313D Air Distribution and Ducting 2 — Questions and Answers
Question 1: What is the purpose of a static pressure tap in a duct system?
- To measure air velocity directly
- To measure the pressure of air against the duct wall (Correct answer)
- To balance airflow between supply and return
- To detect refrigerant leaks in the air handler
Correct answer: To measure the pressure of air against the duct wall
A static pressure tap measures the pressure exerted by air against the duct wall, perpendicular to flow direction.
Question 2: Which duct shape provides the most efficient airflow per unit of material used?
- Square
- Rectangular with 2:1 aspect ratio
- Round (Correct answer)
- Oval
Correct answer: Round
Round ducts have the lowest surface-area-to-cross-section ratio, minimizing friction losses and material use.
Question 3: A residential duct system has excessive external static pressure. What is the most likely result?
- Increased airflow and better comfort
- Reduced airflow and potential equipment damage (Correct answer)
- Higher humidity levels in the supply air
- Faster refrigerant cycle times
Correct answer: Reduced airflow and potential equipment damage
Excessive external static pressure restricts airflow, causing the blower to work harder, reducing CFM and potentially damaging the motor.
Question 4: When installing flexible duct, what maximum sag per foot of run is generally acceptable?
- 1/2 inch per foot (Correct answer)
- 1 inch per foot
- 2 inches per foot
- No sag is acceptable
Correct answer: 1/2 inch per foot
ACCA and SMACNA guidelines allow no more than 1/2 inch of sag per foot of flexible duct to prevent airflow restriction.
Question 5: What term describes the pressure loss caused by air changing direction in a duct fitting?
- Velocity pressure drop
- Dynamic pressure loss (Correct answer)
- Static regain
- Equivalent length loss
Correct answer: Dynamic pressure loss
Dynamic pressure loss refers to energy lost when air changes direction or velocity through fittings such as elbows and tees.
Question 6: A supply register is delivering noticeably warm air on one side and cool air on the other side. What is the most likely cause?
- Undersized return duct
- Poor air mixing due to stratification upstream (Correct answer)
- Refrigerant overcharge
- Incorrect thermostat setpoint
Correct answer: Poor air mixing due to stratification upstream
Temperature stratification in the duct upstream of the register can cause uneven discharge temperatures across the grille face.
Question 7: What is the recommended maximum length for a flexible duct run without a support hanger?
- 2 feet
- 4 feet (Correct answer)
- 6 feet
- 8 feet
Correct answer: 4 feet
Flexible duct should be supported at intervals no greater than 4 feet to prevent excessive sagging and flow restriction.
313D Air Distribution and Ducting 2
What is the purpose of a static pressure tap in a duct system?