313D Airflow and Duct Sizing 3 — Questions and Answers
Question 1: A rectangular duct measures 12 inches by 8 inches. What is its equivalent round duct diameter?
- 8.8 inches
- 9.6 inches (Correct answer)
- 10.7 inches
- 11.3 inches
Correct answer: 9.6 inches
The equivalent diameter for a 12×8 rectangular duct using the hydraulic diameter formula is approximately 9.6 inches.
Question 2: What is the primary reason flexible duct should be fully stretched during installation?
- To prevent condensation buildup
- To minimize airflow resistance from inner liner folds (Correct answer)
- To allow thermal expansion
- To improve fire resistance
Correct answer: To minimize airflow resistance from inner liner folds
Compressed or sagging flex duct creates corrugated inner surfaces that dramatically increase friction and reduce airflow.
Question 3: According to ACCA Manual D, what is the maximum recommended length for a flexible duct run?
- 10 feet
- 14 feet (Correct answer)
- 20 feet
- 25 feet
Correct answer: 14 feet
ACCA Manual D recommends limiting individual flex duct runs to no more than 14 feet to control friction losses.
Question 4: What happens to system airflow when the external static pressure (ESP) exceeds the blower's rated ESP?
- Airflow increases above design CFM
- Airflow decreases below design CFM (Correct answer)
- The blower speed automatically adjusts
- System efficiency improves
Correct answer: Airflow decreases below design CFM
When external static pressure exceeds design limits, the blower delivers less airflow than intended, reducing system capacity.
Question 5: Which duct leakage test measures total system leakage including leakage to conditioned space?
- Duct blaster total leakage test (Correct answer)
- Leakage to outside test
- Pressure pan test
- Blower door test
Correct answer: Duct blaster total leakage test
The total leakage test measures all duct leakage regardless of where it occurs, including leaks inside the conditioned envelope.
Question 6: A supply duct system has a design CFM of 1,200. Using the rule of thumb, approximately how many CFM should each 6-inch branch supply?
- 50 CFM
- 75 CFM
- 100 CFM (Correct answer)
- 125 CFM
Correct answer: 100 CFM
A 6-inch round duct at standard residential velocities typically delivers approximately 100 CFM.
Question 7: What is the effect of a poorly sealed duct boot connection at a register?
- Increased supply air temperature
- Conditioned air loss into unconditioned spaces reducing efficiency (Correct answer)
- Higher indoor humidity
- Increased blower RPM
Correct answer: Conditioned air loss into unconditioned spaces reducing efficiency
Leaky boot connections allow conditioned supply air to escape into wall cavities or attic spaces, wasting energy and reducing delivery.
313D Airflow and Duct Sizing 3
A rectangular duct measures 12 inches by 8 inches.
What is its equivalent round duct diameter?