313D Air Distribution and Ducting 3

Question 1

Which insulation vapor barrier placement is correct for a duct located in a hot, humid unconditioned attic?

Accepted Answer

Vapor barrier on the outside of the insulation facing the attic

Answer

In a hot-humid attic, the vapor barrier must face outward to block moisture from the warm attic air from reaching the cold duct surface.

Question 2

A duct system uses a trunk-and-branch layout. What is the primary advantage of this design?

Accepted Answer

Simplest to fabricate and requires less material than extended plenum

Answer

Trunk-and-branch systems use a central trunk with smaller branch takeoffs, reducing material compared to running full-size ducts to every outlet.

Question 3

What does the term 'aspect ratio' refer to in rectangular duct design?

Accepted Answer

Ratio of duct width to duct height

Answer

Aspect ratio is the ratio of a rectangular duct's width to its height; lower ratios (closer to 1:1) are more efficient.

Question 4

Which type of fire damper is required in a duct penetrating a fire-rated wall and also a smoke barrier?

Accepted Answer

Combination fire/smoke damper

Answer

Where a duct penetrates both a fire-rated wall and a smoke barrier, a combination fire/smoke damper satisfying both requirements must be installed.

Question 5

When sizing a return air grille, what is the generally recommended face velocity to minimize noise?

Accepted Answer

300–500 FPM

Answer

Return grilles are typically sized for 300–500 FPM face velocity to keep noise levels acceptable in residential applications.

(313D) Residential Air Conditioning Systems Mechanic Certification Practice Test