313D Air Distribution and Ducting 3 — Questions and Answers
Question 1: Which insulation vapor barrier placement is correct for a duct located in a hot, humid unconditioned attic?
- Vapor barrier on the inside of the duct liner
- Vapor barrier on the outside of the insulation facing the attic (Correct answer)
- No vapor barrier is needed in attic installations
- Vapor barrier only on the return duct
Correct answer: Vapor barrier on the outside of the insulation facing the attic
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?
- Easiest to balance airflow to individual rooms
- Simplest to fabricate and requires less material than extended plenum (Correct answer)
- Eliminates the need for dampers entirely
- Provides the lowest static pressure of any layout
Correct answer: Simplest to fabricate and requires less material than extended plenum
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?
- Ratio of duct length to duct perimeter
- Ratio of duct width to duct height (Correct answer)
- Ratio of supply CFM to return CFM
- Ratio of static pressure to velocity pressure
Correct answer: Ratio of duct width to duct height
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?
- Type A fire damper only
- Smoke damper only
- Combination fire/smoke damper (Correct answer)
- No damper is required if duct is metal
Correct answer: Combination fire/smoke damper
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?
- 100–200 FPM
- 300–500 FPM (Correct answer)
- 600–900 FPM
- 1000–1200 FPM
Correct answer: 300–500 FPM
Return grilles are typically sized for 300–500 FPM face velocity to keep noise levels acceptable in residential applications.
Question 6: A technician measures 0.80 inches W.C. total external static pressure on a system rated for 0.50 inches W.C. What corrective action should be taken first?
- Increase blower speed immediately
- Inspect the duct system for restrictions, undersized ducts, or dirty filters (Correct answer)
- Add refrigerant to lower head pressure
- Replace the air handler with a higher-capacity unit
Correct answer: Inspect the duct system for restrictions, undersized ducts, or dirty filters
Elevated external static pressure most often results from duct restrictions, undersizing, or a clogged filter rather than a blower or refrigerant issue.
Question 7: What is the function of a balancing damper installed in a branch duct?
- To shut off airflow completely in a zone
- To throttle airflow to achieve the design CFM for that branch (Correct answer)
- To prevent backdraft when the system is off
- To direct airflow upward toward the ceiling diffuser
Correct answer: To throttle airflow to achieve the design CFM for that branch
Balancing dampers are adjustable devices used to throttle flow in individual branches until design airflow rates are achieved throughout the system.
313D Air Distribution and Ducting 3
Which insulation vapor barrier placement is correct for a duct located in a hot, humid unconditioned attic?