HVAC Ductwork 3 — Questions and Answers
Question 1: What ASHRAE standard primarily governs acceptable ventilation rates and duct design for commercial HVAC systems?
- ASHRAE 62.1 (Correct answer)
- ASHRAE 90.1
- ASHRAE 55
- ASHRAE 15
Correct answer: ASHRAE 62.1
ASHRAE 62.1 specifies minimum ventilation rates and indoor air quality requirements, including duct design criteria, for commercial buildings.
Question 2: A technician notices supply air temperature drops 8°F between the air handler and a distant register. What is the most likely cause?
- Oversized blower motor
- Inadequate duct insulation allowing heat gain or loss (Correct answer)
- Incorrect thermostat calibration
- Dirty air filter
Correct answer: Inadequate duct insulation allowing heat gain or loss
Significant temperature drop along supply ducts typically indicates insufficient insulation allowing heat transfer between the duct and surrounding unconditioned space.
Question 3: What is the recommended maximum air velocity in a residential supply trunk duct to minimize noise?
- 400 FPM
- 700 FPM
- 900 FPM (Correct answer)
- 1,200 FPM
Correct answer: 900 FPM
Residential supply trunk ducts are typically designed for 700–900 FPM to balance efficiency with acceptable noise levels; 900 FPM is a common upper limit.
Question 4: When installing flexible duct, what is the maximum sag allowed per SMACNA guidelines between supports?
- 0.5 inches per foot of length
- 1.5 inches per foot of length (Correct answer)
- 3 inches per foot of length
- No sag is permitted at all
Correct answer: 1.5 inches per foot of length
SMACNA recommends flex duct supports no more than 5 feet apart, with no more than 1.5 inches of sag per foot of unsupported duct span.
Question 5: A VAV (Variable Air Volume) box modulates airflow by:
- Changing supply air temperature
- Varying the duct static pressure upstream
- Adjusting a damper to control CFM to the zone (Correct answer)
- Switching between heating and cooling coils
Correct answer: Adjusting a damper to control CFM to the zone
A VAV box uses a motorized damper to throttle airflow to the zone, maintaining zone temperature while the central AHU runs at constant temperature.
Question 6: Which duct material is most resistant to moisture and best suited for use in humid crawlspaces?
- Fiberglass duct board
- Galvanized sheet metal (Correct answer)
- Standard flex duct with fiberglass lining
- Uncoated black iron duct
Correct answer: Galvanized sheet metal
Galvanized sheet metal resists corrosion and moisture far better than fiberglass-lined products, which can grow mold in persistently humid crawlspaces.
Question 7: What is the purpose of a duct balancing damper installed near a branch takeoff?
- Prevent backdrafting from combustion appliances
- Allow manual adjustment of airflow to individual zones or rooms (Correct answer)
- Provide fire-rated separation between zones
- Filter particulates before they enter the branch
Correct answer: Allow manual adjustment of airflow to individual zones or rooms
Balancing dampers at branch takeoffs allow technicians to throttle airflow to individual rooms, balancing the system so each zone receives the designed CFM.
HVAC Ductwork 3
What ASHRAE standard primarily governs acceptable ventilation rates and duct design for commercial HVAC systems?