313D Airflow and Duct Sizing 3

Question 1

A rectangular duct measures 12 inches by 8 inches. What is its equivalent round duct diameter?

Accepted Answer

9.6 inches

Answer

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?

Accepted Answer

To minimize airflow resistance from inner liner folds

Answer

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?

Accepted Answer

14 feet

Answer

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?

Accepted Answer

Airflow decreases below design CFM

Answer

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?

Accepted Answer

Duct blaster total leakage test

Answer

The total leakage test measures all duct leakage regardless of where it occurs, including leaks inside the conditioned envelope.

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