Airflow and Duct Sizing Flashcards

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

Answer: 9.6 inches

The equivalent diameter for a 12×8 rectangular duct using the hydraulic diameter formula is approximately 9.6 inches.

What is the primary reason flexible duct should be fully stretched during installation?

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.

According to ACCA Manual D, what is the maximum recommended length for a flexible duct run?

Answer: 14 feet

ACCA Manual D recommends limiting individual flex duct runs to no more than 14 feet to control friction losses.

What happens to system airflow when the external static pressure (ESP) exceeds the blower's rated ESP?

Answer: Airflow decreases below design CFM

When external static pressure exceeds design limits, the blower delivers less airflow than intended, reducing system capacity.

Which duct leakage test measures total system leakage including leakage to conditioned space?

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.

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?

Answer: 100 CFM

A 6-inch round duct at standard residential velocities typically delivers approximately 100 CFM.

What is the effect of a poorly sealed duct boot connection at a register?

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.