Air Distribution and Ducting Flashcards

Which fitting produces the least pressure drop when connecting a branch takeoff from a trunk duct?

Answer: Wye fitting with a gradual angle

A wye fitting with a gradual divergence angle smoothly redirects airflow with minimal turbulence, producing the lowest pressure drop of these options.

A technician notices that the first branch off a supply trunk delivers much more air than branches further downstream. What is the most likely cause?

Answer: The first branch has a lower resistance path and no balancing damper

Without balancing dampers, air preferentially takes the path of least resistance, over-supplying close branches and starving distant ones.

What is the correct procedure for sealing a connection between a flexible duct collar and a sheet metal fitting?

Answer: Use duct tape rated for duct use (UL 181B-FX) and pull the outer jacket over the collar

UL 181B-FX tape must be used to seal the inner liner to the collar, and the outer jacket and insulation should then be pulled over and secured with a tie or clamp.

A zoning system with multiple motorized dampers closes all zones simultaneously. What device prevents damage to the air handler from this condition?

Answer: A barometric relief damper or bypass damper

A bypass or barometric relief damper allows air to recirculate when all zone dampers are closed, preventing excessively high static pressure from damaging the blower.

Which duct construction standard is most commonly referenced for residential and light commercial sheet metal ductwork in the United States?

Answer: SMACNA HVAC Duct Construction Standards

SMACNA HVAC Duct Construction Standards define material gauges, joint types, and reinforcement for sheet metal ductwork fabrication.

When running ductwork through a garage that is attached to the living space, what is required by most building codes?

Answer: Ducts must be sealed and constructed of sheet metal or equivalent material that resists fire penetration

Codes require that ducts in attached garages be constructed of minimum 26-gauge steel or equivalent and be sealed to prevent CO and combustion gases from entering the living space.

What is the effect of installing a supply diffuser with too small a free area relative to the CFM being delivered?

Answer: Noise complaint due to high face velocity and excessive throw

An undersized diffuser forces air through a small opening at high velocity, generating noise and projecting air too far, causing drafts.