw.
- D. 12 feet of throw is less than the 18-foot distance, so it is not sufficient.
Manual T Register Throw
Manual T evaluates air distribution devices by matching rated throw (distance air travels before reaching terminal velocity) to room dimensions for proper air mixing.
- Terminal velocity is typically defined at 50 fpm
- Throw shorter than room depth causes poor mixing and comfort issues
- Register selection must match both CFM and room geometry
Memory trick: If the throw falls short, comfort goes wrong.